Whenever we sell a pyrometer to an aluminum mill we start the conversation by telling the customer that the two of us are having an argument, but that we just don’t realize it yet. We then go on to tell them that the way to resolve this argument is to seek the truth. There are certain things that can cause a thermocouple to read the wrong temperature value and there are things that may cause a pyrometer to read the wrong value as well. By seeking the truth, then we can learn together how to best produce a repeatable and reliable temperature reading. The customer always agrees to this.

Then, the plant gathers all of their thermocouple probes and we test them all using a NIST certified hotplate. We document the probe readings. Next, we go out to the side of the coil and we let them take a temperature reading using their probe. We then take the probe from them, and push it into the coil, then we remove the probe, move it over about 2 inches, and push it in again a little harder. We compare our readings to theirs. Usually, our reading is significantly higher. This is usually enough to convince them that their thermocouple probes are neither accurate nor repeatable.

Thermocouple readings can be influenced by a number of different factors which may or may not represent the desired target surface temperature. Common influences on thermocouple temperatures include ambient air temperature, thermal inertia (starting temp of the probe), thermal mass of the probe, surface oxidation, loss of heat through conduction, and also the applied pressure of the thermocouple on the surface. 

After some use, thermocouples can also get dull or oxidize and will not be as accurate as they once were, leading to lower reported temperature values. Moreover, some thermocouples will read 40-50°F below the actual temperature right out of the box. Some have even read as much as 100°F too low. The best way to check the accuracy of the thermocouple or probe is to check it against a NIST certified hotplate. Once you determine the accuracy of the thermocouple probe against the hot plate, you can then make a fair comparison between the thermocouple probe and the pyrometer.

Care must also be take using a non-contact pyrometer. Selecting the appropriate wavelength and wavelength technology allows us to avoid certain optical obstructions such as steam, flames, smoke, dust and more to make an accurate and repeatable measurement.

Ready to get rid of your thermocouples? Download our free wavelength selection guide to figure out which wavelength technology is right for your process.

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