using thermocouple in a Multimeter without temperature option - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: August 17, 2014, 01:24:57 pm
content: Hey all,I have a Fluke 21 (Rev I) the original 21, which does not have a temperature option for a thermocouple. I also have a Mastech M92X one-hung-low which also does not have any temperature option.Can I still use a thermocouple to measure temperature? For example, plug it in and determine via the Voltage or mVoltage reading what the temperature is? If I can, what calculations do I need from the Thermocouple data sheet to determine temperature?Thanks,

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2014, 01:58:39 pm
content: The extra thermocouple junctions formed where the thermocouple plugs into the multimeter require cold-junction compensation for this to work which can be provided by something like a Fluke 80TK thermocouple module.

author: Lightages
author_rank: Unknown
timestamp: August 17, 2014, 04:08:18 pm
content: As David Hess said, yes but not very well. The Fluke 80TK will make a good job of it but it costs as much as a good multimeter itself. It is probably better to get a separate cheap temperature meter or another multimeter something like this:http://www.ebay.com/itm/Digitek-DT-2843R-AC-DC-True-RMS-Multimeter-with-Backlight-and-Temperature-/171080136251orhttp://www.ebay.com/itm/Brymen-BM257-Digital-Multimeter-6000-count-Brand-New-Fluke-alternative-/200922627340

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2014, 06:14:51 pm
content: My expensive handheld multimeter has cold junction compensation built in and with a thermocouple to banana jack adapter, it is about as accurate as the Fluke 80TK thermocouple modules that I have. A side effect of the cold junction compensation is that the multimeter itself shows both the thermocouple temperature and the ambient temperature simultaneously.I picked up the Fluke 80TK thermocouple modules that I have on Ebay for cheap. With two of them, I can use them with 3 of my TM500 series multimeters to display two temperature and the difference in temperature simultaneously.

author: plesa
author_rank: Frequent Contributor
timestamp: August 17, 2014, 06:21:35 pm
content: If the range -200 to +600 °C is comply with your requirements I recomends to use the pt100 rather than themocouple. There is no cold junction error.On ohm range ( 1mA current source ) you just needs to create conversion table based on your pt100.

author: Wytnucls
author_rank: Super Contributor
timestamp: August 17, 2014, 06:29:05 pm
content: Quote from: trs80 on August 17, 2014, 01:24:57 pmTheoretically, it can be done, but it won't be very accurate, as the voltages are very small.A K-type thermocouple has an approximate conversion of 41uV per degree Celsius.The voltage reading will be relative to the unknown cold junction temperature. In a lab, that would be close to the ambient temperature.A more accurate solution, but less practical, is to keep the cold junction at 0 degrees in an ice bath.

author: mzzj
author_rank: Super Contributor
timestamp: August 17, 2014, 06:47:06 pm
content: As others already pointed out buying separate meter for temperature is probably the best option. But you can also use existing hardware assuming they have sufficient uV-range resolution for thermocouples.1. Immerse the cold junction to ice water bath. Laborous but best in accuracy. Standard method in any temperature calibration laboratory.2. Guestimate the cold junction temperature and compensate your math accordingly. Viable method if 5 K error is acceptable.3. Use separate meter to measure cold juntion/ambient temp and calculate based on that.4. Ignore the cold juntion temperature if 25K or something like that error is acceptable(ie when measuring over 1500K temperatures with K-type the 25K error is pretty much nothing)NIST website has thermovoltage vs. Temperature tables and equations for calculating the voltage to temperature.

author: made2hack
author_rank: Frequent Contributor
timestamp: August 17, 2014, 07:56:46 pm
content: the price of the 80TK (+ shipping + customs to EU) is high, I mean, close to buying a second hand Fluke that already has the temperature option. Not sure if it is the right option.I'm thinking of a 5$ ebay one hung low K-Type thermometer such as this oneTM 902C Digital LCD K Type Thermometer Single Input + Thermocouple ProbeI think a forum member did a review and it looks acceptable for my needs ( I want to use it in a toaster oven for reflow).http://youtu.be/c-QOXHjHhjs

author: IanB
author_rank: Super Contributor
timestamp: August 17, 2014, 09:15:34 pm
content: Quote from: trs80 on August 17, 2014, 07:56:46 pmYou could also try searching for the Extech 381277. It's less expensive than the Fluke option.

author: paul18fr
author_rank: Contributor
timestamp: August 18, 2014, 01:18:31 pm
content: Quote from: trs80 on August 17, 2014, 01:24:57 pmThe tables and the polynomials can be found in ASTM E230 standardwithout cold junction, you measure the e.m.f resulting of the temperature you would like to measureplusthe room one due to the junction of the thermocouple with the multimeter -> depends on the accuracy you're looking for ;(quite easy to make the cold junction if you've 2 copper wires and Teflon sheaths)Paul

author: mzzj
author_rank: Super Contributor
timestamp: August 18, 2014, 03:23:00 pm
content: Quote from: paul18fr on August 18, 2014, 01:18:31 pmNIST website have the tables and coefficients for most of the thermocouple types for free, no need to go shopping for ASTM standards.http://srdata.nist.gov/its90/menu/menu.html

author: charlespax
author_rank: Regular Contributor
timestamp: August 25, 2014, 05:07:05 pm
content: Quote from: trs80 on August 17, 2014, 01:24:57 pmYes you can. However, you will probably not have very good precision and your accuracy will be terrible.1. Measure the room temperature. If your multimeter has been in the room for a while, you can assume this as your junction temperature.2. Use your multimeter to measure thermocouple temperature.3. Find your voltage in theK-type lookup table4. Add the room temperature to temperature in the lookup table. This is basically the cold junction compensation.Now you can stick your thermocouple in ice water or put it in a flame and see how the temperature changes. This will not be good for quantified measurements, but it should be fun to play with.