Long-term stability of temperature sensors - Page 1
Forum section: electronics / general

author: cellularmitosis
author_rank: Unknown
timestamp: December 23, 2017, 06:51:52 am
content: I can't seem to find good information on how to rank temperature sensor types by long-term stability.What good is a volt-nut if he can't tell if 23.17C from three years ago is still 23.17C today?This comment from splin seems to be the best info we have on the forum so far:https://www.eevblog.com/forum/testgear/temperature-test-box-for-component-characterization/msg617199/#msg617199QuoteThe particular application I had in mind was mounting a temperature sensor on-board a voltage reference, so that the voltage and temperature data would be useful over long-term comparisons.My assumption was that the long-term stability quality ordering went something like this (best to worst):Platinum RTDThermistor in glass DO35Thermistor in epoxy beadAlso, practically, for this application I probably only need about 3.5 digits of long-term stable temperature comparison, so maybe that eases the requirements?Thoughts?

author: ebclr
author_rank: Super Contributor
timestamp: December 23, 2017, 06:59:33 am
content: I hope you have Trump like pockethttp://nvlpubs.nist.gov/nistpubs/Legacy/TN/nbstechnicalnote1265.pdf

author: 2N3055
author_rank: Super Contributor
timestamp: December 23, 2017, 10:03:06 am
content: It is not that simple.What splin wrote is really good info. Let's read it again.You say simple 3.5 digits.. that's not that easy if last digit has to be stable for 3-5 years without calibration...for instance: 23.55°C. that is 23°C and 550 °mK.. you want last digit to be stable, that means at least 4 times better specs , so you would need 10-12 °mK 3 years stability.And now read what splin wrote. Platinum RTD sensors that can do that are available but expensive.But that is not whole story. You need to have that stability for a complete measurement system. Not only sensor. And have that guaranteed as a whole to be within the specs..Also, it is not how it's done. Logic is the same as with voltage or every other reference... You need to have 3-4 thermometers, and observe them as a group. 10 would be even better...You watch them for a long time, characterize them and you build confidence in your temperature measurement. Calibrate them in groups as you go, keeping records all the time...And then, there is the question: do you really need that stability...Realistically 0.1 °C absolute is more than enough to be in same standards as Fluke or Keysight labs as far as room temp goes. If you ever look at temp gradients at your desk for sensors few centimeters away, you will start to realize temperature is very elusive thing to measure..I presume you would want to temperature characterize something over long term.Tempco characterization is a relative measurement, that needs to be only stable enough over time interval that is longer than time constant of DUT.. few seconds, hours, days...In amateur (and small business ) environment, for environment monitoring in a longterm, I think best thing to do would be to have a group (cluster) of Sensirion SHT35 that actually have long term drift specified. They have good resolution, you get good and also long term specified drift RH measurements too...They are factory tested and calibrated and have guaranteed specs.. And they are the whole temp/RH measurement solution: from sensor, signal conditioning to precision A/D converter..SHT35 are guaranteed to have 0,03°C /year maximum drift. With a group of sensor and good practice, much better than that..It is most economic way to get those specs.. It would be challenging to do it from discrete components by yourself.Anything else would be unrealistic, unless you want to spend more money on temperature than voltage...In which case you just contracted tempnut disease, and all bets are off...Regards,Sinisa

author: MK
author_rank: Regular Contributor
timestamp: December 23, 2017, 10:38:25 am
content: Platinum wire is prone to vibration lattice slip and so needs frequent recalibration, thin film platinum is not as stable either.

author: Andreas
author_rank: Super Contributor
timestamp: December 23, 2017, 11:13:44 am
content: Hello,the question is: what temperature do you want to measure? (Air, PCB or stirring oil bath?)And do you really think you can measure (air / PCB) temperatures better than 0.3K at home with simple sensors?I have here a set of calibrated resistors with I2C connection where you can see the difficulties to measure temperatures.Although all sensors are within 3-4 cm every sensor shows a different temperature. 0.8 deg C span for calibrated sensors.Ok each one is on a separate PCB with different placement of the I2C pull up resistors and level shifters.And I already have removed one LED to reduce self heating on the pressure sensor.Temperature gets more equal (within 0.2 deg C) when I switch on a FAN to equalize the self heating.What you can also see is when the door is opened and warm air enters the roomDepending on sensor the "noise" due to air movement is up to 0.1 or sligthly more deg C.If you put the PCB into a card box and cover all sensors with cloth the noise significantly gets lower.(middle part of the 2nd diagram)By the way: the Guildline Weston Cells (9152) used a termistor to keep the oven with a long term stability of 0.01 K.with best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: December 23, 2017, 11:42:05 am
content: Well, it's not almost impossible, but sure not with cheap ready to use sensors like SHT21, SHT25, TMP112 or MS5611.I do have a temperature measurement system, that consist of two sensor heads, each with two PT100 temperature sensors inside. The complete system is calibrated to within 0,02K over the entire temperature range of -40° ... +150°C, including analog to digital conversion (LTC2440) and linearization function implemented inside the microcontroller. They did a six point calibration to ensure, that the system meets its spec. Including calibration you have to invest a thousand bucks. Can't say anything about longterm stability yet, but I'm sure I will get a picture within the next years.As mentioned you have do differ between PT100 in thin film technology and PT100 with platinum wire, but also the used materials for assembling them. It's not straight forward and many thoughts have to be taken into account, to meet specs at the end.I've started building my own temperature measurement system, based on PT1000 elements and a PCap01 that measures discharge time by using a high quality cap forming up a RC circuit with very low charges flowing thru both elements and thus, almost no self-heating, which is another important factor. At least it needs calibration, whick can be implemented inside the PCap itself. I will calibrate it against the commercial temperature measurement system.-branadic-

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 23, 2017, 03:45:04 pm
content: Well, first you build the triple point cell from the old Scientific American article:https://www.scientificamerican.com/article/tackling-the-triple-point/That's probably the easiest way for impoverished volt-nutter to get a reliable calibration point.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 23, 2017, 04:22:03 pm
content: To check for possible drift of the sensor it might be a good idea to have 2 different sensors (e.g. PT, NTC, Diode). If the sensors drift apart one would be at least warned about drift and could have an idea in the order of magnitude.Inside a reference with relatively low TC, the stability of the temperature measurement is not that critical. The corrections due to normal temperature variations are usually small against the long term drift of the reference. So essentially any temperature sensor, from the cheapest plastic encapsulated transistor, NTC or PTC should be good enough.@brandic:I don't think the capacitor and time to digital way of measuring an RTD is a good idea for a precision application. It will also include the resistance of switches and is thus more prone to drift than other systems. The more usual way is a bridge circuit - for high resolution with low self heating an AC bridge with a transformer for the reference arm is a good idea.

author: branadic
author_rank: Super Contributor
timestamp: December 23, 2017, 04:35:52 pm
content: QuoteThis shows, that you have never tried that approach. At least it all depends on the accuracy you're aiming for. But, in any case you will always make a comparison measurement between 2x PT1000 and a low TC 1k resistor. Thus, the effects you are calling for can be eleminated. The results are pretty good-branadic-

author: Alex Nikitin
author_rank: Super Contributor
timestamp: December 23, 2017, 06:48:56 pm
content: LM35 is specified at 0.08K/1000h typical drift at Tj=Tmax, and available in a metal can - that is what I usually use.CheersAlex

author: 2N3055
author_rank: Super Contributor
timestamp: December 23, 2017, 07:10:58 pm
content: @Alex NikitinSHT35 are guaranteed to have 0,03°C /year maximum drift. And that's for whole temperature measurement chain, direct to digital..It is mostpracticalway to getdecentresults for environment monitoring.Of course, he didn't specify what is the target use...If sensor should go inside Vref case, or be attached to something, then other sensors might be used, like platinum, thermistor, LM35...OP should clarify purpose..Regards,Sinisa

author: Andreas
author_rank: Super Contributor
timestamp: December 23, 2017, 07:47:33 pm
content: Hello,another example of measurement "problems" of temperaturesduring my "tilting" experiment of LTZ1000.Depending on orientation of PCB (NTC versus LTZ1) in all 6 possible orientations of the housingI get up to 0.3 K difference between orientations for the NTC sitting 2 cm away from LTZ1000.And that with good thermal shielding of the LTZ and the PCB in that area and a inner metal housing.with best regardsAndreas

author: zhtoor
author_rank: Frequent Contributor
timestamp: December 23, 2017, 09:13:27 pm
content: or maybe winding a square coil of enamelled copper wire and using two such coils to enclosethe thermal volume and using the "natural" tcr of copper to read the temperature could beused just like Cu50 sensors (like pt100/1000's). only the construction would be a lot *cheaper*.ie; the thermal sensor encloses the *whole* thermal volume / oven.regards.-zia

author: Andreas
author_rank: Super Contributor
timestamp: December 23, 2017, 09:20:31 pm
content: Hello,in my measurements long term stability is not the problem. (measurement duration is below one day).And also absolute accuracy plays no role as long as the changes of temperature are exact.in my case a bulky (PT25 in glass or even below for highest stability) sensormakes more problems reading the temperature of the resistor.In my case the better solution is to keep the wires of the sensor and the wires of the resistor in good thermal contact.with best regardsAndreas

author: khs
author_rank: Regular Contributor
timestamp: December 23, 2017, 10:03:22 pm
content: The question is what could be the reason for the drift of the low cost pt1000.I see three main reasons.1) Mechanical stress.2) Change of the conductibility of platinum.3) Change of the conductibility of the 'coating'.My guess the main reason is mechanical stress, so the change is the value of the resistance only, not the temperature dependence of the platinum.With other words it's not longer a PT1000 it's maybe a PT 1001.5 ora PT 998.4In this case it would be easy to recalibrate the "pt1000" with a triple cell.

author: branadic
author_rank: Super Contributor
timestamp: December 23, 2017, 10:14:32 pm
content: As far as I know it's not pure platinum when talking about thin film RTDs.-branadic-

author: 2N3055
author_rank: Super Contributor
timestamp: December 23, 2017, 10:39:29 pm
content: Many mechanisms..- Morphological changes in film because of mechanical stresses (both thermal and vibrations)- Agglomeration (well, basically lumping of the surface) because of ceramic substrate surface imperfections...- Recrystallization of platinum in film (sputtering creates non structured platinum film)- Changes in terminal attachThin film ones are usually made with sputtering process.. It can be pure platinum. After forming the layer comes the photolithography to form shape and it is usually laser trimmed for precise ones..Usually a passivation layer (coating ) is used.Mostly mechanical/thermomechanical shocks are important, other ones happen mostly at higher temperatures..Depends also on design, manufacturers technology and manufacturing process.Keeping it on a room temp in nonaggressive atmosphere, with no temp or mechanical shocks would make it pretty much very stable...Not as good as wire or foil type, but at least better than max spec.But, as I said, you need to take into account stability of measurement system that you connect sensor to... That might prove to be more challenging...Regards,Sinisa

author: beanflying
author_rank: Super Contributor
timestamp: December 24, 2017, 12:39:57 am
content: PT1000 or PT100's for stability and better long term drift in the higher grades. Never T/C's but they have their place too. Two point calibration (ice and boiling point with ambient pressure correction if needed) to a reasonable level of certainty once a year or even every second year for good quality RTD's is fine for under .1 degree. A lot of the all in one IC's and the like I have my doubts about their long term abilities and the only real way to calibrate them is some form of dry well.I am setting up one of my 34970's to do some testing of various temperature sensors, RTD's and T/C's. but still a work in progress.Some good information here and also the Omega site in generalhttps://www.omega.com/temperature/pdf/rtd_gen_specs_ref.pdf

author: ap
author_rank: Frequent Contributor
timestamp: December 24, 2017, 11:25:31 am
content: The 46000 series in fact would be a very precise and stable sensor. Glass bead sensors, since they are hermetic, are in general, at least at moderate temperatures. It would be nice if it was possible to use the 46000 or 44000or similar thermistors with the 3458A, supporting 5k and 10k thermistors (40653B and C), however I have nowhere seen data specifiying their electrical properties and the parameters (and in the end accuracy) used within the MATH function section of the 3458A. So it kind of looks as if KS had not intended thermistors for precise temperature measurements together with the 3458A.If anybody has any related data, would be nice to discuss them here.

author: Henrik_V
author_rank: Regular Contributor
timestamp: December 24, 2017, 11:42:14 am
content: Quote from: Conrad Hoffman on December 23, 2017, 03:45:04 pmThat is the way to go!(Unless you know someone, who ...)

author: mycroft
author_rank: Regular Contributor
timestamp: December 24, 2017, 12:18:28 pm
content: This maybe of interest Investigation of Long-Term Drift of NTC Temperature Sensors with less than 1 mK Uncertaintyhttp://ieeexplore.ieee.org/document/7281460/]http://ieeexplore.ieee.org/document/7281460/

author: mycroft
author_rank: Regular Contributor
timestamp: December 25, 2017, 01:47:17 am
content: QuoteAn easy way is pulsed excitation. With a low duty cycle you can even over-excite the sensor to obtain a better reading or SNR, as is done with strain gauges.

author: Pipelie
author_rank: Regular Contributor
timestamp: December 25, 2017, 03:05:47 am
content: I have some experiment with SPRT, got some data to share.attached

author: splin
author_rank: Frequent Contributor
timestamp: December 25, 2017, 03:53:06 am
content: Quote from: cellularmitosis on December 23, 2017, 06:51:52 amYou've clearly run into the same issue as I did, viz the almost complete lack of long term stability specifications in temperature sensor datasheets - but not much worse than resistor specifications whose stability specs are almost useless - if indeed there are any at all.[Rant mode] (bypass unless you are particularly bored)As you might have guessed, this is one of my pet peeves - stability is one of the most important specs for sensors (and resistors). Great - but who wants a .001% accurate part that might drift outside that tolerance within a few weeks and become a .1% part within a year? I am heartily sick of datasheets which tout 'excellent', 'outstanding', or 'very good' long term stability but give zero information or even hints as to what that means. A Murata thermistor datasheet states:"1. Excellent solderability and high stability in theapplication's environment2. Excellent long-term stability"It hardly matters to me that I can't parse that to make any sensible differentiation between the two because they aren't quantified and thus are totally meaningless. Maybe it's a translation issue or an industry convention of which I'm unaware but I'm guessing it's just marketing BS (with some element of truth).It's as if the industry expects its customers either to have a good understanding of the stability characteristics of the various types of devices (through experience) or be prepared to evaluate and characterise any parts they may consider using themselves. Why the hell should you have to spend countless hours and dollars doing this when the manufacturers already have this data? Obviously the big customers will get that information and quite likely will insist on doing their own testing and qualification anyway, but that leaves an awful lot of small to medium companies wasting a great deal of money doing mostly unnecessary evaluation work.No doubt many don't bother and the (vast?) majority get away with it because in fact the devices actually are very stable on the whole providing they operate in relatively benign environments For example, the abstract of the paper linked by mycroft cites a jellybean $0.15 NTC -QuoteAnybody expect that oustanding result? Anyone got access to the paper to see if it was a single sample or in unrealistic or highly specific operating conditions?To be fair to the manufacturers it is impossible to fully specify drift rates as they are very much a function of operating conditions including high temperatures and thermal shocks, but that doesn't excuse providing absolutely no data - whether in application notes or white papers if not in the datasheets themselves.[/Rant mode]Back to the original question. Stability is not an end in itself, it mainly determines the frequency of calibrations. So there are several options:1)Have your equipment calibrated professionally. Whilst appropriate for some this is very expensive and clearly hard to justify for enthusiasts for reducing their measurment uncertainties by a relatively small amount.2)Do your own calibration. Lars suggested using the TWP but I think that is also unnecessarily difficult and expensive. Using the ice point should be more than adequate. Take a look at this which suggests that an ice bath can get you within 3 or 4 mK of the TPW (method 3):http://www.burnsengineering.com/local/uploads/files/Approximating_the_TPW_Presentation.pdfYou should also take a look at this website which has some very good information on DIY calibration using the ice point an the melting point of Gallium (actually quite inexpensive):http://www.kandrsmith.org/RJS/Misc/Thermometers/absolute_ds18b20.htmlandhttp://www.kandrsmith.org/RJS/Misc/Hygrometers/absolutetemperature.htmlSingle point calibration of NTCs raises the question of exactly how they drift - if the shape of the transfer curve drifts then it wouldn't be sufficient. Fortunately though it sems that NTC drift is typically thermometric - ie. the temperature change is constant, or an offset across the range. This was from a paper found on the Measurement Specialities website, but having been taken over by TE I can't find an online copy. I have thus attached a copy I made earlier. (And why I always try to take copies of anything I find interesting).The implication of the above is that a single point calibration should be enough to restore the calibration across the whole range.3)Purchasing sensors with sufficient accuracy. This is where it gets difficult and the drift specs become relevant given that high precision sensors are relatively expensive. The US Sensor PR series NTCs are attractive costing $7 to $10 with a +/- 50mK tolerance between 0 and 50C. They are claimed to be highly stable but that isn't quantified. However I found this:http://www.ussensor.com/accelerated-life-testingWhich states:Quote(my bold)This strongly implies that the 50mK tolerance PR series devices are expected to remain in tolerance after harsh testing as do other associated 'quality' pages. This may actually be a well understood industry convention, in which case much of my rant is misplaced, but I haven't seen it stated explicitly anywhere - I'd love to see anything authoritive on this issue though.As stated earlier, the YSI super stable 46000 series, glass encapsulated thermistors do specify (low) typical drifts and are available in 50mK tolerance but are expensive and not widely available.  Measurement specialities, who took over YSI, have now been acquired by TE Connectivity who appear to have obsoleted all except the 46004, 100mK part. And lost all the relevant white papers - assuming you can even navigate their infernal website4)Some combination of 2) and 3). The PR series thermistors are possibly inexpensive enough to allow you to purchase one or more periodically as references to recalibrate other, perhaps low cost parts - at least until you have sufficient history to be sure of the drift characteristics of your reference sensors. Of course that doesn't work if the supplier's turnover is so low that you don't get recently manufactured parts. (Don't buy from anyone who has them in stock? )As I mentioned previously, the KTY8x series of silicon diode sensors are interesting. They need to be calibrated but they do claim to have low drift. The glass encapsulated KTY83 (SOD68/DO-34) and KTY85 (DO80/Mini MELF) parts have been obsoleted but there are some German sellors on ebay selling 30 for $7.5 inc postage. The plastic encapsulated KTY81 etc versions are still available but may suffer more from humidty/hysterisis. The drift is specified in this document along with the relevant Arrenius data:http://www.elenota.pl/datasheet-pdf/138914/Philips/SC17?sid=8cae005bccd92bb89ead005554f44553Once again this document seems to have been lost by NXP, robbing designers of valuable information. (I hate the loss of such data whether inadvertently as is probably the case here, or much worse, willfully).From the above document a maximum drift of 250mK for a KTY85 after 10,000 hours @ 125C is shown and from the Arennius equation, 450,000 hours @62.5C. By my calculation that also equates to 5.7mK after 10,000 hours at 62.5C or .1mK/10,000h @ 20C. That would appear to be outstanding for long term, low temperature measurements which is presumably the OP's intent. Of course there's a caveat which is that the Arennius equation is only part of the drift story. This Siemens KTY document gives a bit more insight:http://www.b-kainka.de/Daten/Sensor/gentemp.pdfPage 5 shows the drift charatceristics at high temperature with a not surprising large change over the first 500h and relatively little thereafter, suggesting a burn in before calibration would be a good idea.So where does this leave us? Those who deal with this stuff on a regular basis will undoubtably have a much better handle on what's really important and how realistic datasheets really are so hopefully some will enlighten us. Clearly long term drift of temperature sensors is a very small issue in the case of the LTZ1000 (transistor Vbe) and Fluke 734 (NTC) so drift, in reality may be a non-problem for this application for most sensors.As to your question of types of sensor and order of preference:a) PT100/PT1000 Probably a good choice for higher temperature measurements. But expensive and very hard (almost impossbile) to find any drift data for affordable (none lab grade secondary standard probes) when used at moderate temperatures. Low senstitivity doesn't help. I can't see any reason to choose these for extended temperatures below 50C and possibly even below 100C.b) NTCs. For temperatures below 50C or so, they seem to be an excellent choice - high sensitiivity (10X PT100/PT1000) and capable of excellent stability, and can be very low cost. Anyone know what is used in the Fluke 734?c) Silicon sensors. The SHT35 looks like a reasonable choice but the initial accuracy of +/-300mK isn't good (compared to a +/-50mK NTC) and the drift @ 30mK/y, in unspecified conditions, isn't that impressive. In reality it may be a star performer but there is only so much you can conclude from a datasheet. It isn't particularly cheap either and needs to be mounted on a PCB which could be a serious limitation. The KTY sensors look very good and whilst not as sensitive as NTCs are still twice as good as PT100x types, thus reducing the performance requirements of the rest of the system. It's also possible that jellybean transistors, such as the 2N3904, may give equally stable results when measuring Vbe.Overall, the allure of that potential 0.492 mK/year of the Murata NCP15XH103D03RC @ $0.5 (1 off) is hard to ignore. It's also hard to ignore that very many similar NTC parts, including the really common place types, may well have similar (drift) characteristics but simply haven't been evaluated in this way.Enough. I'm sure there are lots of grammatical and technical errors in the above - my main excuse is that I'm just recovering from an unpleasant flu type infection, so please forgive any incoherency.Merry Christmas all! Turns out I can't attach the paper as it's too big (1.7MByte v 1.2M limit). PM me if you want a copy.

author: beanflying
author_rank: Super Contributor
timestamp: December 25, 2017, 04:24:59 am
content: Excellent rantAs I kick back and digest Christmas Lunch with a beer in my hand time for a quiet read

author: 2N3055
author_rank: Super Contributor
timestamp: December 25, 2017, 10:23:25 am
content: Yep, very good rant Splin!!Merry Christmas to all!Regards,Sinisa

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: December 26, 2017, 05:42:05 am
content: Just another thought, based on experience:We use LM35A and have had as good or better results than Sensirion parts, and keeping it all analog keeps the system quiet. The LM34/35A are nice in that they are ready to work if you can read an analog voltage accurately on your system anyway.Our situation demands the least amount of digital hash around the measurement area. We feed the temperature measure voltage into the same low emf scanner that's running other analog voltage tests on the system, and just treat it like any other analog Vmeasure in the test script. That's actually much easier for us than trying to integrate an I2C or SPI bus into the test apparatus - and then trying to deal with the resulting noise.We also do a cal test every year and the drift is very minimal, usually < .02C or less, and over 2 or 3 years time it tends to average out to even less - we don't really see a major long term trend up or down. The datasheets are fairly conservative for the "A" parts - they run around $25~ or so. Not the cheapest solution but they work very well - the cans are nice since they don't pick up board stress / vibration nearly as bad as the SMT parts. If you have to, Add a 4-20mA current driver to the part and you're ready to ship the temp data over very long lengths of wire with virtually zero digital noise impact.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 26, 2017, 11:59:21 am
content: Interesting read! Wouldnt have thought that an ice-bath could be simply produced with 2mK uncertainty; i thought the atmospheric variation error would be bigger.

author: splin
author_rank: Frequent Contributor
timestamp: December 27, 2017, 12:21:39 am
content: Quote from: splin on December 25, 2017, 03:53:06 am...QuoteI tried a few online pdf compressors and pdf2go.com did the biz. The quality of the attached is only slightly worse than the original.I also forgot this rather relevant link to a National Bureau of Standards paper, 'An Investigation of the Stability of Thermistors':http://nvlpubs.nist.gov/nistpubs/jres/83/jresv83n3p247_A1b.pdfThe conclusion is to avoid disc type NTCs at all costs, and that bead types can be very stable indeed, particularly if keep below 30C. (See figs 15B-1 and 15B-2 on page 260). Some drifted less than 2mK over 2 years! Unfortunately they didn't reveal the manufacturers and part numbers for the 145 devices tested.

author: SteveP
author_rank: Contributor
timestamp: December 27, 2017, 02:31:41 am
content: The article mentioned by mycroft (back on page 1) may well be an excerpt from an MSc. thesis by Anupama Kulkarni at TU Delft which is freely available online. He tested 9 NTCs and gives product numbers. Test setup is described in detail.I can't seem to create a decent direct link to it, but search for "Low Drift, Wireless Temperature Sensor" and you should find copies from citeseerx and Delft University.HTH,--Steve

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 27, 2017, 03:02:31 am
content: Perhaps another way to imagine the problem is needed to see the point Andreas and others are making.Start with an assumption of a perfect temperature sensor. No drift. No bias errors. Small. Near zero time constant. Cheap.Now instead of trying to make the temperature sensitivity of the voltage reference zero you just use your magic sensor to servo the voltage reference to a fixed temperature. As you start to think through the errors that might occur in such a system you can start to understand the difficulty of making a precision measurement of "the" temperature.If you can control "all" of the variables it is possible to imagine precision measurements of the relative temperature over time, but there are many to control. How does the thermal conductivity and heat capacity of the PWB vary with humidity for example.

author: 2N3055
author_rank: Super Contributor
timestamp: December 27, 2017, 10:07:22 am
content: Quote from: SteveP on December 27, 2017, 02:31:41 amThis one ? :http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1010.1132&rep=rep1&type=pdf

author: SteveP
author_rank: Contributor
timestamp: December 27, 2017, 10:50:15 am
content: Yup. Dunno why I couldn't make that link work. Thanks.

author: massivephoton
author_rank: Contributor
timestamp: January 06, 2018, 02:30:48 pm
content: Just out of curiosity, how much drift should one expect from an old school mercury/quicksilver thermometer?I know there are capillary effects, parallax and variations on the wall thickness of the glass element.But once it is calibrated, what drift level can possibly be achieved?

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 06, 2018, 03:52:20 pm
content: Quote from: SteveP on December 27, 2017, 02:31:41 amNCP15XH103D03RC is the best of the lot according to this article and pretty cheap also:-https://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=248253183&uq=636508288350905852regards and thanks Steve-zia

author: MK
author_rank: Regular Contributor
timestamp: January 06, 2018, 09:05:39 pm
content: Quote from: massivephoton on January 06, 2018, 02:30:48 pmi believe that NPL in teddington still use a lot of glass/mercury thermometers, so they must be pretty stable.

author: branadic
author_rank: Super Contributor
timestamp: January 15, 2018, 06:45:34 pm
content: Inspired by the discussion here I ordered a few NCP15XQ102J03RC from RS Components and connected two of them to the RDC unit of a PCap01 which is able to handle resistance from 250? up to several k? forming a RC circuit and measuring discharge time with several ps resolution. This works pretty good by the looks of it. This gives a simple but high resolution thermometer for temperatures below 0°C up to 60°C without any analog circuit.Results will be shown soon, after some more investigation and calibration.-branadic-

author: cellularmitosis
author_rank: Unknown
timestamp: April 14, 2018, 09:29:19 pm
content: Quote from: mycroft on December 24, 2017, 12:18:28 pmI came across a copy of this paper recently (attached). This is the paper which surprisingly found that the SMD part NCP15XH103D03RC actually drifted less than the glass-encapsulated types (but the glass types still drifted typically less than 2mK per year).QuoteHmm, but I wonder if this is why that SMD part performed so well:QuoteOur results in a non-hermetic environment might not be as good.Also interesting was that the level of excitation voltage also affects drift (0.6VAC performed better than 1.2VAC):QuoteQuoteand apparently DC is also worse than AC:Quote

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: April 16, 2018, 01:21:27 am
content: Quote from: 3roomlab on December 24, 2017, 11:51:57 pmOne way is to measure the thermistor resistance with two different excitation currents, e.g. I and I/sqrt(2), then extrapolate to the zero-power resistance. Once you have a zero-power resistance versus temperature characteristic, you can estimate the thermistor's internal temperature rise due to self-heating at each calibration (temperature) point.This first-order correction works ok if your self-heating is small (e.g. tens of uW as in your case). In principle, you could use more than two excitation currents, to estimate zero-power resistance, but I have not heard of that done in practice.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 22, 2018, 10:40:36 pm
content: Really nice info in this thread!Thank you all.I have a Fluke 5616 Secondary Reference PRT that was bought new from Fluke with a calibration certificate.- The calibrated accuracy is ± 0.011 °C at 0 °C- The short-term repeatability is better than ± 0.010 °C (± 0.004 °C is typical)- The long term drift is ± 0.007 °C at the triple point of water when exposed up to its maximum temperature (420 °C) for 100 hoursSince I am using this probe carefully only for up to 100 °C, I would not expect any measurable long therm drift.Or do I assume too much?What I find interesting as well:All my other 4 wire PT100 sensors have a TCR of 0.00385 Ω/Ω/°C (As far as I know)This Fluke 5616 PRT is listed with 0.003925 Ω/Ω/°CWhat is the reasoning behind a different TCR for a high end probe?

author: Vgkid
author_rank: Super Contributor
timestamp: April 22, 2018, 11:01:23 pm
content: From what I've read, the .003925 (alpha) is the tc of pure platinum. That is why this value is listed for many (s)prt thermometers.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 23, 2018, 12:55:24 am
content: Thanks VgkidI looked a little deeper and found this:the TCR of 0.00385 Ω/Ω/°C for RTD sensors is the European standard, DIN/IEC60751 (IEC751)the TCR of 0.003925 Ω/Ω/°C is the USA standard.I had no idea that the USA and Japan would still use a different standard for RTDs.TCR of highest purity platinum is 0.00393 Ω/Ω/°C

author: texaspyro
author_rank: Super Contributor
timestamp: April 23, 2018, 01:06:47 am
content: Quote from: HighVoltage on April 23, 2018, 12:55:24 amWell, there are now RTDs made of isotopically pure platinum... just send money... lotsa, lotsa money. Not sure how swoopty they are. Send me a few and I'll play with them.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 23, 2018, 07:11:38 pm
content: Quote from: massivephoton on January 06, 2018, 02:30:48 pmJust as a reference point, the old Julie SC-106 standard cell oven used a dual oven setup, both controlled by mercury thermoregulators- nothing more than mercury thermometers with electrodes stuck through the glass. As the mercury rises and falls in the column, contact is made or broken, activating the heaters. Now, the advantage they have over an ordinary thermometer is the reservoir is large and the entire column is thus only a few degrees. They had to be purchased for a specific temperature. AFAIK, they were quite stable, the standard cells providing an immediate indication of any drift.As for regular mercury thermometers, I've never seen one go off calibration unless the mercury column got separated, and that's usually fixable.

author: czgut
author_rank: Contributor
timestamp: March 19, 2019, 06:51:00 am
content: Glass exposed to temperature changes gets stress build into it. The smaller changes of temperature, the better stability. For 0'C to 30'C changes [From measurements at NBS / NIST (Special Publication 250-23)] achievable repeatability + stability/year was about 0.017 deg 3 sigma std dev.