Thermocouple good at room temp is good always?

There isn't much that can go wrong with a thermocouple! They should be near identical and accurate but check the manufacturers spec for linearity. Also make sure you are measuring them correctly, the connections between board and wires can make far more difference to the reading than the junction. Use cold junction compensation if possible.

Brian.

Hi,

thermocouple measurement is a differential measurement of cold junction temperature and thermocouple temperature.
If the cold junction is at room temperature and the thermocouple is at room temperature .. then it measures ... nothing!
So if you want to measure absolute temperature at the thermocouple you need to measure absolute temperature at cold junction.

Klaus

Thanks, i guess a good quick check is to boil the kettle, dip the thermocouple wire end in and see if it measures about 100degC.

The boiling water test might work but you would be measuring 100C vs ambient temperature rather than 0C. Obviously, you can mathematically adjust the figures but to measure accurately you have to at least subtract the measured temperature at the 'cold' end of the thermocouple wires. If you are building this measurement system from scratch or incorporating it into a product, I suggest looking at the data sheets for the AD594 / AD595 which detail how it all works.

After reading your first post I demonstrated how thing could go wrong to a student here. I simply wrapped the two (type J) thermocouple wires around the probes of a DVM set to a low voltage range and it showed a 0V measurement. I then used a hot air gun to heat the thermocouple junction to about 120C and the meter reading went up as expected. I then let it cool down and the reading dropped back to zero. So far, so good. But then I heated nothing but one of the DVM probes and again the reading went up, I let it cool and then heated the other probe, the reading went negative. So you can see the voltage is generated whenever the two wires join to some dissimilar metal.

Those AD594/5 ICs have built in thermometers that compensate for ambient temperature, effectively making the connection to the thermocouple wires appear to be at 0C. You still have to be very careful to connect the wires as close to the IC as possible to minimize the temperature difference between solder joints and the IC die and to use the same solder on both wires. Note that not all solder will readily bond to thermocouple wires.

Brian.

Thanks, we are using a TC-08 (Pico) thermocouple reader into the computer.


Its a few hundred quid and we hope it solves a lot of the thermocouple issues for us.

Thanks, your info on thermocouples is really good....i will store this away, and i am looking at these AD59X chips for future stuff.
As you kindly tell, there is a great deal can go wrong in thermocouple readings.

cupoftea said:

Thanks, we are using a TC-08 (Pico) thermocouple reader into the computer.

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So you also have sufficient cold junction compensation, presumed you are using correct TC connectors.

There is exist a proper way to calibrate thermocouples. Room temp does not measure gain or offset errors exactly but it gives a warm and fuzzy feeling if it reads 21 'C and warmer fuzzy feeling if it reads 25'C

Example Temperature Ranges for Calibration:​

• Type K Thermocouple: Calibration points might include temperatures ranging from around -50°C to 1000°C.
• Type J Thermocouple: Calibration might cover temperatures from around -40°C to 750°C.

Always consult with manufacturers calibration procedure.

When done, typically, the accuracy of a Type K thermocouple is within ±1.5°C or ±0.25% of the measured temperature, whichever is greater, over the temperature range of -40°C to 1000°C.

cupoftea said:

Hi,
If you are about to do thermal tests with thermocouples...then to see if they are ok...can you say they are ok, if when in the thermocouple reader, with the end junction bits loose on the lab table, then if they read room temp properly , then those thermocouples are OK? (at all other temps too). We only need accuracy to within about 4 to 5 degrees.

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Any standards-compliant thermocouple wires will give good enough accuracy for your '4 to 5 degrees' specification,
but your 'thermocouple reader' and extension wires, and thermocouple in combination, can NOT be tested adequately
with only one temperature measured. The best practices for thermometry include regular checks for calibration
of readers, and resistance monitoring to ensure no breakages occur, as well as certification of the
wire and insulation materials.

In particular, a reader at ambient temperature will report ambient temperature with a short across
the thermocouple terminals, which short could be ANY wire whatever, or with polarity of the
thermocouple backward, or with a good thermocouple. That test is too easy to pass.

Thanks, with the TC-08 and out proper thermocouple connectors, that go into the TC-08, it seems that if it reads room temp correctly, and then the temp starts increasing when the fet heats up, then it is highly likely that the reading will be correct to within 4 to 5 degrees C, you would agree?
Taking it that the TC-08 is in good calibration.