Measuring Temperature With A Thermocouple Probe

[RJSakowski]

Last Saturday, I was doing some late season masonry work and, on Saturday night, the temperature dropped lower than expected. Sunday morning I picked up my multimeter which has a thermocouple probe for measuring temperature. As I was measuring the temperature, I noticed that the temperature was slowly rising. At first, I thought that I had damaged the probe but then the light went on (I'm kind of slow before I've had my first cup of coffee).



For the uninitiated, a thermocouple consist of two dissimilar metals joined together. A small voltage is generated at the junction of the two metals and this voltage is temperature dependent. In this case, it was a type K thermocouple consisting of the two alloys chromel and alumel. Following the leads back to the multimeter, there are a series of junctions, eventually ending in copper to copper with each junction generating its own voltage.

Thermocouples can be a extremely precise means of measuring temperature but, in order to do so, the reference junctions have to be contained in a tightly controlled thermal environment. In the case of the multimeter probe, the probe plugs into an adapter which plugs into the multimeter. The pins of the probe are made of the same metal, tin plated brass so the reference junction is within the probe plug.

If you are making a temperature reading where the ambient temperature is lower than the calibration temperature (presumably 68F or 20C), the meter will read high. If you are making a temperature reading where the ambient temperature is higher than the calibration temperature the meter will read low. For example, if the probe plug is allowed to cool to the outside temperature of 50F, the temperature read is 63F.

The takeaway from this is that, if using a thermocouple thermometer, the ambient conditions in which a measurement is made should be taken into consideration in order to make accurate temperature measurements.

Bob

 

[rpmMan]

presumably your adaptor includes some sort of cold junction compensation... thermocouples produce voltage (mv) based on the temperature difference of the "hot" junction (probe end) and the measured junction (cold end) which is actually 2 additional connections/junctions i.e. your +/- connections.
All of them use 0 deg c / 32 deg f as their reference.. you can use any meter capable of measuring mv as long as you can accurately measure ambient temp of your +/- connections and or simply place them in an ice/water bath. If you use the ice bath then you simply look up the mv/temp in a table for the type of tc you are using.. otherwise if you can measure the ambient temp you simply look up that value and add it to your meter reading. You can then look up the temp using that value.
As you note the adaptor (if it has built in cold junction compensation) needs to be kept at a constant temperature or you will see a transient bias in the reading.