Re: [time-nuts] Thermal time constant
Most building insulation foam sold here in UK has an Alu foil cladding on both sides ... Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Predrag Dukic wrote: How well depleted it really is? DU has about 1/3 the U235 of natural U, of which less than 1% is U235. All the isotopes are radioactive, but I don't recall what the relative activities are. I think U238 has a half live of 4E9 years or more, so not very many atoms spontaneously disintegrate every second. Uranium separation is not perfect. Some radioactivity is still left. Not much.. you'd have more trouble if you lived on granite or lived in a brick/stone house. It's an alpha emitter, too, so painting it would provide shielding. It's the fact that it's a heavy metal that's more of an issue for toxicity. Don't know about the neutrons from the spontaneous fission. I don't think it is healthy to have it beside You for a long time. Certainly not 8 pounds :)) At 05:35 11.3.2010, you wrote: Bob Camp wrote: Hi The gotcha is that you are interested in the amount of heat per cubic foot rather than the amount of heat per pound. You need to take the standard heat per weight numbers and convert them to heat per volume numbers. Of the things you can easily get, copper is good. Steel is not as good as copper, but better than aluminum. Bob Gold is your friend. Mercury has convective losses because it's a liquid. Platinum or Osmium would also be ok. You might want something that is dense but lower conductivity.. lead? Depleted Uranium? (DU is available very cheaply if you can take 80,000 pound lots) ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. __ Information from ESET NOD32 Antivirus, version of virus signature database 4933 (20100310) __ The message was checked by ESET NOD32 Antivirus. http://www.eset.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Hi Bob: How about just wrapping the aluminum block in aluminum foil with the shiny side out. Then wrap the Styrofoam cube with the shiny side in before putting it in the aluminum box? Have Fun, Brooke Clarke http://www.PRC68.com Bob Camp wrote: Hi Since styrofoam is being rated as a building insulation, it's reasonable to believe that the material on both sides is up around 1. I highly doubt that somebody tossing styrofoam in walls is going to add a radiation factor ... In our application we're talking about a metal block inside a metal enclosure, polishing the surfaces could drop the emissivity by10X. If the budget allows, you could gold plate the surfaces in addition to polishing them . Bob On Mar 10, 2010, at 8:01 PM, Bruce Griffiths wrote: Oops forgot a factor of 4 Radiative heat transfer for surface with an emissivity of 1 at 300K is about 612uW/square cm/degree C which is equivalent to about 25mm of styrofoam. Which raises the question what's the emissivity of the isothermal surface used when measuring the thermal resistance of a slab of styrofoam? Bruce Bruce Griffiths wrote: Radiative heat transfer (for a surface with an emissivity of 1) is around 150uW/square cm /degreeC at 300K. That's equivalent to about 10cm of styrofoam. It seems unlikely that the radiative heat transfer component is included in the thermal resistance rating for Styrofoam. The radiative component is independent of insulation thickness where the insulation doesnt absorb in the 10-30um infrared region. Adding carbon black to the foam appears to increase the thermal resistance of 25mm thick foam by about 10%. Bruce Bob Camp wrote: Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Hi Sounds cheaper than gold plating. I'd put a couple layers of foil in the 8 of foam as well. Maybe a layer every 2. Bob -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Brooke Clarke Sent: Thursday, March 11, 2010 9:48 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Thermal time constant Hi Bob: How about just wrapping the aluminum block in aluminum foil with the shiny side out. Then wrap the Styrofoam cube with the shiny side in before putting it in the aluminum box? Have Fun, Brooke Clarke http://www.PRC68.com Bob Camp wrote: Hi Since styrofoam is being rated as a building insulation, it's reasonable to believe that the material on both sides is up around 1. I highly doubt that somebody tossing styrofoam in walls is going to add a radiation factor ... In our application we're talking about a metal block inside a metal enclosure, polishing the surfaces could drop the emissivity by10X. If the budget allows, you could gold plate the surfaces in addition to polishing them . Bob On Mar 10, 2010, at 8:01 PM, Bruce Griffiths wrote: Oops forgot a factor of 4 Radiative heat transfer for surface with an emissivity of 1 at 300K is about 612uW/square cm/degree C which is equivalent to about 25mm of styrofoam. Which raises the question what's the emissivity of the isothermal surface used when measuring the thermal resistance of a slab of styrofoam? Bruce Bruce Griffiths wrote: Radiative heat transfer (for a surface with an emissivity of 1) is around 150uW/square cm /degreeC at 300K. That's equivalent to about 10cm of styrofoam. It seems unlikely that the radiative heat transfer component is included in the thermal resistance rating for Styrofoam. The radiative component is independent of insulation thickness where the insulation doesnt absorb in the 10-30um infrared region. Adding carbon black to the foam appears to increase the thermal resistance of 25mm thick foam by about 10%. Bruce Bob Camp wrote: Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Hi Also true here in the US, but not for Styrofoam. Bob -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of David C. Partridge Sent: Thursday, March 11, 2010 8:53 AM To: 'Discussion of precise time and frequency measurement' Subject: Re: [time-nuts] Thermal time constant Most building insulation foam sold here in UK has an Alu foil cladding on both sides ... Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Jack Smith at Clifton Laboratories built a replacement for the 1502 chart recorders. An early prototype can be seen here: http://www.cliftonlaboratories.com/mar_2007.htm On Thu, 11 Mar 2010 04:51:00 +, Mark Sims hol...@hotmail.com wrote: Gawd, tell me about it... I just got through rebuilding the drive roller in half a dozen Tektronix YT-1 and YT-1S chart recorders for the 1502B/1520C/1503B/1503C TDR's -- kc0ukk at msosborn dot com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Hi Mathew: There are a bunch of them, but I like the DS3231 because it has a software frequency tweak and I2C interface. http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4627/t/al http://datasheets.maxim-ic.com/en/ds/DS3231.pdf Hi Bruce: Do you have a value for the thermal resistance of Styrofoam? Have Fun, Brooke Clarke http://www.PRC68.com Matthew Smith wrote: Quoth Brooke Clarke at 10/03/10 10:22... ...like a Dallas 32khz, or one of the newer Maxim units... There's a Maxim part to supersede the DS32kHz? I know that my regular vendors [Farnell, Soanar, Futurlec] in Australia don't stock the DS32kHz, which makes them mega-expensive to acquire. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Hi Bruce: What does m2K/W mean? See: http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm 50 mm it's about 1.5 and for 100 mm it's about 3. Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Bruce Griffiths wrote: Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 J/gm/C so the mass of Aluminium required would be 430gm if the thermal resistance to ambient were 1000K/W. With a thermal resistance of 100C/W you need 4.3Kg of aluminium Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
~3.3E-2 W/mK. If, as I suspect, its transparent to 10-20um infrared then adding intermediate aluminium foil radiation shields may be useful. Some styrofoam insulating panels include carbon black to inhibit infrared transmission: http://www.glasscellisofab.com/sheets/polystyrene/data_sheets/STYROFOAM%20PANELMATE.pdf Foam glass is also black for the same reason and its cells are filled with sulphur dioxide (at least it smalls like that when you cut it). Aerogel foam is also good (not sure about its infrared transparency). Bruce Brooke Clarke wrote: Hi Mathew: There are a bunch of them, but I like the DS3231 because it has a software frequency tweak and I2C interface. http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4627/t/al http://datasheets.maxim-ic.com/en/ds/DS3231.pdf Hi Bruce: Do you have a value for the thermal resistance of Styrofoam? Have Fun, Brooke Clarke http://www.PRC68.com Matthew Smith wrote: Quoth Brooke Clarke at 10/03/10 10:22... ...like a Dallas 32khz, or one of the newer Maxim units... There's a Maxim part to supersede the DS32kHz? I know that my regular vendors [Farnell, Soanar, Futurlec] in Australia don't stock the DS32kHz, which makes them mega-expensive to acquire. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Thermal resistance is measured in degrees (C or K or ..) per watt. Its inversely proportional to area and proportional to thickness. I think the clueless clown who created that table means that for a 1 square meter panel of the specified thickness the thermal resistance is the tabulated value measured in Kelvin/Watt. Bruce Brooke Clarke wrote: Hi Bruce: What does m2K/W mean? See: http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm 50 mm it's about 1.5 and for 100 mm it's about 3. Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Bruce Griffiths wrote: Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 J/gm/C so the mass of Aluminium required would be 430gm if the thermal resistance to ambient were 1000K/W. With a thermal resistance of 100C/W you need 4.3Kg of aluminium Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Hi Bruce: If a square meter of Styrofoam is 1.5 deg C/W then a cubic inch would be 39.37 * 39.37 * 1.5 or 2,325 deg C/W The DS3231 dissipates about 1 mw when running. I'm not sure how to come up with an allowable temp increase, but suspect it's based on not exceeding the max allowed operating temp. But for this part the Temp vs. freq curve is flattest at room temp so best to minimize the temp rise. So: (1 deg C) / (0.001 W) = 1000 deg C/W, the same number you had. That requires 185 g of Aluminum or about 69 cc volume or 4.25 cubic inches. That's a block about 1.62 on a side. How does that look? Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Thermal resistance is measured in degrees (C or K or ..) per watt. Its inversely proportional to area and proportional to thickness. I think the clueless clown who created that table means that for a 1 square meter panel of the specified thickness the thermal resistance is the tabulated value measured in Kelvin/Watt. Bruce Brooke Clarke wrote: Hi Bruce: What does m2K/W mean? See: http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm 50 mm it's about 1.5 and for 100 mm it's about 3. Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Bruce Griffiths wrote: Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 J/gm/C so the mass of Aluminium required would be 430gm if the thermal resistance to ambient were 1000K/W. With a thermal resistance of 100C/W you need 4.3Kg of aluminium Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Hi If a square meter of foam 2 thick is 1.5 C/W then a 1 square inch piece (also 2 thick) would be ~40^2 times better. Your cube has ~2 square inches on a side and six sides. (divide by ~12) The outer cube after you put 2 of foam on it has surfaces 5.6 on a side. That's a surface area of 25 square inches. The 10:1 increase in area needs to be considered.(divide by X in addition). 2325/12 = 193.75 193.76 / (5) = 38.75 C/W The block is going to get a *lot* bigger. Consider using steel instead of aluminum. Bob -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Brooke Clarke Sent: Wednesday, March 10, 2010 4:28 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Thermal Time Constant Hi Bruce: If a square meter of Styrofoam is 1.5 deg C/W then a cubic inch would be 39.37 * 39.37 * 1.5 or 2,325 deg C/W The DS3231 dissipates about 1 mw when running. I'm not sure how to come up with an allowable temp increase, but suspect it's based on not exceeding the max allowed operating temp. But for this part the Temp vs. freq curve is flattest at room temp so best to minimize the temp rise. So: (1 deg C) / (0.001 W) = 1000 deg C/W, the same number you had. That requires 185 g of Aluminum or about 69 cc volume or 4.25 cubic inches. That's a block about 1.62 on a side. How does that look? Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Thermal resistance is measured in degrees (C or K or ..) per watt. Its inversely proportional to area and proportional to thickness. I think the clueless clown who created that table means that for a 1 square meter panel of the specified thickness the thermal resistance is the tabulated value measured in Kelvin/Watt. Bruce Brooke Clarke wrote: Hi Bruce: What does m2K/W mean? See: http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm 50 mm it's about 1.5 and for 100 mm it's about 3. Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Bruce Griffiths wrote: Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 J/gm/C so the mass of Aluminium required would be 430gm if the thermal resistance to ambient were 1000K/W. With a thermal resistance of 100C/W you need 4.3Kg of aluminium Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Surely you meant 1 square inch of styrofoam with the specified thickness?? For a 100 hour (3.6E5 seconds) time constant and 1000 deg C/W thermal resistance you need a thermal capacitance of 3.6E5/1E3 = 360 J/C or 360/0.8371 = 430 gm of Aluminium which has a volume of 430/2.7 = 159 cc. You still have the thermal radiation coupling problem through the infrared transparent insulator. Gold plating the aluminium will help. Even if you solve the thermal radiation problem, the thermal conductance of any leads bringing power or signals to or from the clock will be an major problem even if you use constantan or austenetic stainless steel leads. Bruce Brooke Clarke wrote: Hi Bruce: If a square meter of Styrofoam is 1.5 deg C/W then a cubic inch would be 39.37 * 39.37 * 1.5 or 2,325 deg C/W The DS3231 dissipates about 1 mw when running. I'm not sure how to come up with an allowable temp increase, but suspect it's based on not exceeding the max allowed operating temp. But for this part the Temp vs. freq curve is flattest at room temp so best to minimize the temp rise. So: (1 deg C) / (0.001 W) = 1000 deg C/W, the same number you had. That requires 185 g of Aluminum or about 69 cc volume or 4.25 cubic inches. That's a block about 1.62 on a side. How does that look? Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Thermal resistance is measured in degrees (C or K or ..) per watt. Its inversely proportional to area and proportional to thickness. I think the clueless clown who created that table means that for a 1 square meter panel of the specified thickness the thermal resistance is the tabulated value measured in Kelvin/Watt. Bruce Brooke Clarke wrote: Hi Bruce: What does m2K/W mean? See: http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm 50 mm it's about 1.5 and for 100 mm it's about 3. Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Bruce Griffiths wrote: Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 J/gm/C so the mass of Aluminium required would be 430gm if the thermal resistance to ambient were 1000K/W. With a thermal resistance of 100C/W you need 4.3Kg of aluminium Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Hi The gotcha is that you are interested in the amount of heat per cubic foot rather than the amount of heat per pound. You need to take the standard heat per weight numbers and convert them to heat per volume numbers. Of the things you can easily get, copper is good. Steel is not as good as copper, but better than aluminum. Bob On Mar 10, 2010, at 5:52 PM, p...@pseng.org.uk wrote: Looking at the specific heat of metals: http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html Wouldn't Beryllium be better instead of aluminium? I could foresee a few problems though eg machining holes at home, never mind where to get a block from. Phil ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
A better metric may be stored heat for a dollar's worth of material. Scrap iron is pretty cheap. From: Bob Camp li...@rtty.us To: Discussion of precise time and frequency measurement time-nuts@febo.com Date: 03/10/2010 06:03 PM Subject: Re: [time-nuts] Thermal time constant Sent by: time-nuts-boun...@febo.com Hi The gotcha is that you are interested in the amount of heat per cubic foot rather than the amount of heat per pound. You need to take the standard heat per weight numbers and convert them to heat per volume numbers. Of the things you can easily get, copper is good. Steel is not as good as copper, but better than aluminum. Bob On Mar 10, 2010, at 5:52 PM, p...@pseng.org.uk wrote: Looking at the specific heat of metals: http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html Wouldn't Beryllium be better instead of aluminium? I could foresee a few problems though eg machining holes at home, never mind where to get a block from. Phil ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
p...@pseng.org.uk wrote: Looking at the specific heat of metals: http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html Wouldn't Beryllium be better instead of aluminium? I could foresee a few problems though eg machining holes at home, never mind where to get a block from. Phil Actually in this application minimising the volume of the metal block rather than its weight is probably better so one should use specific heat*density as a figure of merit. METALSpecific heat*density (W/J/cubic cm) Aluminium 2.46 Beryllium3.29 Copper 3.49 Silver 2.41 Iron3.87 Thus copper and iron are better choices than Beryllium they also poses fewer machining diffculties Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Just in case anyone finds a beryllium brick somewhere and tries to use it: Beryllium is very very poisonous. cheers, Neville Michie On 11/03/2010, at 10:08 AM, Bruce Griffiths wrote: p...@pseng.org.uk wrote: Looking at the specific heat of metals: http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html Wouldn't Beryllium be better instead of aluminium? I could foresee a few problems though eg machining holes at home, never mind where to get a block from. Phil Actually in this application minimising the volume of the metal block rather than its weight is probably better so one should use specific heat*density as a figure of merit. METALSpecific heat*density (W/J/cubic cm) Aluminium 2.46 Beryllium3.29 Copper 3.49 Silver 2.41 Iron3.87 Thus copper and iron are better choices than Beryllium they also poses fewer machining diffculties Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
There's at least one known case where someone attempted to grind a beryllium mirror substrate using conventional methods without proper control of dust resulting in his death. Bruce Neville Michie wrote: Just in case anyone finds a beryllium brick somewhere and tries to use it: Beryllium is very very poisonous. cheers, Neville Michie On 11/03/2010, at 10:08 AM, Bruce Griffiths wrote: p...@pseng.org.uk wrote: Looking at the specific heat of metals: http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html Wouldn't Beryllium be better instead of aluminium? I could foresee a few problems though eg machining holes at home, never mind where to get a block from. Phil Actually in this application minimising the volume of the metal block rather than its weight is probably better so one should use specific heat*density as a figure of merit. METALSpecific heat*density (W/J/cubic cm) Aluminium 2.46 Beryllium3.29 Copper 3.49 Silver 2.41 Iron3.87 Thus copper and iron are better choices than Beryllium they also poses fewer machining diffculties Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Hi The same caution applies to the beryllium in beryllium oxide heat spreaders. You find them in old RF transistors and some RF tubes. Bob On Mar 10, 2010, at 6:45 PM, Bruce Griffiths wrote: There's at least one known case where someone attempted to grind a beryllium mirror substrate using conventional methods without proper control of dust resulting in his death. Bruce Neville Michie wrote: Just in case anyone finds a beryllium brick somewhere and tries to use it: Beryllium is very very poisonous. cheers, Neville Michie On 11/03/2010, at 10:08 AM, Bruce Griffiths wrote: p...@pseng.org.uk wrote: Looking at the specific heat of metals: http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html Wouldn't Beryllium be better instead of aluminium? I could foresee a few problems though eg machining holes at home, never mind where to get a block from. Phil Actually in this application minimising the volume of the metal block rather than its weight is probably better so one should use specific heat*density as a figure of merit. METALSpecific heat*density (W/J/cubic cm) Aluminium 2.46 Beryllium3.29 Copper 3.49 Silver 2.41 Iron3.87 Thus copper and iron are better choices than Beryllium they also poses fewer machining diffculties Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Beryllium copper parts are also an issue although the beryllium content is only 0.3%-0.5% by weight. Bruce Bob Camp wrote: Hi The same caution applies to the beryllium in beryllium oxide heat spreaders. You find them in old RF transistors and some RF tubes. Bob On Mar 10, 2010, at 6:45 PM, Bruce Griffiths wrote: There's at least one known case where someone attempted to grind a beryllium mirror substrate using conventional methods without proper control of dust resulting in his death. Bruce Neville Michie wrote: Just in case anyone finds a beryllium brick somewhere and tries to use it: Beryllium is very very poisonous. cheers, Neville Michie On 11/03/2010, at 10:08 AM, Bruce Griffiths wrote: p...@pseng.org.uk wrote: Looking at the specific heat of metals: http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html Wouldn't Beryllium be better instead of aluminium? I could foresee a few problems though eg machining holes at home, never mind where to get a block from. Phil Actually in this application minimising the volume of the metal block rather than its weight is probably better so one should use specific heat*density as a figure of merit. METALSpecific heat*density (W/J/cubic cm) Aluminium 2.46 Beryllium3.29 Copper 3.49 Silver 2.41 Iron3.87 Thus copper and iron are better choices than Beryllium they also poses fewer machining diffculties Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Hi So getting back to the original idea. Taking the other extreme - if you go for ~20 C / W on the insulation: The mass of the block will be ~50X what it was before. 1.6 on a side goes to ~5 on a side. Surface area goes up ~10X. 200 C/W with 2 foam goes to 20 C/W Outer surface area goes to ~1.6X the inner surface area Go to 4 of foam and you are pretty close. Go to 8 of foam and you have some room left for things like wires and the like. Resulting block is a bit under 2 feet on a side Bob On Mar 10, 2010, at 4:48 PM, Bruce Griffiths wrote: Surely you meant 1 square inch of styrofoam with the specified thickness?? For a 100 hour (3.6E5 seconds) time constant and 1000 deg C/W thermal resistance you need a thermal capacitance of 3.6E5/1E3 = 360 J/C or 360/0.8371 = 430 gm of Aluminium which has a volume of 430/2.7 = 159 cc. You still have the thermal radiation coupling problem through the infrared transparent insulator. Gold plating the aluminium will help. Even if you solve the thermal radiation problem, the thermal conductance of any leads bringing power or signals to or from the clock will be an major problem even if you use constantan or austenetic stainless steel leads. Bruce Brooke Clarke wrote: Hi Bruce: If a square meter of Styrofoam is 1.5 deg C/W then a cubic inch would be 39.37 * 39.37 * 1.5 or 2,325 deg C/W The DS3231 dissipates about 1 mw when running. I'm not sure how to come up with an allowable temp increase, but suspect it's based on not exceeding the max allowed operating temp. But for this part the Temp vs. freq curve is flattest at room temp so best to minimize the temp rise. So: (1 deg C) / (0.001 W) = 1000 deg C/W, the same number you had. That requires 185 g of Aluminum or about 69 cc volume or 4.25 cubic inches. That's a block about 1.62 on a side. How does that look? Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Thermal resistance is measured in degrees (C or K or ..) per watt. Its inversely proportional to area and proportional to thickness. I think the clueless clown who created that table means that for a 1 square meter panel of the specified thickness the thermal resistance is the tabulated value measured in Kelvin/Watt. Bruce Brooke Clarke wrote: Hi Bruce: What does m2K/W mean? See: http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm 50 mm it's about 1.5 and for 100 mm it's about 3. Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Bruce Griffiths wrote: Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 J/gm/C so the mass of Aluminium required would be 430gm if the thermal resistance to ambient were 1000K/W. With a thermal resistance of 100C/W you need 4.3Kg of aluminium Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Hi Bob: I like the idea of a six inch on a side cube. An outer aluminum box, styrofoam insulation, and a block of (copper, Aluminum, etc.) in the center. A DS3231 or similar RTC and a PIC with a DB-9 connector on one face. Have Fun, Brooke Clarke http://www.PRC68.com Bob Camp wrote: Hi So getting back to the original idea. Taking the other extreme - if you go for ~20 C / W on the insulation: The mass of the block will be ~50X what it was before. 1.6 on a side goes to ~5 on a side. Surface area goes up ~10X. 200 C/W with 2 foam goes to 20 C/W Outer surface area goes to ~1.6X the inner surface area Go to 4 of foam and you are pretty close. Go to 8 of foam and you have some room left for things like wires and the like. Resulting block is a bit under 2 feet on a side Bob On Mar 10, 2010, at 4:48 PM, Bruce Griffiths wrote: Surely you meant 1 square inch of styrofoam with the specified thickness?? For a 100 hour (3.6E5 seconds) time constant and 1000 deg C/W thermal resistance you need a thermal capacitance of 3.6E5/1E3 = 360 J/C or 360/0.8371 = 430 gm of Aluminium which has a volume of 430/2.7 = 159 cc. You still have the thermal radiation coupling problem through the infrared transparent insulator. Gold plating the aluminium will help. Even if you solve the thermal radiation problem, the thermal conductance of any leads bringing power or signals to or from the clock will be an major problem even if you use constantan or austenetic stainless steel leads. Bruce Brooke Clarke wrote: Hi Bruce: If a square meter of Styrofoam is 1.5 deg C/W then a cubic inch would be 39.37 * 39.37 * 1.5 or 2,325 deg C/W The DS3231 dissipates about 1 mw when running. I'm not sure how to come up with an allowable temp increase, but suspect it's based on not exceeding the max allowed operating temp. But for this part the Temp vs. freq curve is flattest at room temp so best to minimize the temp rise. So: (1 deg C) / (0.001 W) = 1000 deg C/W, the same number you had. That requires 185 g of Aluminum or about 69 cc volume or 4.25 cubic inches. That's a block about 1.62 on a side. How does that look? Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Thermal resistance is measured in degrees (C or K or ..) per watt. Its inversely proportional to area and proportional to thickness. I think the clueless clown who created that table means that for a 1 square meter panel of the specified thickness the thermal resistance is the tabulated value measured in Kelvin/Watt. Bruce Brooke Clarke wrote: Hi Bruce: What does m2K/W mean? See: http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm 50 mm it's about 1.5 and for 100 mm it's about 3. Have Fun, Brooke Clarke http://www.PRC68.com Bruce Griffiths wrote: Bruce Griffiths wrote: Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 J/gm/C so the mass of Aluminium required would be 430gm if the thermal resistance to ambient were 1000K/W. With a thermal resistance of 100C/W you need 4.3Kg of aluminium Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
Re: [time-nuts] Thermal time constant
I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Radiative heat transfer (for a surface with an emissivity of 1) is around 150uW/square cm /degreeC at 300K. That's equivalent to about 10cm of styrofoam. It seems unlikely that the radiative heat transfer component is included in the thermal resistance rating for Styrofoam. The radiative component is independent of insulation thickness where the insulation doesnt absorb in the 10-30um infrared region. Adding carbon black to the foam appears to increase the thermal resistance of 25mm thick foam by about 10%. Bruce Bob Camp wrote: Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Oops forgot a factor of 4 Radiative heat transfer for surface with an emissivity of 1 at 300K is about 612uW/square cm/degree C which is equivalent to about 25mm of styrofoam. Which raises the question what's the emissivity of the isothermal surface used when measuring the thermal resistance of a slab of styrofoam? Bruce Bruce Griffiths wrote: Radiative heat transfer (for a surface with an emissivity of 1) is around 150uW/square cm /degreeC at 300K. That's equivalent to about 10cm of styrofoam. It seems unlikely that the radiative heat transfer component is included in the thermal resistance rating for Styrofoam. The radiative component is independent of insulation thickness where the insulation doesnt absorb in the 10-30um infrared region. Adding carbon black to the foam appears to increase the thermal resistance of 25mm thick foam by about 10%. Bruce Bob Camp wrote: Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Hi Since styrofoam is being rated as a building insulation, it's reasonable to believe that the material on both sides is up around 1. I highly doubt that somebody tossing styrofoam in walls is going to add a radiation factor ... In our application we're talking about a metal block inside a metal enclosure, polishing the surfaces could drop the emissivity by 10X. If the budget allows, you could gold plate the surfaces in addition to polishing them . Bob On Mar 10, 2010, at 8:01 PM, Bruce Griffiths wrote: Oops forgot a factor of 4 Radiative heat transfer for surface with an emissivity of 1 at 300K is about 612uW/square cm/degree C which is equivalent to about 25mm of styrofoam. Which raises the question what's the emissivity of the isothermal surface used when measuring the thermal resistance of a slab of styrofoam? Bruce Bruce Griffiths wrote: Radiative heat transfer (for a surface with an emissivity of 1) is around 150uW/square cm /degreeC at 300K. That's equivalent to about 10cm of styrofoam. It seems unlikely that the radiative heat transfer component is included in the thermal resistance rating for Styrofoam. The radiative component is independent of insulation thickness where the insulation doesnt absorb in the 10-30um infrared region. Adding carbon black to the foam appears to increase the thermal resistance of 25mm thick foam by about 10%. Bruce Bob Camp wrote: Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
There's a small problem with my calculation and the published figures for the thermal resistance of styrofoam. If the measurements include radiative transfers the thermal resistance per unit area of a styrofoam slab should reach a limiting value (when the radiative transfer component dominates) as the thickness is increased. Possibilities 1) My calculation is incorrect. 2) Infrared absorption in thick styrofoam slabs is significant. 3) The tabulated figures for the thermal resistance of styrofoam are merely scaled up from the values measured with thin sheets. This gives misleading values for thick sheets if the tabulated values include radiative transfer. Bruce Bob Camp wrote: Hi Since styrofoam is being rated as a building insulation, it's reasonable to believe that the material on both sides is up around 1. I highly doubt that somebody tossing styrofoam in walls is going to add a radiation factor ... In our application we're talking about a metal block inside a metal enclosure, polishing the surfaces could drop the emissivity by10X. If the budget allows, you could gold plate the surfaces in addition to polishing them . Bob On Mar 10, 2010, at 8:01 PM, Bruce Griffiths wrote: Oops forgot a factor of 4 Radiative heat transfer for surface with an emissivity of 1 at 300K is about 612uW/square cm/degree C which is equivalent to about 25mm of styrofoam. Which raises the question what's the emissivity of the isothermal surface used when measuring the thermal resistance of a slab of styrofoam? Bruce Bruce Griffiths wrote: Radiative heat transfer (for a surface with an emissivity of 1) is around 150uW/square cm /degreeC at 300K. That's equivalent to about 10cm of styrofoam. It seems unlikely that the radiative heat transfer component is included in the thermal resistance rating for Styrofoam. The radiative component is independent of insulation thickness where the insulation doesnt absorb in the 10-30um infrared region. Adding carbon black to the foam appears to increase the thermal resistance of 25mm thick foam by about 10%. Bruce Bob Camp wrote: Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Hi Low density (1 or 2 lb / cu ft) urethane foam is going to be a better insulator than styrofoam. I believe it's reasonably opaque at IR. Bob On Mar 10, 2010, at 8:23 PM, Bruce Griffiths wrote: There's a small problem with my calculation and the published figures for the thermal resistance of styrofoam. If the measurements include radiative transfers the thermal resistance per unit area of a styrofoam slab should reach a limiting value (when the radiative transfer component dominates) as the thickness is increased. Possibilities 1) My calculation is incorrect. 2) Infrared absorption in thick styrofoam slabs is significant. 3) The tabulated figures for the thermal resistance of styrofoam are merely scaled up from the values measured with thin sheets. This gives misleading values for thick sheets if the tabulated values include radiative transfer. Bruce Bob Camp wrote: Hi Since styrofoam is being rated as a building insulation, it's reasonable to believe that the material on both sides is up around 1. I highly doubt that somebody tossing styrofoam in walls is going to add a radiation factor ... In our application we're talking about a metal block inside a metal enclosure, polishing the surfaces could drop the emissivity by10X. If the budget allows, you could gold plate the surfaces in addition to polishing them . Bob On Mar 10, 2010, at 8:01 PM, Bruce Griffiths wrote: Oops forgot a factor of 4 Radiative heat transfer for surface with an emissivity of 1 at 300K is about 612uW/square cm/degree C which is equivalent to about 25mm of styrofoam. Which raises the question what's the emissivity of the isothermal surface used when measuring the thermal resistance of a slab of styrofoam? Bruce Bruce Griffiths wrote: Radiative heat transfer (for a surface with an emissivity of 1) is around 150uW/square cm /degreeC at 300K. That's equivalent to about 10cm of styrofoam. It seems unlikely that the radiative heat transfer component is included in the thermal resistance rating for Styrofoam. The radiative component is independent of insulation thickness where the insulation doesnt absorb in the 10-30um infrared region. Adding carbon black to the foam appears to increase the thermal resistance of 25mm thick foam by about 10%. Bruce Bob Camp wrote: Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Depends on the duration of the experiment. Urethanes arent well known for stability. The low thermal resistance of close cell urethanes is largely due to their better retention of the blowing agent which has lower thermal conductivity than air. Closed cell foam glass (however the sulphur dioxide released when a cell is broken may be an issue) is much more stable than plastic foams the thermal resistance of which slowly deteriorates due to gas diffusion (air diffuse in and blowing agent diffuses out). The gradual permeation of the plastic foam by water vapour can also have a significant effect. Foam glass is sometimes used to insulate very large cryogen tanks. Bruce Bob Camp wrote: Hi Low density (1 or 2 lb / cu ft) urethane foam is going to be a better insulator than styrofoam. I believe it's reasonably opaque at IR. Bob On Mar 10, 2010, at 8:23 PM, Bruce Griffiths wrote: There's a small problem with my calculation and the published figures for the thermal resistance of styrofoam. If the measurements include radiative transfers the thermal resistance per unit area of a styrofoam slab should reach a limiting value (when the radiative transfer component dominates) as the thickness is increased. Possibilities 1) My calculation is incorrect. 2) Infrared absorption in thick styrofoam slabs is significant. 3) The tabulated figures for the thermal resistance of styrofoam are merely scaled up from the values measured with thin sheets. This gives misleading values for thick sheets if the tabulated values include radiative transfer. Bruce Bob Camp wrote: Hi Since styrofoam is being rated as a building insulation, it's reasonable to believe that the material on both sides is up around 1. I highly doubt that somebody tossing styrofoam in walls is going to add a radiation factor ... In our application we're talking about a metal block inside a metal enclosure, polishing the surfaces could drop the emissivity by10X. If the budget allows, you could gold plate the surfaces in addition to polishing them . Bob On Mar 10, 2010, at 8:01 PM, Bruce Griffiths wrote: Oops forgot a factor of 4 Radiative heat transfer for surface with an emissivity of 1 at 300K is about 612uW/square cm/degree C which is equivalent to about 25mm of styrofoam. Which raises the question what's the emissivity of the isothermal surface used when measuring the thermal resistance of a slab of styrofoam? Bruce Bruce Griffiths wrote: Radiative heat transfer (for a surface with an emissivity of 1) is around 150uW/square cm /degreeC at 300K. That's equivalent to about 10cm of styrofoam. It seems unlikely that the radiative heat transfer component is included in the thermal resistance rating for Styrofoam. The radiative component is independent of insulation thickness where the insulation doesnt absorb in the 10-30um infrared region. Adding carbon black to the foam appears to increase the thermal resistance of 25mm thick foam by about 10%. Bruce Bob Camp wrote: Hi That raises the interesting question of weather radiant transfer is already included in the rated thermal resistance. My guess is that the 8 of foam is enough to cover any radiation issues and still get you above 20 C / W. Since you are probably starting with 1 or 2 slabs, including the aluminum foil would be pretty easy. It can't hurt and it might help. Bob On Mar 10, 2010, at 7:29 PM, Neville Michie wrote: I have been wondering about achieving the rated thermal resistance from plastic foam, the problem being that radiant transmission may be very strong through the foam. What happens if you interleave concentric sheets of foam plastic with aluminium foil? (taking care to keep the foil on isothermal surfaces) Will this stop the radiant transfer and leave only the thermal conduction of the plastic foam? cheers, Neville Michie ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and
Re: [time-nuts] Thermal Time Constant
Brooke Clarke wrote: Hi Bruce: What does m2K/W mean? See: http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm 50 mm it's about 1.5 and for 100 mm it's about 3. thermal resistance (you can tell because it's degrees/watt, as opposed to watts/degree which would be conductivity) square meter Kelvin/Watt. and doubling for a doubling in thickness makes perfect sense. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Brooke Clarke wrote: Hi Bruce: If a square meter of Styrofoam is 1.5 deg C/W then a cubic inch would be 39.37 * 39.37 * 1.5 or 2,325 deg C/W The DS3231 dissipates about 1 mw when running. I'm not sure how to come up with an allowable temp increase, but suspect it's based on not exceeding the max allowed operating temp. But for this part the Temp vs. freq curve is flattest at room temp so best to minimize the temp rise. So: (1 deg C) / (0.001 W) = 1000 deg C/W, the same number you had. That requires 185 g of Aluminum or about 69 cc volume or 4.25 cubic inches. That's a block about 1.62 on a side. How does that look? you can also use U.S. R-values.. BTU/hr/ft^2/deg F. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
Bob Camp wrote: Hi The gotcha is that you are interested in the amount of heat per cubic foot rather than the amount of heat per pound. You need to take the standard heat per weight numbers and convert them to heat per volume numbers. Of the things you can easily get, copper is good. Steel is not as good as copper, but better than aluminum. Bob Gold is your friend. Mercury has convective losses because it's a liquid. Platinum or Osmium would also be ok. You might want something that is dense but lower conductivity.. lead? Depleted Uranium? (DU is available very cheaply if you can take 80,000 pound lots) ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
water is pretty good, too and cheap. Coupling to it is easy as well. Don - Original Message - From: jimlux jim...@earthlink.net To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, March 10, 2010 9:35 PM Subject: Re: [time-nuts] Thermal time constant Bob Camp wrote: Hi The gotcha is that you are interested in the amount of heat per cubic foot rather than the amount of heat per pound. You need to take the standard heat per weight numbers and convert them to heat per volume numbers. Of the things you can easily get, copper is good. Steel is not as good as copper, but better than aluminum. Bob Gold is your friend. Mercury has convective losses because it's a liquid. Platinum or Osmium would also be ok. You might want something that is dense but lower conductivity.. lead? Depleted Uranium? (DU is available very cheaply if you can take 80,000 pound lots) ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal time constant
How well depleted it really is? Uranium separation is not perfect. Some radioactivity is still left. I don't think it is healthy to have it beside You for a long time. Certainly not 8 pounds :)) At 05:35 11.3.2010, you wrote: Bob Camp wrote: Hi The gotcha is that you are interested in the amount of heat per cubic foot rather than the amount of heat per pound. You need to take the standard heat per weight numbers and convert them to heat per volume numbers. Of the things you can easily get, copper is good. Steel is not as good as copper, but better than aluminum. Bob Gold is your friend. Mercury has convective losses because it's a liquid. Platinum or Osmium would also be ok. You might want something that is dense but lower conductivity.. lead? Depleted Uranium? (DU is available very cheaply if you can take 80,000 pound lots) ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. __ Information from ESET NOD32 Antivirus, version of virus signature database 4933 (20100310) __ The message was checked by ESET NOD32 Antivirus. http://www.eset.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Quoth Brooke Clarke at 10/03/10 10:22... ...like a Dallas 32khz, or one of the newer Maxim units... There's a Maxim part to supersede the DS32kHz? I know that my regular vendors [Farnell, Soanar, Futurlec] in Australia don't stock the DS32kHz, which makes them mega-expensive to acquire. -- Matthew Smith Smiffytech - Technology Consulting Web Application Development Business: http://www.smiffytech.com/ Blog/personal: http://www.smiffysplace.com/ LinkedIn: http://www.linkedin.com/in/smiffy Skype: msmiffy Twitter: @smiffy ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Thermal Time Constant
Bruce Griffiths wrote: Brooke Clarke wrote: Hi: For some time I've considered surrounding a free running 32678 Hz oscillator (like a Dallas 32khz, or one of the newer Maxim units) with thermal mass and insulation in order to get the time constant into the range of some days. To get a feel for it a simple experiment shows that a half inch diameter brass rod 3.75 long (102 grams) has a thermal time constant of about 6 min 35 seconds when wrapped lightly in a towel. Is there a way to calculate the amount of aluminum and Styrofoam needed to get a time constant of say 100 hours? This came up in relation to WWVB clocks that free run for most of the time. When you compare WWVB clocks it's not uncommon to see tens of seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC Start with the maximum thermal resistance the application can withstand (determined by internal dissipation and acceptable temperature rise above ambient). If for example the dissipation is 10mW and acceptable temperature rise 10C then thermal resistance will be about 1000C/W. The thermal capacity required can then be calculated from the time constant: C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium. The required thickness of styrofoam can then be calculated from the surface area of the aluminium block. Achieving a thermal resistance of 1000C/W may be a little difficult without using radiation shields. Bruce Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 J/gm/C so the mass of Aluminium required would be 430gm if the thermal resistance to ambient were 1000K/W. With a thermal resistance of 100C/W you need 4.3Kg of aluminium Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
