Calibrating thermocouples


Thread Starter


Can anybody recommend a good and fairly inexpensive calibration device for type J thermocouple on a hydraulic molding machine.



Satish Salankimatt

First of all tell me what is the operating range.
If your operating range is less example 300 deg C you can use an Oil bath and a 4 1/2 digit multimeter to calibrate.
If your operating range is high example 600 deg C then you can use dry block heater and again use 4 1/2 digit multimeter to calibrate.
In above cases you can get the temperature of the indicated mV from temperature charts.

You can procure oilbath and dry block which has 0.1 deg C resolution.

Note that in both the above cases the oil bath and the dry bath have to be calibrated by external party with higher resolution and accuracy master. Preferrably 10 times better.

Michael R. Batchelor

An ice bath and boiling water work great, have very high accuracy (physical constants, you know), and they're extremely cheap. Of course, you can't really "calibrate" the *thermocouple*, since you can't actually manipulate the metal content of the two wires at the junction. But you can check and adjust the accuracy of the display
(or whatever the device goes to). What kind of accuracy do you need, and how much money do you want to spend. A small hand held calibrator is a lot easier to throw in a tool bag than two pots of water, etc.
You check their mV versus °C.
Omega do commercialise several types of
temperature bath. If your T/C input directly the black box, and if this last one is a numerical one, then the linearisation is resident in there. The linearising function is considered sufficient for general applications if they are tracable to NIST (formerly NBS).
please note that thermocouples cannot be calibrated. You can only calibrate or compensate the nonlinearity of the thermocouple in your signal conditioning unit.

You have to first check the linearity of the thermocouple against a standard chart. You need to use an oil bath or such devices to heat the t/c in stages and then using an accurate standard temperature gauge, make a chart of the ambient and gauge temperature and measure the emf.
Compare this with a chart of the thermocouple type.

Check weather your signal conditioning unit (transmitter, T/c card, Controller etc.) where the t/c connects has facilities for compensating
for any non-linearities. If so perform such adjustments, else dump the t/c and get an accurate one.


Bob Peterson

In the real world, buying this type of expensive calibration equipment is really unnecessary. The accuracy/precision required is not anywhere near
what such an elaborate setup gives you. If your reading is off by a 1/2 deg C its not likely to matter much in your case.

I suggest a 2 point calbration. One in an ice bath (0 degrees C). The other in boiling water (100 deg C). Adjust gain and span of transmitter so they read correctly at these two points and you will be real close. Get an inexpensive dial thermometer (calibrated if you like) and put in the ice or boiling water bath to check the actual temperature (altitude affects the boiling point slightly, so you may be a bit off if you assume water boils at 100 deg C in your local).

If you want to do a three point calibration, the third point can be an oil bath on an electric burner, using the dial thermometer as the reference.

Always go cheap and simple rather then expensive and complex unless you really need the expensive and complex solution.

Bob Peterson

Jim Sullivan

I'd purchase two items. A hand-held TC meter (Omega, Fluke) and a J-type thermocouple. Get both certified at a cal lab. Temporarily mount the certified TC near the oven sensor. Compare the outputs. Adjust controller as needed. I used this technique to certify aerospace heat-treat ovens to ASM-2750 (a heat-treatment tolerance spec.)