Questions regarding temperature gradient in thermocouples.

[mforoutanfar]

Hi folks!

I was wondering if anyone can help me with a problem I'm having with the concept of thermocouples.

If I understand correctly, there should necessarily exist a temperature gradient in one of the conductive couples in order for the emf to be generated. So how can you make sure that such a gradient exists in your application? I mean, imagine a TC used in a boiler with constant T. Won't the heat eventually propagate through the whole TC and eliminate the temperature gradient?

Some help will be much appreciated.

Thanks!

 

[danw2]

The hot end of the thermocouple is at boiler temperature. Once the thermocouple exits the confines of the boiler the remaining thermocouple wires are lower in temperature, the ambient temperature surrounding the boiler. At some point, the wire terminates.

The gradient from the hot end across the wires to whatever the temperature is at the 'cold' termination end produces the EMF.

The generated EMF corresponds to the values in the thermocouple tables only when the 'oold' end is at the freezing point of water. Otherwise some form of cold junction compensation is necessary for the table values to be of use.

 

[hacksaw]

as described by the previous poster, it is a temperature difference, not the gradient that is significant, but having said that, if the thermocouple wiring is inhomogeneous (poor quality), the gradient can lead to slight measurement errors.

 

[danw2]

>Won't the heat eventually propagate through the whole TC and eliminate the temperature gradient?

No. Is the end of your car's tailpipe at the same temperature as the engine's exhaust manifold? Why not? They're both conducting very hot products of combustion.