Here's a NIST paper on Thermistor stability: http://nvlpubs.nist.gov/nistpubs/jres/83/jresv83n3p247_A1b.pdf
Bruce > > On 06 June 2017 at 01:45 Bob kb8tq <kb...@n1k.org> wrote: > > Hi > > Well, as part of the process of designing them into OCXO’s you do indeed > check their long term stability. > The test is done in an indirect fashion so you only come up with a “it’s > below the limit” sort of number. The > typical process involves running a group of OCXO’s on turn to check the > frequency and then shifting them > off turn to make a sort of thermometer. After a few months of frequency > readings you take them back to turn > for a while. Relative frequency shift math gives you a stability number > for the thermistor and the rest of the > circuitry. You may repeat the run for months / shift process a couple of > times. If the answer isn’t “I can’t see > a difference” you look for a new thermistor. Since it’s a long drawn out > test, the tendency is to stick with a > vendor’s part for quite a while. The parts also tend to be design > specific so what works in my (say SMT) > design may not work well in your (say chip and wire) design. > > Bob > > > > > > On Jun 5, 2017, at 9:20 AM, romeo987 <romeo...@westnet.com.au> > > wrote: > > > > Hi, guys > > I have been following time nuts and volt nuts for some time out of > > interest and fascination. Although my personal backyard hobby is more along > > a volt nuts line, the two worlds often collide - like in this discussion of > > temperature sensors, and in particular their long term stability. NTC > > thermistors appear to be very commonly used in ovens used to stabilize > > voltage references (solid state as well as chemical) . I have long wondered > > about their stability. If, as Bruce asserts, "high quality thermistors can > > achieve drifts of around 1mK/month" then it appears that this level of > > drift is a significant factor in the "apparent" aging of, say, a bank of > > Weston cells (which is still my best backyard shot at a voltage reference). > > > > I have had no luck with Google; Bruce's statement is the first > > quantified allusion that I have seen to this subject. Is there any actual > > data available on the long term performance of NTC sensors? > > > > Roman > > > > > > > > > > On 5 Jun 2017, at 9:53 AM, Bruce Griffiths > > > <bruce.griffi...@xtra.co.nz> wrote: > > > > > > The other issue that needs to be considered is the drift in > > > temperature sensor characteristics when operated at a constant > > > temperature (as is typical in a continuously operated crystal oven). High > > > quality thermistors can achieve drifts of around 1mK/month. Its unlikely > > > that something as complex as an AD590 will achieve a similar drift > > > (1nA/month in a operating current of 300uA or so at 25C). High quality > > > PRT sensors drift even less than thermistors when operating at constant > > > temperature. > > > > > > Bruce > > > > > > > > > > > > > > .On 05 June 2017 at 11:59 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. > > > > > _______________________________________________ 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.
