Are natural gas and propane thermocouples the same or are they different for each gas?
They both should be the same.
I have only worked on natural gas and not propane.
The thermocouples work off of sensing the heat present and not the type of gas.
If heat is present than a miili amp of current is sent to the gas valve to allow gas thru the valve. If there is not heat present at the thermocouple then than gas will not flow thru the gas valve. Safety feature.
The mechcanical thermal couples should be the same. The valves are slightly different, gas pressure of liquid propane and natural gas are different. Electrical thermal couples are dissimilar metals welded together, and produce microvolts/degree of temperature, based on how designed.