Re: [volt-nuts] Best way to measure micro Ohms
Coincidental to all this micro-ohms and nanovolt talk, I've been doing some severe large scale garage cleaning to thin stuff out. I found that audio amplifier that I mentioned earlier, that is good for some LIA reference driver applications. I also found my low-level measurement notebooks, including the datasheet and my notes about its operation and modifications. I found that the exact same datasheet I got online years ago is still available, so here it is, if anyone is interested. http://www.toacanada.com/assets/files/BG-10_IM.pdf I also dug out the old Keithley 148 nanovoltmeters, etc, and couldn't resist fooling around with them for a bit. I'll have more to say later, but in new threads. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Ed, It is not sinusoidal AC. It is a DC voltage which is allowed to settle before it is measured, It is then measured a few times to get an average. The excitation current is then turned off to allow the resistors to cool before the DC voltage is reversed and the process is continued. The key factor is that all measurements are taken in a stable state. There is a difference between reversing DC and AC. Years ago I ask a guy how he got resistance values plotted to as many digits as he did on a set of bridge resistors. He told me he used an HP 3457A in statistics mode and measured the voltage drop on the sent then revised the current exciting the bridge and took the same measurements again. He then averaged the values together to get the ratios of the 2 bridge arms. He called it reversing DC. The first time I remember hearing the term. AC current sources are inherently less stable than DC current sources and AC measurement system are inherently less accurate than DC system. If you use AC rather than DC you are going to give up at least one and maybe 2 least significant digits. You would not be having this discussion if you were willing to throw that much accuracy away. The standard resistor used in the test typically has a DC documented value as well as a documented temperature coefficient and an undocumented AC value with an undocumented frequency coefficient and undocumented AC temperature coefficient. True AC measurements are not nearly as accurate as DC measurements. HP 3458A has a mode for measuring AC where it takes a bunch of DC measurements at high speed and calculates the root mean square. It works best on repetitive low noise sinusoidal signals. For the correct signal, it is the most accurate AC mode. For other signals, there is an AC to DC converter that feeds a DC measurement system. In that case, the vast majority of error comes from the converter. If the converter method gives you a tighter range of readings than the DC method you know you have dirty signal/distorted when compared to a sine wave or not very repetitive. It works a lot like a sampling O-scope. Both of which depend on a solid trigger signal. On Thu, Sep 28, 2017 at 11:17 AM, ed breya wrote: > Hmm. Alternating the direction of the current repeatedly and processing > the results - sure seems like that is fundamentally an AC measurement too, > despite using DC measurement equipment. > > Ed > > > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/m > ailman/listinfo/volt-nuts > and follow the instructions there. > -- *John Phillips* ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
On 28 September 2017 at 19:17, ed breya wrote: > Hmm. Alternating the direction of the current repeatedly and processing > the results - sure seems like that is fundamentally an AC measurement too, > despite using DC measurement equipment. > > Ed I guess the point is current/voltage can be measured accurately before the direction is changed, whereas that's less so with AC. But I don't need great accuracy in trying to sort out if there are contact resistance problems caused by things such as copper slip, or aluminum oxides. I need to measure very low resistances, but not with great accuracy. For me at least, I have no Fluke 5520A, or low resistance meter. The Agilent programmable DC power supplies I have are not designed for precision measurements. But the lock-in amplifier I already had, and the audio amplifier was cheap. Sometimes you have to make do with what you have, or can justify buying. Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Hmm. Alternating the direction of the current repeatedly and processing the results - sure seems like that is fundamentally an AC measurement too, despite using DC measurement equipment. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Any technology that does not DC must have some other overriding redeeming factors given the AC measurements are not nearly as accurate DC and AC currents are just as inaccurate. Best bridges I have used take several reading with half of them opposite polarity and average them together. Sometimes called the reversing DC method. Given what you are trying to measure your temperature and skin effects (not RF skin) induce more error than a good instrumentation system. The current duty cycle can make a big difference when you are looking this deep into the mud. The current source should be a fast settling DC supply of about 10 amps. More than 10 amps and stability and accurate usually degrades. The DC voltage measurement system should look for a stable DC level before taking a usable current is then turned off then reversed for the next reading. Several reading are filtered and averaged together. Use something like a Fluke 5520A. I believe the 488 bus control can reverse the polarity, if not use an external relay to reverse the DC current. Different current levels may be used to see where you get the most stable readings. Use a standard resistor in the same range as the UUT. 2 HP 3458As can simultaneously read the standard restore and the UUT. 1 3458 may work in ratio mode but probably will not give good readings at these voltage levels. In any case, each input should be switched between the standard and UUT. All of the hardware would be controlled by a basic program running in excel to load the data into a spreadsheet for filtering and analysis. In the end, the ratio between the standard resistor and the UUT is all the counts. Well not quite. The stability of each instrument will make a difference. The absolute current and voltage calibration kindof cancel out as long as the meters are close to calibration stable and linear. Make sure the voltage measurements are all taken in the same range. If you do not want to get this elaborate there are several 10 amp micro-ohm metes to be had. The good ones do the reversing DC with math internally and are quick enough to do minimal sample heating. Any that put out a constant 10 amps while measuring are inferior. On Wed, Sep 27, 2017 at 3:51 PM, Dr. David Kirkby (Kirkby Microwave Ltd) < drkir...@kirkbymicrowave.co.uk> wrote: > On 27 September 2017 at 22:22, ed breya wrote: > > > I just noticed this discussion recently, so I'm late to the party, but > > that never stops me from adding my one-cent's worth. > > > > David, regardless of the aluminum and other material issues, I think your > > initial idea of using a lock-in analyzer is definitely the way to go. I'm > > very fond of LIAs, although I seldom need or use them, so my opinion is > > somewhat biased. I have five - two Ithaco 391A orange-band, a PAR 5204, > an > > SR830 and SR850. > > > > > Yes, me too. I have used a number over the years, the nicest of which was > the Stanford Research SR830. The EG&G 7260 I own has rather user-hostile > interface. There's not even a power switch on the front, and I'm not sure > if there's even on on the rear. > > The following link might interest others who don't know about these > instruments > > http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/AboutLIAs.pdf > > > > > > > If you use an audio power amplifier for driving the experiment, you can > > rig it up so that the LIA can be used to measure the drive current as > well > > as the resulting voltage drop. Let's say the amplifier is for 8 ohms, so > > you put a few ohms in series with the output, then from there into a > > precision one-ohm sampling resistor, then into the RUT, forming a voltage > > divider. The RUT is expected to be in the micro-ohm region, which is many > > thousands of times smaller than the sampling R, so its tiny voltage drop > > will be negligible, allowing the sample voltage to be a good > representation > > of the test current. You could also just treat the whole thing as a > voltage > > divider and calculate the "exact" results. > > > > I did purchase a Radio Shack 100 W Public address I will set this up > later. > > > > > > This could be fun and interesting. There are plenty of pieces and > > variables involved to experiment with to optimize the measurement, and > lots > > of other tricks available to enhance it if necessary. > > > > Ed > > > Yes, with lock-in amplifiers there are an almost infinite set of > possibilities of how to do the actual measurement. I don't know if the > reference output is a sine wave or square wave on this unit. I have the > option of using a Stanford Research DS345 30 MHz function generator to > generate a sine wave if needed. There are almost a million things that one > can change. > > Dave > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/volt-nuts > and follow the instructions there. > -- *John Phillips*
Re: [volt-nuts] Best way to measure micro Ohms
On 27 September 2017 at 22:22, ed breya wrote: > I just noticed this discussion recently, so I'm late to the party, but > that never stops me from adding my one-cent's worth. > > David, regardless of the aluminum and other material issues, I think your > initial idea of using a lock-in analyzer is definitely the way to go. I'm > very fond of LIAs, although I seldom need or use them, so my opinion is > somewhat biased. I have five - two Ithaco 391A orange-band, a PAR 5204, an > SR830 and SR850. > Yes, me too. I have used a number over the years, the nicest of which was the Stanford Research SR830. The EG&G 7260 I own has rather user-hostile interface. There's not even a power switch on the front, and I'm not sure if there's even on on the rear. The following link might interest others who don't know about these instruments http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/AboutLIAs.pdf > > If you use an audio power amplifier for driving the experiment, you can > rig it up so that the LIA can be used to measure the drive current as well > as the resulting voltage drop. Let's say the amplifier is for 8 ohms, so > you put a few ohms in series with the output, then from there into a > precision one-ohm sampling resistor, then into the RUT, forming a voltage > divider. The RUT is expected to be in the micro-ohm region, which is many > thousands of times smaller than the sampling R, so its tiny voltage drop > will be negligible, allowing the sample voltage to be a good representation > of the test current. You could also just treat the whole thing as a voltage > divider and calculate the "exact" results. > I did purchase a Radio Shack 100 W Public address I will set this up later. > > This could be fun and interesting. There are plenty of pieces and > variables involved to experiment with to optimize the measurement, and lots > of other tricks available to enhance it if necessary. > > Ed Yes, with lock-in amplifiers there are an almost infinite set of possibilities of how to do the actual measurement. I don't know if the reference output is a sine wave or square wave on this unit. I have the option of using a Stanford Research DS345 30 MHz function generator to generate a sine wave if needed. There are almost a million things that one can change. Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
I just noticed this discussion recently, so I'm late to the party, but that never stops me from adding my one-cent's worth. David, regardless of the aluminum and other material issues, I think your initial idea of using a lock-in analyzer is definitely the way to go. I'm very fond of LIAs, although I seldom need or use them, so my opinion is somewhat biased. I have five - two Ithaco 391A orange-band, a PAR 5204, an SR830 and SR850. If you use an audio power amplifier for driving the experiment, you can rig it up so that the LIA can be used to measure the drive current as well as the resulting voltage drop. Let's say the amplifier is for 8 ohms, so you put a few ohms in series with the output, then from there into a precision one-ohm sampling resistor, then into the RUT, forming a voltage divider. The RUT is expected to be in the micro-ohm region, which is many thousands of times smaller than the sampling R, so its tiny voltage drop will be negligible, allowing the sample voltage to be a good representation of the test current. You could also just treat the whole thing as a voltage divider and calculate the "exact" results. The voltage on the RUT is measured at whatever gain is needed. The voltage on the sample resistor will be plentiful at 1V/A, and both signals will have very low source R, and minimal noise. Since both signals can be measured by the LIA, the uncertainties in assessing each part with different equipment are much reduced. The reference frequency should be as low as possible, limited by the amplifier's low-end capability, and selected so it and its harmonics land as far as possible from the power line frequency and its harmonics, for say up to n=15, or whatever is practical. This will help to reduce line interference from nearby sources, and ground loops, and from the amplifier. Especially at low frequencies, the amplifier may show a lot of line harmonics when driven to high levels - the filter capacitors in its power supply can only do so much, and audio PAs are likely not all that great in terms of PSRR. Turning on the LIA's line notch filter will also help, at least with the fundamental. The frequency needs to be very low in order to minimize the parasitic currents that will cause errors, especially considering that this setup is dividing on the order of a million in a single stage. If this appears to cause problems, you can reduce the large division ratio by using a much smaller sample resistor, and treating it as a divider for calculation purposes. Alternatively, adding some appropriate shielding, or splitting the division into isolated sections can greatly reduce the effects. To avoid signal ground loops, measuring the drive current and the RUT voltage should be separate operations, each carried on its own BNC cable to the LIA, while the other is completely disconnected and out of the way - having no common-grounding or cable bundling or fancy signal routing/switching is best. The weakest link ground-loop-wise may be the necessity of carrying the reference drive between the LIA and power amplifier input, likely sharing the same ground as the output. This could force you to set it up for differential measurement of the RUT signal. The special audio PA that I have for such purposes has its ins and outs transformer-coupled, which helps a lot. This could be fun and interesting. There are plenty of pieces and variables involved to experiment with to optimize the measurement, and lots of other tricks available to enhance it if necessary. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
I guess I would have to ask, "How do you know everything was done 'right'?" The evidence you have stated makes a pretty good case that it wasn't. The neutral entering a sub panel at most has the same current going through it as one of the line input leads, though in a 240V sub panel, load balancing makes it typically much lower. If the neutral was installed correctly, and yet still cold flowed, then one or both of the line inputs should have cold flowed too. If they didn't, then I don't believe the neutral was installed correctly. The grades and finishes of the aluminum used in the electric supply systems are well specified and regulated through UL, and the NFPA. If the wrong grade, temperature, or alloy of aluminum was used; or the wrong service equipment, fastener, or torquing was used; or NoAlOx was omitted; then it wasn't done "right". As to aluminum oxide, unless it is removed, and prevented, a good electrical connection to the aluminum is impossible. NoAlOx does both. It prevents the oxygen from reaching the joint, and the grit embedded in it breaks through any oxide layer, and mechanically raises fresh aluminum to the surface to make a gas tight joint. Even though the manufacturer says it isn't necessary to mechanically remove the oxide layer, I do it anyway. I have never had an aluminum connection (made in this way) fail. -Chuck Harris Dan Kemppainen wrote: > Aluminum... > > Even with everything done right, with approved hardware, aluminum wiring will > tend to > 'creep' (Aluminum mushing out of the way of the lug screw or clamp. > > Just recently I checked the main lugs on my 150A garage sub panel, and sure > enough > the neutral lug was loose. This was about 8 years after installation, with > virtually > no use (only one circuit used for lights until now.) > > However I believe this is only a problem with the softer grades of aluminum. > If your > enclosure were out of 7075, or 2024 my guess is you wouldn't see aluminum > creep at > all. 6061-T6 or similar would probably be OK also. It's the soft stuff that > moves > easily. They all form the oxide layer, so that's still a problem... > > In a pinch, I may be able to machine something, but shipping to there from > here is > probably expensive... > > Dan ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Aluminum... Even with everything done right, with approved hardware, aluminum wiring will tend to 'creep' (Aluminum mushing out of the way of the lug screw or clamp. Just recently I checked the main lugs on my 150A garage sub panel, and sure enough the neutral lug was loose. This was about 8 years after installation, with virtually no use (only one circuit used for lights until now.) However I believe this is only a problem with the softer grades of aluminum. If your enclosure were out of 7075, or 2024 my guess is you wouldn't see aluminum creep at all. 6061-T6 or similar would probably be OK also. It's the soft stuff that moves easily. They all form the oxide layer, so that's still a problem... In a pinch, I may be able to machine something, but shipping to there from here is probably expensive... Dan On 9/20/2017 8:28 AM, volt-nuts-requ...@febo.com wrote: An aluminum electrical connection needs a few things to be reliable: 1) a "springy" fastener 2) mechanical precleaning 3) an oxygen blocking coating. In the US, aluminum conductors are allowed for certain usages. We used to allow 14 and 12AWG receptacle wiring, but too many houses burned down. The receptacles were redesigned for Cu or Al, but the codes remained stubbornly against the practice. A few more times where copper prices go through the roof, and the codes will change. For larger conductors, the wire, or bar, is brightened up with Emory paper, or a stainless steel (important!) brush, and then is covered with "Gorilla Snot", or some sort of NoAlOx grease. NoAlOx is a grease made of an oxygen resistant heavy oil, and a coarse emory grit. I like to again rough things up after the NoAlOx is liberally applied. Finally, the conductors are tightened to specified torque using a springy fastener... The springy fastener is often simply an ordinary fastener with a "Bellview Washer" stack to give it compliance. The big thing that makes high current aluminum joints fail is thermal expansion. If the fastener isn't springy, the aluminum expands from the heat, finds it cannot go in the direction of the tightened fastener, and flows elsewhere. When the joint cools, and the aluminum under the fastener shrinks, the joint is now loose, and will arc when current is once again applied, evaporating more aluminum out of the joint. Soon the fire department will be coming... if you are lucky. NoAlOx prevents this issue, if you use a springy fastener. -Chuck Harris ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
An aluminum electrical connection needs a few things to be reliable: 1) a "springy" fastener 2) mechanical precleaning 3) an oxygen blocking coating. In the US, aluminum conductors are allowed for certain usages. We used to allow 14 and 12AWG receptacle wiring, but too many houses burned down. The receptacles were redesigned for Cu or Al, but the codes remained stubbornly against the practice. A few more times where copper prices go through the roof, and the codes will change. For larger conductors, the wire, or bar, is brightened up with Emory paper, or a stainless steel (important!) brush, and then is covered with "Gorilla Snot", or some sort of NoAlOx grease. NoAlOx is a grease made of an oxygen resistant heavy oil, and a coarse emory grit. I like to again rough things up after the NoAlOx is liberally applied. Finally, the conductors are tightened to specified torque using a springy fastener... The springy fastener is often simply an ordinary fastener with a "Bellview Washer" stack to give it compliance. The big thing that makes high current aluminum joints fail is thermal expansion. If the fastener isn't springy, the aluminum expands from the heat, finds it cannot go in the direction of the tightened fastener, and flows elsewhere. When the joint cools, and the aluminum under the fastener shrinks, the joint is now loose, and will arc when current is once again applied, evaporating more aluminum out of the joint. Soon the fire department will be coming... if you are lucky. NoAlOx prevents this issue, if you use a springy fastener. -Chuck Harris Mitch Van Ochten wrote: > I once repaired a Valhalla 2555A Current Calibrator > <https://valhallascientific.com/DataSheets/2555A_Data_Sheet.pdf> (good for > up to 100A output). It uses aluminum bus bars inside to route the current. > The junctions between the bars had become higher than normal impedance and it > could no longer deliver 100A. I disassembled all joints, cleaned them with > emery cloth, then applied a drop of Caig Deoxit and re-assembled. That was > over four years ago and no complaints from the customer so far. > > > Best regards, > > mitch > > -Original Message- > From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Andre > Sent: Wednesday, September 20, 2017 3:54 AM > To: Discussion of precise voltage measurement > Subject: Re: [volt-nuts] Best way to measure micro Ohms > > Hi, just my $0.02 worth. > I have some instrumentation amplifiers here also looked into low resistance > connections for my other projects. > > If I recall correctly you need to look at the electrochemical series. For > interconnects on Al you want a metal similar on the ES. > The oxide is a problem but if you connect it properly eg with an oil droplet > and clamp connnector using compatible vernier it should be fine. > > Looking at how wiring in the US is done might give you some ideas. > > Kind regards, -Andre > > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
I once repaired a Valhalla 2555A Current Calibrator <https://valhallascientific.com/DataSheets/2555A_Data_Sheet.pdf> (good for up to 100A output). It uses aluminum bus bars inside to route the current. The junctions between the bars had become higher than normal impedance and it could no longer deliver 100A. I disassembled all joints, cleaned them with emery cloth, then applied a drop of Caig Deoxit and re-assembled. That was over four years ago and no complaints from the customer so far. Best regards, mitch -Original Message- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Andre Sent: Wednesday, September 20, 2017 3:54 AM To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Best way to measure micro Ohms Hi, just my $0.02 worth. I have some instrumentation amplifiers here also looked into low resistance connections for my other projects. If I recall correctly you need to look at the electrochemical series. For interconnects on Al you want a metal similar on the ES. The oxide is a problem but if you connect it properly eg with an oil droplet and clamp connnector using compatible vernier it should be fine. Looking at how wiring in the US is done might give you some ideas. Kind regards, -Andre From: volt-nuts on behalf of Charles Steinmetz Sent: 19 September 2017 21:30 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Best way to measure micro Ohms David wrote: > A practical problem is the tools available to me. The U-channel was > machined by someone in my radio club, and the rest I made myself using > nothing more than a drill and hand tools. It would be nice to make > more out of one piece, but it would require better tools than I have > readily available. There are certainly engineering companies that > could do a better job, but it would be quite costly. If you can make a decent drawing of what you need (it doesn't have to be a draughtsmanlike job, as long as the form and measurements are clearly shown), I bet you can find someone in your radio club, or a personal friend, or someone on a list you frequent to make it for a nominal cost (you would probably need to pay for the raw material, although I don't charge for anything that comes out of my scrap pile and this is true of many other home shop machinists). HSMs are behind every third or fourth garage door in the UK. Another option would be to build up the parts as you have done, then have someone TIG weld the pieces together. Best regards, Charles ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Hi, just my $0.02 worth. I have some instrumentation amplifiers here also looked into low resistance connections for my other projects. If I recall correctly you need to look at the electrochemical series. For interconnects on Al you want a metal similar on the ES. The oxide is a problem but if you connect it properly eg with an oil droplet and clamp connnector using compatible vernier it should be fine. Looking at how wiring in the US is done might give you some ideas. Kind regards, -Andre From: volt-nuts on behalf of Charles Steinmetz Sent: 19 September 2017 21:30 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Best way to measure micro Ohms David wrote: > A practical problem is the tools available to me. The U-channel was > machined by someone in my radio club, and the rest I made myself using > nothing more than a drill and hand tools. It would be nice to make more out > of one piece, but it would require better tools than I have readily > available. There are certainly engineering companies that could do a better > job, but it would be quite costly. If you can make a decent drawing of what you need (it doesn't have to be a draughtsmanlike job, as long as the form and measurements are clearly shown), I bet you can find someone in your radio club, or a personal friend, or someone on a list you frequent to make it for a nominal cost (you would probably need to pay for the raw material, although I don't charge for anything that comes out of my scrap pile and this is true of many other home shop machinists). HSMs are behind every third or fourth garage door in the UK. Another option would be to build up the parts as you have done, then have someone TIG weld the pieces together. Best regards, Charles ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
David wrote: A practical problem is the tools available to me. The U-channel was machined by someone in my radio club, and the rest I made myself using nothing more than a drill and hand tools. It would be nice to make more out of one piece, but it would require better tools than I have readily available. There are certainly engineering companies that could do a better job, but it would be quite costly. If you can make a decent drawing of what you need (it doesn't have to be a draughtsmanlike job, as long as the form and measurements are clearly shown), I bet you can find someone in your radio club, or a personal friend, or someone on a list you frequent to make it for a nominal cost (you would probably need to pay for the raw material, although I don't charge for anything that comes out of my scrap pile and this is true of many other home shop machinists). HSMs are behind every third or fourth garage door in the UK. Another option would be to build up the parts as you have done, then have someone TIG weld the pieces together. Best regards, Charles ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
David wrote: Thanks. You have confirmed what I was thinking - it is *probably* the oxide causing the problem. It's not a waveguide in the normal sense of the word, transmitting a TE or TM wave down a hollow tube, but more like a coaxial line transmitting something close(ish) to a TEM wave. The outer conductor is uncoated aluminum and rectangular in cross section. The inner conductor is brass. See pictures attached Looking at the pix, there appear to be lots of aluminum joints due to the "built-up" construction, maximizing the potential for the sort of troubles you are having. I would re-make the piece in brass, doing everything possible to use as few pieces as possible (for example, by milling recesses into one piece rather than building up a compound piece from more easily machined sub-parts). I would also plate everything in silver. Best regards, Charles ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
On 19 September 2017 at 20:32, Charles Steinmetz wrote: Looking at the pix, there appear to be lots of aluminum joints due to the > "built-up" construction, maximizing the potential for the sort of troubles > you are having. I would re-make the piece in brass, doing everything > possible to use as few pieces as possible (for example, by milling recesses > into one piece rather than building up a compound piece from more easily > machined sub-parts). > A practical problem is the tools available to me. The U-channel was machined by someone in my radio club, and the rest I made myself using nothing more than a drill and hand tools. It would be nice to make more out of one piece, but it would require better tools than I have readily available. There are certainly engineering companies that could do a better job, but it would be quite costly. > > I would also plate everything in silver. Yes, I was thinking that. I contacted one local plating company, and asked them about silver plating. The lady said they did not do brass, but did aluminum. This struck me as odd, as they had plated brass for me before - but that was gold, not silver. From what I read, aluminum is a lot more tricky to plate than brass. I sent them a drawing some time ago, but got no response. I will have to chase them up. The conductivity of aluminum is better than brass, and since I am mainly interested in low frequencies (1.8 to 28 MHz), the skin depth will be deeper than it is possible to silver plate. So the "RF resistance" might well be set by the material its constructed from, rather than any plating. Of course, if the plating stops an oxide building up, that should cut the losses. But certainly something is not quite right, as the RF losses are higher than expected. I was keen to see if any of those could be explained by DC / low frequency losses. Best regards, Charles Dave, G8WRB ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
On 18 September 2017 at 23:28, george wrote: > The reason that DC is used commercially to measure resistance is simple, > if you use AC you may well get the reactive component as well as the > resistance coming into play. > That may not be an an issue with a dual-phase lock-in amplifier, as the phase angle of the voltage can be resolved. In fact, it hints at something I have long thought about - using a lock-in amplifier as an LCR meter. I do however have a decent HP 4284A LCR meter, but it can't read very low impedances in the micro ohm range. I've put a couple of offers in on micro-ohm meters, but also bought a 150 W public-address (PA) audio amplifier for £25 (around $35). With that, and the lock-in amplifier, I should be able to make measurements, although I accept the uncertainty will be higher than a dedicated micro ohm meter. > > Such low resistance measurements commercially are normally only made on > high current power distribution networks as part of a periodic test regime > where you need to determine the quality/resistance of such things as bus > bar joints/connections and loop resistance. > > It is not a good idea to use copperslip around aluminium, there is an > aluminium based version that should be used, but, be warned, just like > copperslip it is an insulator, try putting your meter probes, set for > resistance, into a tub of both, I have. I do not know just why but the > aluminium version is just like sand, it gets everywhere when you use it. > So does Coperslip! It sure is messy, but I was given it free, and it stopped a leak. > > To check your joint I would use a four wire Kelvin set up using say 10 > Amps from my constant current bench supply and then use my Keithly 616 > digital electrometer to measure the voltage/s present across the joint, a > simple application of Ohms law will then give the resistance. > I don't have such an instrument, whereas the audio amplifier cost me very little, and I already have the lock-in amplifier. I did buy a lower powered (15 W, $5) amplifier from China, but thought by the time I box it up with a PSU, it would cost more than a PA amplifier with a built in mains supply. The PA amp, being in the UK, should also arrive a lot quicker than the units from China. > > 73 George G6HIG > Dave, G8WRB ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
If this stuff is what I think it is, it does contain a sand of sharp, probably aluminum oxide, abrasive. The idea is when you bolt the connection together, the abrasive will break through the aluminum oxide layer on the conductors, and will mushroom out the aluminum as the abrasive burrows in, making lots of pure aluminum metal contacts. Usually, it also contains a dense oil to keep oxygen away, and prevent further oxidization of the aluminum. -Chuck Harris george wrote: > The reason that DC is used commercially to measure resistance is simple, if > you > use AC you may well get the reactive component as well as the resistance > coming > into play. > > Such low resistance measurements commercially are normally only made on high > current power distribution networks as part of a periodic test regime where > you > need to determine the quality/resistance of such things as bus bar > joints/connections and loop resistance. > > It is not a good idea to use copperslip around aluminium, there is an > aluminium > based version that should be used, but, be warned, just like copperslip it is > an > insulator, try putting your meter probes, set for resistance, into a tub of > both, > I have. I do not know just why but the aluminium version is just like sand, > it > gets everywhere when you use it. > > To check your joint I would use a four wire Kelvin set up using say 10 Amps > from > my constant current bench supply and then use my Keithly 616 digital > electrometer > to measure the voltage/s present across the joint, a simple application of > Ohms > law will then give the resistance. > > 73 George G6HIG ___ volt-nuts > mailing > list -- volt-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the > instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
The reason that DC is used commercially to measure resistance is simple, if you use AC you may well get the reactive component as well as the resistance coming into play. Such low resistance measurements commercially are normally only made on high current power distribution networks as part of a periodic test regime where you need to determine the quality/resistance of such things as bus bar joints/connections and loop resistance. It is not a good idea to use copperslip around aluminium, there is an aluminium based version that should be used, but, be warned, just like copperslip it is an insulator, try putting your meter probes, set for resistance, into a tub of both, I have. I do not know just why but the aluminium version is just like sand, it gets everywhere when you use it. To check your joint I would use a four wire Kelvin set up using say 10 Amps from my constant current bench supply and then use my Keithly 616 digital electrometer to measure the voltage/s present across the joint, a simple application of Ohms law will then give the resistance. 73 George G6HIG ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Dave, It looks like IET updated the design of the instrument with better switching. If you download the manual from the IET website, they still show the older model. I wonder if the newer model incorporates a front switch that enables the pulsed mode that was added by the owner on EEVBlog? I can't tell by the photo. Todd On Mon, Sep 18, 2017 at 5:44 PM, Dr. David Kirkby (Kirkby Microwave Ltd) < drkir...@kirkbymicrowave.co.uk> wrote: > On 18 September 2017 at 18:36, Todd Micallef wrote: > > > Dave, > > > > Another meter is the Cambridge LOM-510A. I am not sure if it is in your > > budget but there has been a review made on EEVBlog. There is one > currently > > on eBay with a current amplifier that I have never seen before today. It > > may be worth reviewing if it meets your needs. > > > > Todd > > > > > Hi Todd, > Something struck me as odd about the *model* number LOM-510A. > > When I google the Keithley 580, I see a *paid advert* for an IET Labs > LOM-510A. > > http://www.ietlabs.com/lom-510.html?gclid=CjwKEAjw3f3NBRDP_ > NHS9fq53n4SJACKIfEYZT_jyIk_TKn0gqEkzvfxwKASIpIZTcDFJfJlMwvnRRoCZYHw_wcB > > But it looks nothing like the Cambridge LOM-510A on eBay > > http://www.ebay.com/itm/Cambridge-510A-Micro-Ohmmeter- > DLRO-Includes-NEW-leads-NIST-Calibrated-LOM-510A-/291956733548 > > It seems a bit odd for two manufacturers (IET Labs and Cambridge > Instruments) to both have the exact same model number (LOM-510A) for an > instrument with identical functionality (micro ohms). Yet photographs of > the two instruments show they look similar, but are not identical. The IET > Labs one appears to have a couple of extra switches. > > Just to make something appear even more odd, the IET Labs has a link to a > review on EEVblog. > > http://www.eevblog.com/forum/testgear/ie/30/ > > but the picture shows Cambridge Instruments, but the title shows IET Labs. > > > > Dave > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/volt-nuts > and follow the instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
On 18 September 2017 at 18:36, Todd Micallef wrote: > Dave, > > Another meter is the Cambridge LOM-510A. I am not sure if it is in your > budget but there has been a review made on EEVBlog. There is one currently > on eBay with a current amplifier that I have never seen before today. It > may be worth reviewing if it meets your needs. > > Todd > Hi Todd, Something struck me as odd about the *model* number LOM-510A. When I google the Keithley 580, I see a *paid advert* for an IET Labs LOM-510A. http://www.ietlabs.com/lom-510.html?gclid=CjwKEAjw3f3NBRDP_NHS9fq53n4SJACKIfEYZT_jyIk_TKn0gqEkzvfxwKASIpIZTcDFJfJlMwvnRRoCZYHw_wcB But it looks nothing like the Cambridge LOM-510A on eBay http://www.ebay.com/itm/Cambridge-510A-Micro-Ohmmeter-DLRO-Includes-NEW-leads-NIST-Calibrated-LOM-510A-/291956733548 It seems a bit odd for two manufacturers (IET Labs and Cambridge Instruments) to both have the exact same model number (LOM-510A) for an instrument with identical functionality (micro ohms). Yet photographs of the two instruments show they look similar, but are not identical. The IET Labs one appears to have a couple of extra switches. Just to make something appear even more odd, the IET Labs has a link to a review on EEVblog. http://www.eevblog.com/forum/testgear/ie/30/ but the picture shows Cambridge Instruments, but the title shows IET Labs. Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Dave, Another meter is the Cambridge LOM-510A. I am not sure if it is in your budget but there has been a review made on EEVBlog. There is one currently on eBay with a current amplifier that I have never seen before today. It may be worth reviewing if it meets your needs. Todd On Mon, Sep 18, 2017 at 12:31 PM, Dr. David Kirkby (Kirkby Microwave Ltd) < drkir...@kirkbymicrowave.co.uk> wrote: > On 18 Sep 2017 00:43, "Charles Steinmetz" wrote: > > > > > Also, since you said the waveguide is aluminum (and didn't say anything > about plating), be aware that aluminum exposed to air is covered by a thin > aluminum oxide layer (Al2O3), which forms within seconds after a new > surface is exposed. This layer is thin -- generally about 4 nm -- but the > bulk resistivity of Al2O3 is very high, so there is a finite and variable > resistance across the interface between two joined pieces of aluminum > (depending on the area of the joint, the joining pressure, and the extent > to which the joining method produces a clean [oxide-free], gas-tight > interface between the joined surfaces). > > > > Best regards, > > > > Charles > > Thanks. You have confirmed what I was thinking - it is *probably* the > oxide causing the problem. > > It's not a waveguide in the normal sense of the word, transmitting a TE or > TM wave down a hollow tube, but more like a coaxial line transmitting > something close(ish) to a TEM wave. The outer conductor is uncoated > aluminum and rectangular in cross section. The inner conductor is brass. > See pictures attached (I made them small, so quality his not great, but it > should not too use much bandwidth) > > Attached are a couple of pictures, and also S11 measured on a VNA, with one > connector shorted Since this is a reflection measurement, the EM wave > travellels along this twice, so about half the loss would be in each > direction. It is only a rough measurement, but a transmission measurement > showed similar results, but half as much attenuation, as it is only being > attenuated one way. > > Maybe I need to use brass, or silver plate the aluminum. > > The purpose of this was to measure the loss of a very low loss liquid > dielectric, but from discussions I had with someone at NPL, such a > structure is not suitable if the loss is very low. > > Anyway, I have put it an offer on a Keithley microohm meter. I notice there > are a lot of Chinese ones at quite low priced. I've no idea how good/bad > they are. But they are much more modern and cheaper than an *affordable* > Kiethley meter. A Keithley 2002 is well outside my budget. > > Dave > > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/volt-nuts > and follow the instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
David wrote: Can anyone explain why commercial instruments use DC, despite that small DC voltages will be developed by unwanted thermocouples? I would have thought that using AC was a no-brainer no very low resistance measurements, but commercial instruments don't use to use AC. Difficulty of measuring AC, compared to DC, is one reason, as has been mentioned. But the main reason is that skin effect (and usually to a much lesser extent, inductance) is a significant factor at surprisingly low frequencies, *particularly* when the expected value is in the micro- to deci-ohm range. Also, since you said the waveguide is aluminum (and didn't say anything about plating), be aware that aluminum exposed to air is covered by a thin aluminum oxide layer (Al2O3), which forms within seconds after a new surface is exposed. This layer is thin -- generally about 4 nm -- but the bulk resistivity of Al2O3 is very high, so there is a finite and variable resistance across the interface between two joined pieces of aluminum (depending on the area of the joint, the joining pressure, and the extent to which the joining method produces a clean [oxide-free], gas-tight interface between the joined surfaces). Best regards, Charles ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
I use a Keithley 2182 and 6221 nano-ohm setup at work. It is a combination of a reversing precision current source and a nanovoltmeter with embedded software to manage the process. I can reliably measure into the 50 nano-ohm regime.The surface chemistry of the metal joint is very important. Both nickel and aluminum have thin tenacious oxides. It takes a compliance setting of >65 V to punch through nickel oxide films on pressure contacts.If a bolted contact has sufficient contact pressure to crack the film yo can obtain dry circuit conduction. My experience is that a silver plated contact surface will have a decade lower contact resistance than an otherwise identical nickel plated one.Keithley has an excellent Low Level Measurement Handbook and appnotes for download. Howard Davidson Sent from Yahoo Mail for iPad On Sunday, September 17, 2017, 4:41 PM, Charles Steinmetz wrote: David wrote: > Can anyone explain why commercial instruments use DC, despite that small DC > voltages will be developed by unwanted thermocouples? I would have thought > that using AC was a no-brainer no very low resistance measurements, but > commercial instruments don't use to use AC. Difficulty of measuring AC, compared to DC, is one reason, as has been mentioned. But the main reason is that skin effect (and usually to a much lesser extent, inductance) is a significant factor at surprisingly low frequencies, *particularly* when the expected value is in the micro- to deci-ohm range. Also, since you said the waveguide is aluminum (and didn't say anything about plating), be aware that aluminum exposed to air is covered by a thin aluminum oxide layer (Al2O3), which forms within seconds after a new surface is exposed. This layer is thin -- generally about 4 nm -- but the bulk resistivity of Al2O3 is very high, so there is a finite and variable resistance across the interface between two joined pieces of aluminum (depending on the area of the joint, the joining pressure, and the extent to which the joining method produces a clean [oxide-free], gas-tight interface between the joined surfaces). Best regards, Charles ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
A DMM with good low resistance capability will have an "offset compensated ohms" feature. A large current is used and then a small current is used. The slope of the line formed by V-I points is the true resistance. Measuring tiny DC voltages is easier than measuring tiny AC voltages. More bandwidth is more cost for an amplifier. Pass an amp or more through the device and use a KE 2182A to measure the microvolts. Since I do not own a 2182A, I use a KE 147. You will need a DMM on the analog output of the 147 if you want digits of resolution. When I do low resistance, I am just looking to bracket the resistance, not measure to 6 1/2 digits. The thermal offsets can require a lot of time to stabilize. I built a Tetrajunction demo device that is in the tens of nano-ohms. It takes all day to set up the measurement and have the offsets stabilize and take a few measurements. -Brian Smith On Sun, Sep 17, 2017 at 1:23 PM, Dr. David Kirkby (Kirkby Microwave Ltd) < drkir...@kirkbymicrowave.co.uk> wrote: > I want to measure the resistance between two bits of aluminum. Each are 40 > x 30 mm across. One is 250 mm long, the other is 8 mm long. I'm wondering > is surface oxides are on the faces, so despite being held together with > bolts, the resistance is perhaps not as long as I would expect. There's > also a layer of "copperslip" between these, to provide a waterproof joint. > That might be adding unnecessary resistance. > > What sort of instrument is (if any) capable of measuring this? I have a 6.5 > digits HP 3457A with a 30 Ohm 4-wire mode, but the uncertainty is 0.0065% + > 20315 counts. Those 20315 counts are a lot! > > I can't seem to see much in the way of commercial instruments for very low > resistance measurements. I would have thought an AC source was needed, yet > they all seem to use DC. Why? > > I've thought of hooking a signal generator up to an audio amplifier capable > of driving a few amps, passing that through the joint, then using an EG&G > 7260 lock-in amplifier to measure an AC voltage across the joint. > > Any better suggestions? > > Can anyone explain why commercial instruments use DC, despite that small DC > voltages will be developed by unwanted thermocouples? I would have thought > that using AC was a no-brainer no very low resistance measurements, but > commercial instruments don't use to use AC. > > > Dr. David Kirkby Ph.D CEng MIET > Kirkby Microwave Ltd > drkir...@kirkbymicrowave.co.uk > http://www.kirkbymicrowave.co.uk/ > Tel: 01621-680100 / +44 1621-680100 (0900 to 2100 UK time) > > Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT, > UK. > Registered in England and Wales, company number 08914892. > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/volt-nuts > and follow the instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
On 17 September 2017 at 21:58, Mitch Van Ochten < mi...@vincentelectronics.com> wrote: > The Keithley 2002 uses DC but automatically takes a reading of any offset > voltage and subtracts it (offset compensation). Rated accuracy on the 20 > ohm range (2 years) is +/- 26 ppm, and with 10 averages it has a resolution > of 0.1 microohms. > > > Best regards, > > mitch > Also very pricy! I've made an offer on a Keithley 580 micro ohm meter, and ordered a couple of $3 audio amplifier boards. I will try one with the lock-in amplifier. It it does not work, or works poorly, it will not have broken the bank. Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
On 17 September 2017 at 20:12, wrote: > The question is what accuracy you need. > No a lot. I just want to find out if there's any voltage drops that are significantly higher than I would expect. The unit makes an RF transmission line, and the loss at RF is significantly higher than predicted by a computer model, which takes into account the skin depth of the materials. I'm wondering if there's something odd going on. I suspect the problem is the current in the aluminum is not being computed properly due to the oxide on the surface. But I just wanted to make sure there was no unexpected DC resistance. I don't think there will be, but I want to climate that possibility. > The classical way to do that (achieving high accuracy) is to apply a known > accurate current (say 10A) and measure the voltage drop accross the rod > with a nanovoltmeter. > As the piece of aluminum is isothermal you should not expect a big > thermovoltage. You could also compensate for this by reversing the current > and take the average, also by nulling the voltage reading prior to applying > any current. Generating precisely known AC currents (low uncertainty) is > difficult (i.e. measuring it precisely), therefore DC currents are ususaly > used also in metrology for this. > If you do some internet search you will find metrology reports about this. > If you do not have a nanovoltmeter you could build a measurement amplifier > with not that much of an effort (based on chopper amp or low drif precision > opamp) > The only nV meter I have is the lock-in amplifier, which has a full-scale sensitivity of 2 nV to 1 V in a 1-2-5 sequence. The only instrument I have able to measure > 3 A of current is a handheld multimeter. One of my power supplies can supply 35 A, and has an ammeter in it. I don't have any particularly accurate means of measuring DC current outside the limited of the 3457A. In terms of simplicity, getting a $10 audio amplifier from China and using the lock-in amplifier is the way to go, but I accept a metrologist would not like that idea! Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
The Keithley 2002 uses DC but automatically takes a reading of any offset voltage and subtracts it (offset compensation). Rated accuracy on the 20 ohm range (2 years) is +/- 26 ppm, and with 10 averages it has a resolution of 0.1 microohms. Best regards, mitch -Original Message- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Dr. David Kirkby (Kirkby Microwave Ltd) Sent: Sunday, September 17, 2017 1:23 PM To: Discussion of precise voltage measurement Subject: [volt-nuts] Best way to measure micro Ohms I want to measure the resistance between two bits of aluminum. Each are 40 x 30 mm across. One is 250 mm long, the other is 8 mm long. I'm wondering is surface oxides are on the faces, so despite being held together with bolts, the resistance is perhaps not as long as I would expect. There's also a layer of "copperslip" between these, to provide a waterproof joint. That might be adding unnecessary resistance. What sort of instrument is (if any) capable of measuring this? I have a 6.5 digits HP 3457A with a 30 Ohm 4-wire mode, but the uncertainty is 0.0065% + 20315 counts. Those 20315 counts are a lot! I can't seem to see much in the way of commercial instruments for very low resistance measurements. I would have thought an AC source was needed, yet they all seem to use DC. Why? I've thought of hooking a signal generator up to an audio amplifier capable of driving a few amps, passing that through the joint, then using an EG&G 7260 lock-in amplifier to measure an AC voltage across the joint. Any better suggestions? Can anyone explain why commercial instruments use DC, despite that small DC voltages will be developed by unwanted thermocouples? I would have thought that using AC was a no-brainer no very low resistance measurements, but commercial instruments don't use to use AC. Dr. David Kirkby Ph.D CEng MIET Kirkby Microwave Ltd drkir...@kirkbymicrowave.co.uk http://www.kirkbymicrowave.co.uk/ Tel: 01621-680100 / +44 1621-680100 (0900 to 2100 UK time) Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT, UK. Registered in England and Wales, company number 08914892. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
The question is what accuracy you need. The classical way to do that (achieving high accuracy) is to apply a known accurate current (say 10A) and measure the voltage drop accross the rod with a nanovoltmeter. As the piece of aluminum is isothermal you should not expect a big thermovoltage. You could also compensate for this by reversing the current and take the average, also by nulling the voltage reading prior to applying any current. Generating precisely known AC currents (low uncertainty) is difficult (i.e. measuring it precisely), therefore DC currents are ususaly used also in metrology for this. If you do some internet search you will find metrology reports about this. If you do not have a nanovoltmeter you could build a measurement amplifier with not that much of an effort (based on chopper amp or low drif precision opamp) > Gesendet: Sonntag, 17. September 2017 um 19:23 Uhr > Von: "Dr. David Kirkby (Kirkby Microwave Ltd)" > > An: "Discussion of precise voltage measurement" > Betreff: [volt-nuts] Best way to measure micro Ohms > > I want to measure the resistance between two bits of aluminum. Each are 40 > x 30 mm across. One is 250 mm long, the other is 8 mm long. I'm wondering > is surface oxides are on the faces, so despite being held together with > bolts, the resistance is perhaps not as long as I would expect. There's > also a layer of "copperslip" between these, to provide a waterproof joint. > That might be adding unnecessary resistance. > > What sort of instrument is (if any) capable of measuring this? I have a 6.5 > digits HP 3457A with a 30 Ohm 4-wire mode, but the uncertainty is 0.0065% + > 20315 counts. Those 20315 counts are a lot! > > I can't seem to see much in the way of commercial instruments for very low > resistance measurements. I would have thought an AC source was needed, yet > they all seem to use DC. Why? > > I've thought of hooking a signal generator up to an audio amplifier capable > of driving a few amps, passing that through the joint, then using an EG&G > 7260 lock-in amplifier to measure an AC voltage across the joint. > > Any better suggestions? > > Can anyone explain why commercial instruments use DC, despite that small DC > voltages will be developed by unwanted thermocouples? I would have thought > that using AC was a no-brainer no very low resistance measurements, but > commercial instruments don't use to use AC. > > > Dr. David Kirkby Ph.D CEng MIET > Kirkby Microwave Ltd > drkir...@kirkbymicrowave.co.uk > http://www.kirkbymicrowave.co.uk/ > Tel: 01621-680100 / +44 1621-680100 (0900 to 2100 UK time) > > Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT, > UK. > Registered in England and Wales, company number 08914892. > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
In message , "Dr. David Kirkby (Kirkby Microwave Lt d)" writes: >I want to measure the resistance between two bits of aluminum. Each are 40 >x 30 mm across. One is 250 mm long, the other is 8 mm long. The dominant factor in the resistance you want to measure is the pressure on the contacting surfaces. If the contact pressure is high enough, making the joint "gas tight", you can treat it as one contiguous piece of aluminium and just do the math (= differential equations for complex geometries) If the contact pressure is too low, trying to measure the joint resistance is a fools errand, because it will not be stable over any parameter (voltage, current, humidity, vibration, air composition, ...) The only known way to measure ohmic resistance in the micro-ohm domain with any precision is caloriometric methods, and they are experimentally troublesome to no end. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
[volt-nuts] Best way to measure micro Ohms
I want to measure the resistance between two bits of aluminum. Each are 40 x 30 mm across. One is 250 mm long, the other is 8 mm long. I'm wondering is surface oxides are on the faces, so despite being held together with bolts, the resistance is perhaps not as long as I would expect. There's also a layer of "copperslip" between these, to provide a waterproof joint. That might be adding unnecessary resistance. What sort of instrument is (if any) capable of measuring this? I have a 6.5 digits HP 3457A with a 30 Ohm 4-wire mode, but the uncertainty is 0.0065% + 20315 counts. Those 20315 counts are a lot! I can't seem to see much in the way of commercial instruments for very low resistance measurements. I would have thought an AC source was needed, yet they all seem to use DC. Why? I've thought of hooking a signal generator up to an audio amplifier capable of driving a few amps, passing that through the joint, then using an EG&G 7260 lock-in amplifier to measure an AC voltage across the joint. Any better suggestions? Can anyone explain why commercial instruments use DC, despite that small DC voltages will be developed by unwanted thermocouples? I would have thought that using AC was a no-brainer no very low resistance measurements, but commercial instruments don't use to use AC. Dr. David Kirkby Ph.D CEng MIET Kirkby Microwave Ltd drkir...@kirkbymicrowave.co.uk http://www.kirkbymicrowave.co.uk/ Tel: 01621-680100 / +44 1621-680100 (0900 to 2100 UK time) Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT, UK. Registered in England and Wales, company number 08914892. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.