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(probably operator error/ignorance...)

Trying to read room temperature to verify accuracy of the meter readout w/ K type thermocouple -

I have four devices: Tektronix DMM916 multimeter w/ K type thermocouple, Klein Tools CL390 multimeter w/ K type thermocouple, Cooking meat probe, Wall thermostat

Readings are 58F, 59F, 70, 70 respectively

The room temperature is right around 70F

Note - Putting the tip of the thermocouple between two fingers, (or even under my tongue) does not increase the temperature very much. Putting the cooking meat probe under my tongue raises its temp to 97F.

Question - Why are the two multimeters reading similar to each other but different from the other two devices?

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  • \$\begingroup\$ How is the reference temperature measured? \$\endgroup\$
    – D Duck
    Oct 28, 2023 at 16:22
  • \$\begingroup\$ Possibly due to multimeter probing...please describe the conditions under which those K thermocouples were connected to multimeter - for example: did you use an iso-thermal block? This is somewhat the same question that @DDuck asks. \$\endgroup\$
    – glen_geek
    Oct 28, 2023 at 16:41
  • \$\begingroup\$ Thanks DDuck and glen-geek, This is where user ignorance comes into play. I did not use an iso-thermal block. The probes are connected directly to the meter. \$\endgroup\$ Oct 28, 2023 at 17:20

2 Answers 2

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The multimeters do not have a pure thermocouple-metal path from thermocouple to meter. This means the junctions between meter and thermocouple pollute the reading with their own temperatures. Even the cheapest dedicated thermometer with a proper k-type plug will best an expensive DMM with generic 4 mm connectors.

Even with the Tek DMM, a temperature measurement function on a DMM, conducted through other than a proper K-type socket, is best regarded as an indication, rather than a measurement. I notice the manual calls for a 'temperature probe adapter', did you use one? I'm not quite sure what metal-changing shenanagins they could play in here, but they might attempt an isothermal change from k-type to copper, at a temperature of the input sockets. Ideally, you would set up a known cold junction temperature when you move from thermocouple metal to copper, so if their ambient sensor was right at the input socket pins, that's about the best you could do with a DMM input.

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  • \$\begingroup\$ Thanks Neil_UK, really interesting. By chance I only looked at the Klein Tools manual which did not mention the need for a temperature probe adapter or other. Kind of comical that my $12.95 meat cooking probe outdoes my $300 Tek DMM in terms of temperature accuracy. :( \$\endgroup\$ Oct 28, 2023 at 17:31
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    \$\begingroup\$ But the Tek DMM does so many other measurements with better accuracy... \$\endgroup\$
    – Solar Mike
    Oct 28, 2023 at 17:53
  • \$\begingroup\$ Update - I looked in the manual for the DMM 916 to see the adapter and probe it requires. Turns out that's actually what I'm using. (image here on ebay ebay.com/itm/314678820095) So I guess I'm back to my original question, Aside from a defective probe, adapter or meter circuits, why is this meter reading so much different that the thermometer and thermostat? Remembering that the Klein tools meter is reading essentially the same as the Tek DMM. \$\endgroup\$ Oct 28, 2023 at 17:59
  • \$\begingroup\$ @tomRmalcolm 11F, that's about 6C. That's not many uV. What's the accuracy specification for the meter + probe adapter? \$\endgroup\$
    – Neil_UK
    Oct 28, 2023 at 18:17
  • \$\begingroup\$ was not able to locate accuracy specs for the probe and adapter. The meter temperature accuracy said "2 degrees C". Not sure what that means, as it did not say +/- 2 degrees C. \$\endgroup\$ Oct 28, 2023 at 19:29
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Thermocouple temperature measurement produces a voltage that is proportional to the temperature of electrically joined "dissimilar metals.

When the copper meter probes of a DMM are attached, two more temperature dependent junctions are formed that also produce a voltage that adds to the measuring-junction voltage.

These voltages must be subtracted from the measured voltage in order to get a true reading. Placing the extra junctions on an isothermal block (also called a cold junction) of a known temperature allows the determination of the voltage to be subtracted.

There is a lot of information found by a Google search for "cold junction compensation".

All About Circuits has some good information. Sample images shown below.

enter image description here enter image description here

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