Sheated thermocouple not responding?

Question

I have a lab thermocouple that is connected to control. It has steel sheath ( don't know if its grounded or not), the tip is 1/2 inch long and the wiring is covered by steel woven with K type connector.

When I put the tip into hot liquid, it does not show any temperature change, but when I touch the top part near the end of wiring, it reacted and showing my body temperature.

I checked using ohm meter and it shows connection from the sheath to the end of the line. and other directly exposed thermocouple showing correct temperature reading.

any idea if there is special requirements to operate sheated thermocouples?

thanks

http://www.capgo.com/Resources/Temperature/Thermocouple/ThermoSheath.GIF

Answer 1

For a possible explanation, see the ref. For a thermocouple to function, the far end (away from the junction) of the dissimilar-metal wiring from the junction must be at a reference temperature to produce an accurate reading, and in any case must be at a different temperature from the junction to produce any reading at all. If the wires from the sheathed portion to the meter are not dissimilar (e.g., are copper) then the "reference" end of the dissimilar-metals wiring is inside the sheathed portion, and will be at the same temperature as the junction when immersed; thus you see no reading. You might check to see if there is a reading if you only partially immerse the tip.

Answer 2

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