Electronics > Beginners
Thermocouople thermometers.
6PTsocket:
I have a K type thermocouple temperature function in my DMM. It is even ice bath calibrateable. Simple explanations I have found compare the temp of the reference thermocouple to the measurement thermocouple. This explains why taking the dmm outside on a cold day produces drifting readings as the reference thermocouple in the dmm gets cold, too. I also have an instant read kithchen thermometer that does not seem to have this problem. They make expensive thermocouple thermometers for BBQ competitions where they are outside in all kinds of weather and are still accurate. I would doubt the reference is in some kind of temperature controlled oven. Anybody know how they do it? Thanks.
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IanB:
Other thermometers may use a variable resistance element (thermistor) rather than a thermocouple. The reference junction temperature measurement in a thermocouple meter will also use a thermistor.
beanflying:
I have a couple of K type Thermocouples fitted into a Coffee Roaster that go through a pair of Multimeters for Logging data. I had to move the meters themselves away from the roaster as the ambient was killing the accuracy. So the key is to keep the electronics close to ambient temps to improve accuracy.
One more modern method https://www.maximintegrated.com/en/app-notes/index.mvp/id/4026
A more traditional approach https://www.omega.co.uk/techref/thermoref.html
The Omega Site is well worth having a good look around too :)
David Hess:
A thermocouple thermometer has *two* thermocouples in series. One is at the probe's measurement point and the other is at the "cold junction block" inside the multimeter where the thermcouple wires connect to the internal circuitry. The temperature of the cold junction block is measured with another temperature sensor to compensate for its effect on the thermocouple's measurement.
If the thermocouple probe attaches to the multimeter banana jacks using an adapter, then it is part of the cold junction block also and it takes longer for the cold junction block temperature to settle.
6PTsocket:
--- Quote from: David Hess on December 26, 2018, 10:59:07 am ---A thermocouple thermometer has *two* thermocouples in series. One is at the probe's measurement point and the other is at the "cold junction block" inside the multimeter where the thermcouple wires connect to the internal circuitry. The temperature of the cold junction block is measured with another temperature sensor to compensate for its effect on the thermocouple's measurement.
If the thermocouple probe attaches to the multimeter banana jacks using an adapter, then it is part of the cold junction block also and it takes longer for the cold junction block temperature to settle.
--- End quote ---
Thanks. Look at the link from beanflying (above) it gets heavily into cold junction compensation methods from hardware, software and lookup tables. I understand the basic circuit. It was the compensation that I was ignorant about.
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