How does a thermocouple sensor work??

As I understand it, most pyro guages are thermocouples.
If it's a true thermocouple, it is made of two dissimilar metals, and generates a small voltage when there is a temperature difference between the two metals. The guage end measures this voltage and displays it as a temperature.
So if measuring resistance of the sender, the resistance of the thermocouple will not change much with temperature. If you have a meter which measures millivolts, you should be able to measure a voltage across the thermocouple when it is heated.
Conversely, the guage should respond to a small voltage across it. But I wouldn't recommend using a battery to check the guage, as it might overload the meter (the output of the thermocouple is usually only millivolts)
If the sender is a thermistor, the resistance varies with temperature, and the guage measures this resistance. You should be able to see the resistance change with temperature when measured with an ohm meter.
Gerry

From memory 25 C at the thermocouple tip equates to 1.0 mV dc for a Type K thermocouple therefore if EGT is 250 C you should be measuring approx 10 mV across the 2 wires of the Thermocouple.

Cheers

Darryl

Mine is a WWG pyrometer and puts out 4.04 millivolts at 100 degrees Celsius and virtually doubles each time up to 37.3 millivolts at 900 degrees. I reckon it should read close enough to zero ohms at low temps as it is shorted at the sensor end by a sort of spot weld.

I guess you could heat it up and measure the small voltage across the terminals to check the sensor. (thats why the gauge is so expensive, it reads such very low voltages.)

Hope this helps. At least the sensor should be the the cheapest part.

The two wires coming out of the thermocouple should not have continuity with the outside shroud...disconnect the gauge from it and measure if there are any ohms resistance between either wire and ground.
Also, joining the two wires from the gauge together should result in ambient temperature being displayed, as these two wires are also discimilar....
If the gauge doesnt read ambient, you have a problem upstream.
However 999 out of 1000 times its the thermocouple that dies...
Andrew

Hi Signman,

If it is a thermocouple it should read a very LOW resistance because it is effectively a short circuit caused by the connection two wires of different but conductive metals.
A common fault with industrial thermocouple is open circuit caused by mechanical fatigue or corrosion.

As others have said a thermocouple produces a "known" temp vs mV relationship due the difference between the hot junction (sensor end) and the cold junction (meter end).

For a Type K Chromel vs Alumel thermocouple if the meter terminals are at 25 Deg C the reading will be 4.095 mV if the sensor is at 100 Deg C. The mV reading will vary as the cold junction temperature changes even if the sensor remains at 100 Deg C. So lets say for a practical experiment if you were to put the sensor in a container of BOILING water you should measure about 4 mV or 0.004 V DC.

Regards,

Rosscoe

Hi Signman,

Haven't been on here for a while so I've only just noticed your post. Looks like you been given enough info on the theory of thermocouples. As far as I know all EGT gauges use type K TCs. While I can't guarantee this info will apply to all brands of EGT TC, this what have measured on the ones I supply:

Mine are MIMS construction (Mineral Insulated, Metal Sheathed) with the actual thermoelectric junction insulated from the metal sheath (which is 310 stainless-steel, good for use up to 1100 C). So, each wire should be insulated from the sheath (open circuit). My TCs have 2.5m fibreglass insulated, stainless-steel braided leads and typically measure between 11 and 15 Ohms between the leads at ambient temperature.

I've never been sure whether the thermoelectric voltage developed by the TC affects the resistance reading but I've measured them with several different digital multimeters and always get the same range of resistance.

So before despatch I check each TC to ensure open-circuit between each wire and the sheath/braid and 11 to 15 Ohms between the wires. Then I connect them up to the digital indicator and check for 0 C reading in an ice slurry.

Hope this is of some help.
Ian