Hi If your objective is a resolution of < 0.001 C at something < 1 second, the current crop of digital sensors don’t quite do what you need to do. They are a terrific way to do wide range measurements that might feed into some sort of correction algorithm. A conventional thermistor bridge falls apart if you try to run it -55 to +125. The range of resistances involved results in significantly lowered resolution at the end(s) of the range.
Bob > On Jun 4, 2017, at 8:18 PM, Adrian Godwin <artgod...@gmail.com> wrote: > > Where do digital sensors (e.g. ds1820 and some more recent parts from TI) > fit into this ? > > > On Mon, Jun 5, 2017 at 12:59 AM, Attila Kinali <att...@kinali.ch> wrote: > >> Moin, >> >> This discussion is kind of getting heated. >> Let's put some facts in, to steer it away from >> opinion based discussion. >> >> On Sun, 4 Jun 2017 08:44:33 -0700 >> "Donald E. Pauly" <trojancow...@gmail.com> wrote: >> >>> I stand by my remark that thermistors have been obsolete for over 40 >>> years. The only exception that I know of is cesium beam tubes that >>> must withstand a 350° C bakeout. Thermistors are unstable and >>> manufactured with a witches brew straight out of MacBeth. Their >>> output voltages are tiny and are they inconvenient to use at different >>> temperatures. >> >> If you really mean thermistors, and not, as Bob suggested thermocouples, >> then I have to disagree. The most stable temperature sensors are >> platinum wire sensors. The standards class PRT's are the gold standard >> when it comes to temperature measurement, for a quite wide range >> (-260°C to +960°C) and are considered very stable. They offer (absolute) >> accuracies in the order of 10mK in the temperature range below 400°C. >> Even industrial grade PRT sensors give you an absolute accuracy better >> than 0.1K up to 200-300°C. The "cheap" PT100 are more of the order of >> 1-10°C >> accuracy... all numbers just using a two-point calibration. >> >> For more information on this see [1] chapter 6 and [2] for industrial >> sensors. >> >> NTC sensors have a higher variablity of their parameters in production >> and are usually specified in % of temperature relative to their reference >> point, which is usually 25°C. Typical values are 0.1% to 5%. Additionally >> there is a deviation from the reference point, specified in °C, which >> is usually in the order of 0.1°C to 1°C. >> >> The NTC sensors are less accurate than PT sensors, but offer the advantage >> of higher resistance (thus lower self-heating), higher slope (thus better >> precision). Biggest disadvantage is their non-linear curve. Their price >> is also a fraction of PT sensors and due to that you can have them in >> many different forms, from the 0201 SMD resistor, to a large stainless >> steal pipe that goes into a chemical tank. NTCs are the workhorse in >> todays temperature measurement and control designs. >> >> The next category are band-gap sensors like the AD590. Their biggest >> advantage is that their 0 point is fix at 0K (and very accurately so). >> Ie they can be used with single point calibration and achieve 1°C accuracy >> this way. Their biggest drawback their large thermal mass and large >> insulating case, because they are basically an standard, analog IC. >> Ie their main use is in devices where there is a lot of convection and >> slow temperature change. Due to their simple and and quite linear >> characteristics, they are often used in purely analog temperature >> control circuits, or where a linearization is not feasible. >> But only if price isn't an issue (they cost 10-1000 times as >> much as an PTC). Their biggest disadvantage, beside their slow >> thermal raction time, is their large noise uncorrelated to the >> supply voltage, and thus cannot be compensated by ratiometric measurement. >> They are also more suceptible to mechanical stress than NTC's and PT's, >> due to their construction. Similar to voltage references (which they >> actually are), their aging is quite substantial and cannot be neglected >> in precision application. >> With a 3 point calibration, better than 0.5°C accuracy can be achieved >> (modulo aging) within their operating temperature range, which is >> rather limited, compared to the other sensor types. >> >> I don't know enough about thermocouples to say much about them, beside >> that they are cumbersome to work with (e.g. the cold contact) and >> produce a low voltage (several µV) output with quite high impedance, >> which makes the analog electronics difficult to design as well. >> >> >> With todays electronics, the easiest sensors to work with are NTC and >> PT100/PT1000 as most high resolution delta-sigma ADCs have direct support >> for 3 and/or 4 wire measurement of those, including compensation for >> reference voltage/current variation. Using a uC as control element >> also opens up the possibility to linearize the curve of NTCs without >> loss of accuracy. Usually measurement precision, with a state-of-the-art >> circuit, is limited by noise coupling into the leads of the sensor >> and noise in and around the ADC. (see [3-5]) >> >> >>> Where did you get the idea to use a 1 k load for an AD590? >> >> Jim was refering to a circuit _he_ used in a satellite. Not to your >> circuit. >> >>> The room temperature coefficient of an AT crystal is -cd 100 ppb per >>> reference cut angle in minutes. (-600 ppb/C° for standard crystal) >>> The practical limit in a crystal designed for room temperature is >>> about 0.1' cut accuracy or ±10 ppb/C°. If you have access to an >>> atomic standard, you can use feed forward to get ±1 ppb/C°. If the >>> temperature can be held to ±0.001° C, this is ±1 part per trillion. >>> This kind of accuracy has never been heard of. >> >> It has been heard of. The 8607 was spec'ed to <2e-10 p-p deviation >> over temperature range (-30°C to 60°C). Also, to hold the temperature >> stable to 0.001K in a room temperature environment (let's say 10K >> variation), >> you need a thermal gain of >10k. That's quite a bit and needs considerable >> design effort. Most OCXO design's I am aware of are in the order of 100 >> (the DIL14 designs) to a few 1000 for single ovens, to a few 10k for >> double ovens. The only exception is the E1938 which achieves >1M. >> But that design is not for the faint hearted. I don't remember seeing >> any number, but i would guess the 8607 has a thermal gain in the >> order of 100k to 1M as well, considering it being a double oven in >> a dewar flask. >> >> Also, what do you mean by atomic standard and feed forward? >> If you have an atomic standard you don't need to temperature >> stabilize your quartz. You can just simply use a PLL to lock >> it to your reference and achieve higher stability than any oven >> design. >> >>> Feed forward also >>> allows you to incorporate the components of the oscillator into the >>> thermal behavior. It does no good to have a perfect crystal if the >>> oscillator components drift. >> >> Beyond tau=100s, the temperature and moisture sensitivity of the >> electronics, combined with the aging of the electronics and the >> crystal will be the limit of stability. Of course, this is under >> the assumption that you achieved a thermal noise limited design >> and thus the 1/f^a noise of the oscillator is negligible in the >> time range considered. >> >> >> Attila Kinali >> >> [1] "Traceable Temperatures - An Introduction to Temperature Measurement >> and Calibration", 2nd edition, by Nicholas and White, 2001 >> >> [2] "Thin-film platinum resistance thermometer for use at low temperatures >> and in high magnetic fields", Haruyama, Yoshizaki, 1986 >> >> [3] "Completely Integrated 4-Wire RTD Measurement System Using a Low Power, >> Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0381 >> http://www.analog.com/CN0381 >> >> [4] "Completely Integrated 3-Wire RTD Measurement System Using a Low Power, >> Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0383 >> http://www.analog.com/CN0383 >> >> [5] "2- 3- 4- Wire RDT (Pt100 to PT1000)Temperature Measurement" >> Ti Presentation >> http://www.ti.com/europe/downloads/2-%203-%204-Wire% >> 20RTD%20Measurement.pdf >> >> >> -- >> You know, the very powerful and the very stupid have one thing in common. >> They don't alters their views to fit the facts, they alter the facts to >> fit the views, which can be uncomfortable if you happen to be one of the >> facts that needs altering. -- The Doctor >> _______________________________________________ >> 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.