Temperature test box for component characterization  - Page 1
Forum section: electronics / testgear

author: janaf
author_rank: Frequent Contributor
timestamp: February 24, 2015, 01:46:26 pm
content: Getting deeper into volt-nuttery, I realize I need a test box for characterizing temperature dependency of components like resistors, zeners, LTFLU...So far these ideas are just beyond sketching.I have followed Andres thread on TC measurements of precision resistors, beginnig to end. The basic principle seems very good, doing measure relative known characteristics at stable temperautre, using the same reference voltage for the measurement device / DAC as for the test signal for the DUT.https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg615711/#msg615711What I want:- Very good temperature control / stability for DUT. <0.01C?- Ability to measure sub?ppm levels of changes in voltage / resistance / current versus temperature.- Moderate temperature range like 0 to 75C.- Multiple components tested in parallel?- Moderate physical size. Like a shoe-box?- Automated test cycle, PC interfaced.Have got :- A peltier unit.- Some big fat chunks/slabs of (tellurium) copper- Some DAC boards: LTC2498, LTC2442, ADS1282 & DMMs- Various PID temperature regulators (12V)- Precision NTCs and LM35- Various decent voltage references, 5V, 2.5V...- Galvanic isolation USB interface- Lots of Li-XX battery packsPeltier unit:http://www.ebay.com/itm/THERMOELETRIC-PELTIER-JUNCTION-COOLER-ASSEMBLY-LWKW-0317-/220865136946?pt=LH_DefaultDomain_0&hash=item336c968932My plan is to:- Remove the smaller heat sink from the Peltier untit- Replace it with a copper slab with holes drilled for temperature sensors & cables- Add screw-on copper slab "lid" with routed cavity.- Add foam insulation cap surrounding the copper slabs and peltier unit except outer heat sink.Build a second shielded box, temperature controlled at moderate temperature, for reference components, DAC boards etc.All electronics could be run on battery but Peltier has to on mains => 12V converter. The temperature controller sensor needs to interface with Peltier via Isolated SSR / FET / transistor driver.Comments welcome!To be continued.

author: janaf
author_rank: Frequent Contributor
timestamp: February 24, 2015, 08:21:01 pm
content: Reserved for measurement results.

author: janaf
author_rank: Frequent Contributor
timestamp: February 24, 2015, 08:21:30 pm
content: Peltier module arrived.I've split the module using a kitchen spatula and gentle brute force.The larger heat sink is approx 125x125 mm (5"x5")The Peltier is the common 40x40mm size surrounded by two types of quite thick insulation foam.I will add more foam, out to the full 125x125. With 25mm foam on all sides, the copper slab can be 75x75mm. Leaving 7.5 mm copper walls, I end up with 60x60 "test chamber" which I think is quite enough for my needs.

author: splin
author_rank: Frequent Contributor
timestamp: February 24, 2015, 08:41:57 pm
content: Do you need good long term temperature stability? I'm guessing that absolute temperature accuracy is not too important but if you want to compare results over long periods then the stability of the temperature sensor is important. If it is only to be used to characterise temperature coefficients/sensitivities etc. it doesn't matter but it would be a shame to do a lot of testing over long periods and then have relatively large temperature uncertainties such that you couldn't accurately characterise long term drifts of voltage references, resistors etc.Naturally sensors with proper drift specifications aren't very common. Whilst Platinum RTDs can be excellent, common low cost ones aren't especially good (or don't specify the drift) whilst lab grade ones are very expensive and difficult to use accurately.They aren't easy to find, or cheap ($40+), but the best I've found is the YSI (now owned by Measurment Specialities) 46000 "Super stable" series with a claimed typical thermometric drift < 10mK over 100 months (Tamb = 25C or 70C). They also are available with interchangeablity tolerances down to 50mK ($180!). The 45000 series are cheaper but are specificed as < 50mK drift over 10 months (Tamb = 100C).More easily obtainable are US Sensor PR series, also interchangeable to .05K; however, whilst they spout a lot of hype about their high stability, they don't actually specify it anywhere that I can find. They probably are excellent but it would be an act of faith to rely on it.Much cheaper are NXP KTY82 and KTY83 diode based sensors. The latter are specified at 1 ohm drift over 10,000 hours (Tamb =175C), equivalent to 128mK at 25C. The drift should be way less when used at < 70C, but who knows exactly how much?Recalibrating periodically is another option, but horribly expensive if you can't do it yourself and not particularly easy at temperatures other than at the triple point of water. Personally I'd pay for at least one super stable thermistor and use it to calibrate cheaper working sensors such as the KTY83 or an NTC thernistor.I wonder what sensors Fluke used in their voltage calibrators?

author: babysitter
author_rank: Frequent Contributor
timestamp: February 25, 2015, 12:41:52 am
content: Don't use bare 100% PWM with low frequency. During the 0V phase, heat will flow back from where it came, and during the 100% phase, you have maximum heat generation due to U*I.There are dedicated Peltier controller chips, just look at the linear website.

author: janaf
author_rank: Frequent Contributor
timestamp: February 25, 2015, 09:42:53 am
content: For now; no need for very accurate / long time stable temperature measurements. LT35 and some US sensor NPTs will do. Later, who knows...Will not use low frequency PWM for temperature.

author: janaf
author_rank: Frequent Contributor
timestamp: February 26, 2015, 03:14:58 pm
content: In the mean time, a simple aging oven:A 100 ohm power resistor 10W in a small thermospower at 12v (ie near 100mA)Stabilizes at near 110CWith a proper cork it will reach 120C easy.I'll add a thermostat or controller just in case. It will run on a regular 12V adapter. As it consumes only a bit more than 1W it can be left on for months, years.

author: janaf
author_rank: Frequent Contributor
timestamp: March 20, 2015, 09:37:55 am
content: Quick and dirty #1I was asked to do some measurements on wire-wound resistors from Pettis and hermetic foils from Vishay. Here are pics of a quick-and dirty setup I made for this, while I'm waiting for components for a more permanent unit.A block of copper, 50x50x18mm, 0.4kgTwo ceramic 100 ohm heater resistorsTwo thermocouplesDUT, two resisotrsThe stuff above is stuck on the copper block, using semi-soft, heat resistant silicone rubber.The copper block and parts will be wrapped with insulation and placed in a large thermos / DewarOne of the thermocouples is connected to a PID regulator, output power is via a DC SSROne of the thermocouples is connected to a PID regulatorOutput power is via a DC SSR, 0-30VDC, as neededThe other thermocouple is for temperature loggingResistors will be connected 4-wire to DMMsSorry, no cooling, no PT-100 available this time.Data ETA, this weekend.

author: janaf
author_rank: Frequent Contributor
timestamp: March 23, 2015, 07:02:46 pm
content: Comments please!I have been doing some thinking about measurements of the ppm differences over temperature. Andreas's thread about temperature characteristics of precision resistors uses a clever circuit, using the REF voltage of the ADC for the resistance measurements.I found something similar in a MAX197 app note AN-270 (attached), figure 5. The nice thing about these circuits is that they compensate for any drift in the reference voltage, by measuring ratios. Probably nothing new under the sun. The current through the resistor / RTD / DUT is given by I=Vref/R_iset (i.e I=2.5V/25K=0.1mA)Anyway, I rolled my own, with a Instrumentation amp instead. It's attached asRTD_circuit_2.An example: in the case of a 1000 ohm resistor with 0.1mA current (reasonable?) there will be a voltage drop of 0.1V over the test resistor. This voltage can be amplified by a factor 10 or a bit more to improve SNR etc. With the components / values in the figures, the voltage will be nominally 2.6V before and "always" 2.5V after the resistor.However, I realized that if we add a very stable reference resistorsee RTD_circuit_3, also 1K, with it's own current loop, we can generate another 2.6V reference point, while we can be sure the 2.5V on the lower side is still 2.5V. Now we can connect the 2.6V of the reference and the 2.6V of the DUT, both to the instrumentation amp, amplify by, say, a factor 1000, and get a very much amplified signal from ppm level changes of the DUT.As far as I can see, the whole circuit will compensate for any REF voltage changes as well as minor changes to the R_iref resistor, while R_ref has to be rock stable over a measurement cycle.The price is two op-amps, a reference resistor and a negative power rail.Comments please!

author: Andreas
author_rank: Super Contributor
timestamp: March 23, 2015, 10:04:48 pm
content: Hello Jan,sorry but I do not understand how the compensation works,and where you will really attach your measurement instrument.Did you try to simulate this with LTSPICE?Further point: does the (temperature dependant) gain of the instrument amplifier change the output?With best regardsAndreas

author: janaf
author_rank: Frequent Contributor
timestamp: March 24, 2015, 06:15:05 am
content: Quote from: EmmanuelFaure on March 24, 2015, 03:52:55 amDamned, I thought I was firstYes, it's a kind of Wheatstone bridge.

author: janaf
author_rank: Frequent Contributor
timestamp: March 24, 2015, 06:40:25 am
content: Quote from: Andreas on March 23, 2015, 10:04:48 pmQuote from: Andreas on March 23, 2015, 10:04:48 pmThe ADC connects to out, ref and ground.The measurement is between the known voltage on the positive side of the Ref resistor and the positive side of the DUT. So I'm measuring the variation of the DUT (over temperature / time), not the actual voltage/resistance over it. Thebig fat assumptionis that the rest of the circuit is constant enough to make meaningful measurements.However, the good part is that the change in voltage over DUT needs to be measured with a few digits accuracy only. Instead of measuring the voltage over the DUT down to single ppm, I'd be measuring the variation; something like 2 digit accuracy and 3 digit resolution should be enough to see the characteristic of the resistor.As you point out, the gain of the InAmp is a weakness & error source. I must have a highly stable resistor there.On the positive side; the noise from the voltage reference would be common mode to the InAmp and therefore largely suppressed.SPICE: coming...

author: babysitter
author_rank: Frequent Contributor
timestamp: March 24, 2015, 06:52:06 am
content: Consider making a second (quite) thermostatted environment where you put all the parts that you want stable, e.g. your differential voltmeter amplifier, the thermostat circuit, reference resistors...Hendrik

author: mzzj
author_rank: Super Contributor
timestamp: March 24, 2015, 07:35:56 am
content: Quote from: splin on February 24, 2015, 08:41:57 pmOne of the better options amongst Pt100 sensors would be thishttp://www.distrelec.de/en/Resistance-thermometer-TDI-P100-3045/p/17668845Pricing is around same ballpark or cheaper than YSI 4600 series.

author: janaf
author_rank: Frequent Contributor
timestamp: March 24, 2015, 08:06:05 am
content: Quote from: babysitter on March 24, 2015, 06:52:06 amWill do!

author: blackdog
author_rank: Frequent Contributor
timestamp: March 24, 2015, 08:51:08 am
content: Hi, Some remarks from me :-)I made some months ago a temp sensor calibrator, and klik on the link to see the topic on the Dutch Forum Circuitsonline, use google translate!http://www.circuitsonline.net/forum/view/123403If you want good control over the temperature, then it is important that you have a good link between the sensor and heater.I use the NTC for controling the temperature and de PT1000 between the holes for the D.U.T. goes to the TEK DMM4050 Multimeter for checking of the set temperature.I hoop this info helps :-)Kind regarts,Blackdog

author: janaf
author_rank: Frequent Contributor
timestamp: March 24, 2015, 01:04:06 pm
content: Thanks.If you go back to post #7 you can see the quick-and-dirty thermal block I'm using to start with, waiting for the real thing.

author: janaf
author_rank: Frequent Contributor
timestamp: March 24, 2015, 01:15:56 pm
content: Tatatata!I'm doing real time measurements on the forum now!I have set up a quick and dirty test with two resistors:One 1K, 1%, wirewound from PettisOne 1K, 1%, VHP101T hermetic foil from VishayLook in post #7 for some more details on the setup.The temperature is set with a PID controller. I wait until the set temperature has been reached, wait at least five minutes, then take an average over one minute.UPDATE: Yo, there was something wicked going on with the first measurements of resistor B. I have updated the attached file, it looks much more consistent now. I can't really explain what happened with the first series.UPDATE: Finished for today. Well after midnight here. I attach an updated file.Final plot: I attach the final version, have removed some garbage from the first plot. This is as good as it gets with a quick-and-dirty setup.I really need to automate the process and finish the final setup. It took 8+ hours to do it manually. Not realistic to do that except in very special cases.I'm quite sure it would be sufficient to 5-7 temperature points in all, to see the general trends.I need to do measurements in a temperature controlled of the room, these weren't. At these levels (ppm) the room temperature influences the DMMs (by 1-2ppm/C)Bottom line: With the box method, resistor A (Pettis) has a TCR of+0.7ppm/Cwhile resistor B (VHP101T) has a TCR of-0.24ppm/Cover the measured range. Hysteresis was very low or zero for both, levels below measurable in this setup.PS Taking things apart showed that one of the leads of the Pettis resistor was touching a thermocouple which explains the early "strange" results. After fixing that, the resistor behaved very well, much better than specified.PS2: The price of the VHP101T is about five times higher than the Pettis resistor.PS3: As the absolute accuracy of the resistors for the LTZ1000ACH is not important, I specified each at 1% tolerance. Both where delivered better than 0.01%!According to my DMM: Pettis; 1.00009 kOhm, VHP101T; 1.00002 kOhm

author: janaf
author_rank: Frequent Contributor
timestamp: March 25, 2015, 08:03:41 am
content: So I'm finished now. It was a good learning experience and gave me ideas for the later, final, setup.The Pettis resistor performed much better (0.7ppm/C) than specs (5ppm/C?) at this temperature range.The VHP101T result was near its "typical" specification of 0.2ppm/C with best part of the curve lower than room temperaure.Comments?

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 25, 2015, 08:58:29 am
content: Quote from: DiligentMinds.com on March 24, 2015, 05:37:32 pmHi Ken,I'm also searching for a better 40k resistor for my 3458A, but your statements are incorrect in several aspects.The 40k needs to have a T.C. as low as 0.1ppm/K (currently specified ~1.3ppm/K)This will improve the "Temperature Coefficient With ACAL" parameter ONLY! (by a factor of 10)Anyhow, all Ohm ranges will still have a very bad T.C. of 3ppm/K, prohibiting a useful Ohm Transfer Stability specification.To really improve the Ohm mode for T.C., you would have to replace ALL resistors which are involved in the Ohm measuring function, by low T.C. ones, i.e. BMF types R307 = 300, R308 = 3.00k, R310=10.00k, and the complete resistor array RP300, containing 30k, 60k, 600k, 6M, 10k, 0.3k, 3k. If I did not forget any, that would make 10 additional low T.C. resistors, see picture.Additionally, because the timely drift specification is so bad, the 40k would need to be a hermetically sealed, AND oil filled type. VHPxxx types give a typical annual drift of 2ppm/6years.PWW resistors are not suitable, as these have typical drift rates of 20ppm/year, maybe below 5ppm/year for selected ones.This 40k resistor is part of the DCI / current ranges, which are also quite unstable, maybe also affecting the ACAL OHM function.Therefore, I would also like to replace most of the current shunt resistors by <1ppm/K ones.At least, R 207=40k (0.1ppm/K 2ppm/6yr.) , R206 = 500k, R208 = 4k53, R209 = 634, R210 = 90, R211 = 9, R212 = 1 Ohm, 2W.Frank

author: janaf
author_rank: Frequent Contributor
timestamp: March 25, 2015, 09:52:54 am
content: Quote from: babysitter on March 24, 2015, 06:52:06 amI realize I have the same problem for my DMMs, in case I use those, like I did in the Q&D measurements. There are reference resistors in the DMM that are temperature sensitive. It seems like the measured resistance value changes by 1-2ppm/C. I can compensate by running internal calibrations now and then but don't know how far that helps. I should put a thermostat on the fan of the DMM mainframe. I have seen the 3458A owners have similar problems.

author: babysitter
author_rank: Frequent Contributor
timestamp: March 25, 2015, 10:25:51 am
content: I know in the meantime I must appear like a DB6NT sales pusher, but let me point to the QH40A (QH60A) crystal heater again to temperature stabilize small things, e.g. reference resistors!

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 25, 2015, 10:43:52 am
content: Quote from: janaf on March 25, 2015, 09:52:54 amNo wonder, as the Ohm measurement topology is very similar between all the 6 1/2 7 1/2 bench meters and the 3458A, see my description just above!In the lower grade meters, even worse quality resistors than in the 3458A are used.Therefore, you won't get these meters stable in Ohm mode...It's a pity even for the 34470A, as obviously they used an even better 10k BMF reference resistor than in the 3458A, but as the specification clearly indicates, they don't manage to either transfer its high stability to Ohm modes (due to downgraded ACAL functionality), nor did they use appropriately temperature stable resistors for Ohm mode..Frank

author: janaf
author_rank: Frequent Contributor
timestamp: March 25, 2015, 11:48:22 am
content: One problem is that to do a TCAL, you need to know the TCR of the resistor, either by having a very repetitive supply or by measuring the resistors individually.The best is of course to solve the root of the problem but with my DMMs I can't upgrade the resistors, there's no documentation available, at least not that I can find. :-|My DMM modules have on-board temperature sensors, readable by software so I could make a curve fit and correct output for temperature. But I might have to calibrate the dependency for each resistance range.Or build a temperature controlled cabinet for the whole thing.....

author: Andreas
author_rank: Super Contributor
timestamp: March 25, 2015, 08:22:10 pm
content: Hello,when looking closer at your RTD_circuit_3.png I can only see one regulated current through the DUT.The regulated current of DUT is proportional to the VRef and proportional to the 25K resistor.The Reference resistor has at the lower pin the same 2.5V.But the current in reference resistor is not regulated:At the upper pin I see that U3 is amplifying the offset of U1 with his open loop gain.(So actually amplifying the difference of offset of U1 + U3).Or do I get anything wrong?So which of the setups do you use for your T.C. measurements.And how much offset drift do the OpAmps have?A standard OpAmp has about 5uV/K offset drift. So with 100mV over the DUT resistor 1 deg C temperature difference of a standard OpAmp will give about 50ppm/K error.With best regardsAndreas

author: janaf
author_rank: Frequent Contributor
timestamp: March 25, 2015, 09:57:17 pm
content: In the measurements so far, I've been using plain DMMs, no additional circuitry.In RTD_circuit_3, could have used it's own 25K current resistor but instead I used the U2 buffer which senses the voltage of R_iset. So U2 will sink current from Rref and keep the voltage the same as on the low side of DUT. In this way, both resistors use the same 25K for current setting, while U1 and U3 have the same reference voltage.I thought this was better than using two different R_iset which would cause errors if they drift.So U3 and U2 create a copy of U1 with the same current in both but one through the Rref and one through REF, voltage difference sensed by the InAmp.But right now, Im not sure. I might use the simpler RTD_circuit_2. Will see...

author: janaf
author_rank: Frequent Contributor
timestamp: March 25, 2015, 10:13:26 pm
content: The 1K0 foil resistor I measured the other day looks really good. My DMM say it's 21 ppm above 1K at 23C. So I'm considering keeping it as a reference, instead of putting it in a LTZ1000 circuit.The TCR of the resistor is negative ,so by ovenizing it, elevating temperature, it could be possible to trim it to "exactly" 1K0. Trimming by 20 ppm does seem to require quite high temperature, but as my DMMs have not been calibrated for years, I don't know how accurate the 21 ppm offset is....What do you think?

author: Vgkid
author_rank: Super Contributor
timestamp: March 26, 2015, 12:29:50 am
content: ^^^The same way Fluke does it in their "A Poor Man's Resistance Bridge"paperhttp://support.fluke.com/calibration-sales/download/asset/9010017_a_w.pdf

author: janaf
author_rank: Frequent Contributor
timestamp: March 26, 2015, 09:50:19 am
content: As a sanity check, I re-did measurements on the same Ultraohm Plus / Pettis resistor. Results essentially the same as before, TCR around +0.6ppm/C and hysteresis, if there its below my horizon of interest.Generally, as Andreas measured a TCR of -1.1 on the same type of resistor, I think we can simply say the are well within specs and for any better info or we'd need to hand-pick and measure each resistor individually for a circuit.Edwin, would you say that, without any hand-pick / trimming, if the resistors are made from the same wire at the same time, the measurements would typically be similar? (sorry, I only have one 1K). Or is the kind of spread you see between my measurements and Andreas's measurements something you'd expect?

author: janaf
author_rank: Frequent Contributor
timestamp: March 26, 2015, 12:23:46 pm
content: Quote from: ltz2000 on March 18, 2015, 11:36:09 amIt looks like my 1K VHP101T is at least as good as the RS9010 in TCR and could be as good in absolute value as well.For now I'll keep it, put it in an adjustable oven and then hope to have it tested

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: March 26, 2015, 05:01:42 pm
content: Janaf,The resistors I made for you and Andreas all come from the same spools of wire, i.e. 120R all from the same spool, 1K are all from the same spool, ect. for a total of four different spools of wire. Unlike other PWW resistor designs, all of my resistors exhibit the same characteristics without regard to the wire size, the 120R resistors have the same characteristics as the 1K, 12K or 70K, the majority (typically 60% or more) of the TCR will be within 0±1PPM/°C, therefor I can correctly claim that as a typical TCR with the remaining resistors within 0±3PPM/°C. I don't play word games with the specs and I use actual measured TCRs as the basis for the TCR spec, no box or butterfly hide and seek. The <1PPM/°C yield can be higher than 60% of course. Since PWW manufacturing is significantly different than film/foil, I cannot target the TCR characteristic directly, a given spool of wire may give me a tighter yield on TCR over another spool as such but that is a function of the wire supplier, the only control I have over them is my specification for the wire's TCR which is very specific, it is one of the reasons of many why I can turn out a significant low TCR yield without tweaking. I do not know of any other PWW resistor supplier that has as good a TCR yield as I do nor are they as linear in TCR, I repeat linear, that is inherent in the design and has been verified by 30 years of production and by customers. That linearity has been used to discover unknown non-linearity in circuits and measurements. The curve remains flat until near the outer temperature range when there is a slight change, this is mainly due to the alloy's characteristics.Vishay does a lot of tweaking to their processes to produce lower TCRs, however, evidence would indicate that they do not have any consistent control over those processes that yield very low TCRs, particularly in the fact that few of their low TCR resistors are within the claimed 'typical' TCR spec. Whether Vishay actually does any 'cherry-picking' of parts from a given yield for those really low TCR parts, I cannot say for sure, I would definitely not expect Vishay to admit to it publicly or even privately for that matter. There is nothing wrong with selective picking of parts to meet a customer's specifications so I really don't know why Vishay denies doing it.Yes, if a customer asks for a TCR yield tighter than my standard yield, then yes, they would have to be 'cherry-picked' from the batch. I can do things which can improve the yield of low TCR resistors in sets, for instance, if possible depending on the spread of the values, the 1K and 12K resistor set for example. One of my customers uses this to advantage, he has reported TCR tracking of better than <0.2PPM/°C over a 60°C range (all that he needed), copper adhesive tape was wrapped around the pair. I was not asked to guarantee a particular tracking TCR for this application, only what could I do to possibly improve tracking without high expense. As I've mentioned before, the tighter the specs, the lower the TCR specified, the higher the cost. While I do think that Vishay's oil filled are overpriced, that is the only way they are going to come close and I'd bet a dollar to a donut that those oil filled wonders are indeed cherry-picked. Are there resistors with TCRs close to zero in my yields, yes, how many? I really don't know, the distribution of TCR within a given range appears to be somewhat random in nature, there has never been any large scale measurements to mathematically indicate what kind of distribution is actually present. Given the fact that Vishay's foil resistors are made from essentially the same alloy as mine, they would also have a distribution of both + and - TCRs in their yields, what they are actually selling may be wholly different from that.

author: janaf
author_rank: Frequent Contributor
timestamp: March 26, 2015, 05:24:20 pm
content: Thanks for the info Edwin. It's interesting that they are from the same wire as it gives us a chance to compare!So back to the question; Can resistors from the same batch / wire have fundamentally different TCR, even if small? His has -1.1, mine +0.7 ppm/C.Please note that I'm not complaining, both are much better than specs, but I'm curious and interested to know what to expect.Another possibility is of course that have differences in measurement setup, that we have actually measurement errors / uncertainties. Then also something was learned....

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: March 26, 2015, 06:21:00 pm
content: Yes, the range of TCR, while controlled by the wire's specified TCR (my specification) keeps the resultant TCRs within the specified 0±3PPM/°C, there are other process factors involved which cannot be controlled tightly enough (at least currently) to guarantee an even smaller TCR range than the ±3 PPM/C, the fact that I can control those parameters well enough to ensure a majority of the TCR is within ±1PPM/°C without tweaking or cherry-picking gives credit to the resistor's design.You are correct in that measuring very low TCRs is difficult particularly when a high quality resistance bridge is not being used, there are a lot of tiny gremlins that can sneak into the measurements that create error and at times, they are not presented to the operator. Using a bridge minimizes the number of parameters that must be tightly controlled for accuracy, the two most important is measurement accuracy and temperature, these two parameters generally have excellent repeatability and once any other sources of error are identified and/or eliminated (such as thermal EMF), the TCR becomes relatively easy to measure with very small error. Using indirect methods to measure TCR unfortunately increases the likely hood of error sources as there are more of them to control than with a bridge. The biggest advantage of the resistance bridge is that it presents an direct reading of the resistance change very accurately, the temperature then only requires repeatability as absolute accuracy is not needed. Only repeatability of the temperature points and most thermometers are very good at that. Personally, I still use mercury thermometers, they are accurate and their repeatability is quite high to the point of almost eliminating any error.I am not surprised at your TCRs, in fact I would have been a bit surprised if they had been between 2 or 3 PPM/°C.For a bit more information, check my post over in the T.C. Measurements today.

author: janaf
author_rank: Frequent Contributor
timestamp: March 26, 2015, 07:22:35 pm
content: More measurement results attached.- Ultrohm P (Pettis) 1K, same one for the third and last? time.- S102K foil 1K. Measured 0.5ppm/C (specified 1ppm/C) and seems very linear at this range, but has some hysteresis? Actually, I made a first measurement, not shown, with higher hysteresis, then cycled this one 25-75C a couple of times and it seems to have helped. I have a batch of the S102K so I can measure several to see what the spread looks like. It's specified 0.05% but the first one measured at +0.1%. However, its a NOS batch, could be decades old....

author: Andreas
author_rank: Super Contributor
timestamp: March 26, 2015, 08:48:04 pm
content: Quote from: janaf on March 26, 2015, 07:22:35 pmHello Jan,could be some humidity effect which is baked out at higher temperatures.On voltage references they give also the hysteresis from the 2nd or 3rd temperature cycle.But this is not honest since under room temperature conditions the epoxy will soak humidity from the environment.With best regardsAndreas

author: lars
author_rank: Regular Contributor
timestamp: March 26, 2015, 09:09:29 pm
content: As Andreas I guess it is some humidity effect on the S102K. I have two S102JT 1K that I have followed about 15 years. They show a variation of about 20ppm for a seasonal change of humidity between 25 and 60%RH. I remember it was difficult to find the temperature coeficient due to "hysteresis". I also have two S102JT 100ohm that has maybe 5ppm seasonal variations and two S102JT 10K that has about 20ppm seasonal variation for the same variation of humidity.Lars

author: lars
author_rank: Regular Contributor
timestamp: March 26, 2015, 10:10:56 pm
content: Quote from: janaf on March 25, 2015, 10:13:26 pmHello Jan,I have a 1kohm VHP203 resistor that is -13ppm+-10ppm with +0.2+-0.2ppm/C that I have checked against a calibrated Fluke 5700A and Fluke 8508A. If we happen to live close to each other in Sweden we can perhaps meet. Please contact me off list.For info the 1kohm VHP203 drifted about -5ppm the first year and the next four years about -2ppm (0.5ppm/year) against a GR1440 resistor I have. As my resistor comparator is a HP3456A, my resolution is limited to 1ppm but as I check them several times a year the statistics helps a little to get a better estimate of the drift.One guess is that the first year drift was because I soldered wires to the resistors without any heat clamp. I also have four other VHP203 that drifted in the same way except a 100ohm that has about double the drift (with temperature coefficient between -0.1 and -0.9ppm/C). The year after I soldered the VHP203 I soldered two Alpha HK 10kohm hermetically sealed resistors that seemed very similar to the VHP. Now I used a heat clamp. They have drifted less than -2ppm against the GR1440 the last 4 years!If your resistor is 20ppm high it should be possible to parallell it with 50Mohm. As the sensitivity is only 1:50000 for that parallell resistor(s) it should be ok with standard metal film types.Lars

author: babysitter
author_rank: Frequent Contributor
timestamp: March 27, 2015, 05:23:33 am
content: Quote from: lars on March 26, 2015, 10:10:56 pmRight this week at work I did a similar thing, 120R || (2K+390R) using a Vishay S102 as 120R and a UPW50 as 2K, and a found-in-a-junkbox 390R to make a little no-brain lab standard in a box simulating a Pt100 close to body temperature.

author: babysitter
author_rank: Frequent Contributor
timestamp: March 27, 2015, 05:25:47 am
content: Quote from: babysitter on March 27, 2015, 05:23:33 amAlso, I abused eevblog knowledge (pointer to the BPR resistors) to build a small oven...

author: janaf
author_rank: Frequent Contributor
timestamp: March 27, 2015, 07:48:32 am
content: Quote from: lars on March 26, 2015, 10:10:56 pmHello Lars.I do not really trust the 20ppm high as it's based on my DMMs which have not been calibrated for years. So I should have it (mod the resistor) calibrated before any mod, to check my DMMs. As the hermetic seems to have very low hysteresis, I'd could keep it at room temperature, only fire it up if/when really needed. They are specified to drift typical? 2ppm/6 years.Jan

author: janaf
author_rank: Frequent Contributor
timestamp: March 27, 2015, 07:58:53 am
content: Quote from: Andreas on March 26, 2015, 08:48:04 pmAs I have a batch of these I can treat / measure different ways to see if there is a difference. Time allowing?