Thermocouples - how do they work - Page 1
Forum section: electronics / general

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 04, 2020, 05:05:55 pm
content: So this started out with me boiling water to see if my 121GW meter read it out at 100C, original thread here:https://www.eevblog.com/forum/testgear/eevblog-121gw-multimeter-issues/msg2854342/#msg2854342I wanted to move it out of the meter issues area because I dont think its a meter issue as far as I know. Currently I think its my lack of knowledge in the area.qft1967 responded:Quoteand then ThickPhilM responded with a link to a wiki:https://en.wikipedia.org/wiki/Thermocouple#Type_KI am now more confused as I was before after reading that, or at the least have more questions than this original post started with.Taken from the wiki:QuoteDoes this 121GW meter use its internal temp as Tref, or does the Type K thermocouple have a Tref somewhere as a complete package? I would think if its a complete package it would be possibly embedded in the bulky package for the terminals that plug into the meter?So I was originally going off this sites info for the reading of the thermocouple and I am assuming this site is correct(but I may have the incorrect site) :https://www.thermocoupleinfo.com/type-k-thermocouple.htmWhen I looked up 19C for my ambient temp, it shows it should have voltage of .758mV. I measured a voltage of aprox .1mV which puts the temp somewhere between 2-3C. This would make sense a bit I suppose if the meters internal temp was the reference, but then even if this was the case, should I not be reading 0mV because Tref and Tsense are equal?The reading also seems to be about 6DegF off from my home temps, which also seems to match what I seen when I tried to boil water and the meter showed it boiled at 97C and not 100C, again about 3C error. Could by Tref be off or out of calibration? Can it be adjusted?

author: retrolefty
author_rank: Super Contributor
timestamp: January 04, 2020, 05:20:14 pm
content: I worked with TCs for several decades in industrial process control systems. I found the most useful information on their theory and application from Omega Eng. company. Their web site is much different now then when I retired but I did find this starting point:https://www.omega.com/en-us/resources/thermocouple-junction-principles

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2020, 07:17:58 pm
content: The normal thermocouple probes don't contain an extra temperature sensor or could junction compensation. So the meter has to use it's internal temperature for cold junction compensation. This may not be very accurate as the actual cold junction is outside the meter, where je TC wires goes to copper, but the best they can do. To keep the error small one should keep temperature gradients near the meter small.Because of electrical safety in normal voltage mode, one can not use the more suitable special TC connectors, that have the cold junction inside. So a DMM has a hard time to be accurate with TC. It is no common to have a special TC mode for use with an external cold junction compensation circuit or ice bath. In this case one usually has to measure µV and than calculation the temperature from a table.The boiling temperature depends on air pressure and thus altitude. 97 C may be about correct at some 3000 ft (just a crude estimate).

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 04, 2020, 08:08:56 pm
content: Yeah that’s what I found, 97C would be around 3k ft elevation, which I am not. Shouldn’t this temp probe be more accurate than 3deg at 100C?

author: SeanB
author_rank: Super Contributor
timestamp: January 04, 2020, 08:17:50 pm
content: K type thermocouples ( actually pretty much any thermocouple) are not really meant to be used around room temperature, so 3K will be quite within the accuracy spec. If you are measuring around that point there are much better methods and probes you can use, thermocouples work best with high temperature differences, so for really hot things, or for cryogenic applications.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 04, 2020, 10:00:16 pm
content: So given that the only meter I have that measures temperatures is the 121GW.I was hoping it would be more accurate than 3degC. I don’t necessarily need a really accurate measurement, but I would like to know what I am measuring with is reliable.Given I am using the 121GW, there doesn’t seem to be a way to calibrate it. Working in a range of 0-300C it does seem a very small section of the K type range. It was my hopes that by boiling water I could adjust the reading to 100C, I suppose I could just take what ever reading the meter gives and add three to it, but this seems pretty hackish.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2020, 10:07:16 pm
content: Thermocouples are not very accurate, but should be better than 3 K at 100 C. So at least some of the Error is likely due to the meter, especially the cold junction compensation. Due to the limitation with the plugs a few K off error is not surprising. Part of this could also come from the calibration / adjustment of the meter - the temperature part may be slow and thus not that accurate in a fast factory calibration. Is there a internal temperature function, and if so is this reasonable accurate? With just a copper short (instead of the TC) the TC function should read the cold junction temperature. Just adding 3 K is a first approximation and can work reasonable as type K is relatively linear.I would expect most of the error to be due to the cold junction compensation in the meter. For higher accuracy it needs a special TC connector and thus a separate thermometer, not at CAT III DMM.Even 6 digit bench DMMs have similar limitations for thermocouple - accuracy just is limited with normal 4 mm plugs.A test point at 0 C would be less sensitive to the actual thermocouple and thus a better test for the meter itself.Generally thermocouples are tend to be not very accurate. So they are mainly used when there is not good alternative: e.g. at high temperatures or of if it needs to fast or very small. Low temperatures are usually more difficult, as the Seebecke effect tends to get small at low temperature. Thermocouples also don'T get better for large differences it is only that the cold junction error may than be less important.For 100 C a PT1000 probe would be the more accurate choice.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 04, 2020, 10:21:40 pm
content: So I did do a test in some ice water and the meter showed 0.0C. Seems good there.Could that be a possible firmware upgrade? The ability to interchange temp probes to the meter?Maybe pick the one from a menu, and give an option to offset the value for a quick calibration.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2020, 08:52:53 am
content: If the meter shows 0 C for the ice water test, this would indicate that the cold junction reading could be OK. I could still be the problem with a temperature difference between the actual junction temperature (at the plugs) and the sensor, that could be different between the tests. This could be visible when measuring a constant temperature and moving the meter / probe cable and for the test warm the plugs with the hands.If the cold junction is OK the difference could be a problem with the scale factor, either due to a poor thermocouple quality or calibration / software problem. One could check the meter part by applying a small DC voltage (some 10 mV) and measure this in thermocouple and mV range. One could also measure the thermocouple (at 100 C) in voltage mode and do the calculation to temperature by hand to check.When measuring in water with non isolated wires, one may have to keep the water clean, so it is not too conductive. There can be a small electrochemical voltage (could 100 mV range) between the 2 TC materials and thus a small current flowing through the water, that can cause a small error from the wire resistance. Usually the error should not be severe as the conductivity of water is low.With a more rigid shielded thermocouple this is not a problem, but than thermal conduction through the probe can be a problem so that the actual temperature of the probe can be different from the water if not immersed far enough.

author: beanflying
author_rank: Super Contributor
timestamp: January 05, 2020, 10:29:51 am
content: Much better place out here for a chatAs I put in the 121GW thread type K's are around +-2C at 100'ish plus some more for the meter. Without some other T/C's and a second better meter you are only guessing at what is at issue. Also as suggested the Omega site is a good read.I did some testing with my Sika Calibrator against the 121GW and it stacked up well within expectationshttps://www.eevblog.com/forum/testgear/sika-18600e-dryblock-temperature-calibrator-teardown-pictures-testing/msg2249781/#msg2249781Zero point testing herehttps://www.eevblog.com/forum/testgear/kaye-140-4-ice-point-reference/msg2414400/#msg2414400Both of those were just with 'decent' Ali Express T/C's and based on follow ups with a four wire RTD on my Keysight 34461A well under 0.5C for the zero point accuracy.Look at two wire RTD's generally PT1000 to keep the resistance higher if you want to stick with the 121GW as your device then use tables or formulas to come up with a temperature.If you are more serious then look at four wire RTD's and even an Arduino/Max31865 for a cheap solution or strap it to a four wire bench meter if you can run to that. The Arduino option is worth doing as a learning/playing experiment regardless.