Why do temperature-voltage curves of all thermocouple types pass from origin?

Question

Notice that the temperature in the graph is in ^oC, not even in ^oK.

0^oC is the border between temperatures of water and ice under 1 atm pressure. Why do thermocouples give 0V voltage at 0^oC temperature? Is there any relation ship between water and thermocouples, are the thermocouples designed to behave like this, or is this just a coincidence?

Answer 1

The thermocouple voltage is from the difference in temperature between the sense junction and a reference junction; the reference junction always exists and may be the connection of the thermocouple wires to your voltmeter; note that brass, copper, plating materials all form additional intermediate junctions that cancel if they are at a uniform temperature. Thermocouple tables are based on the reference junction being at 0C (ice-point reference, a convention) so that is why the sense junction temperatures in these tables are all 0V at 0C. Omega Engineering website has great reference documentation on thermocouples that explains all about reference and intermediate junctions. http://www.omega.com/techref/

There are many temperature compensation methods. One is to put the reference junction in ice slush, a stable and known reference point. Another is to produce a voltage to compensate for the actual junction temperature so that the measured voltage will be the "table temperature" with no offsets. Another method is to use an above-ambient and stabilized junction temperature in which case a fixed offset can be used to compensate. While this may all sound complicated, thermocouples are frequently used for extreme temperatures, and the accurate measuring or stabilization of a reference junction can be done with lower temperature electronic methods.

Answer 2

The temperature on the X axis is the temperature difference between the two thermocouple junctions. That's not the absolute temperature. If both junctions are at 0^oC, the thermocouple voltage would be 0V. If both junctions are at 100^oC, the thermocouple voltage would be 0V still.

Answer 3

Voltage in thermocouple is function of difference between temperature of "hot" and "cold" part of thermocouple. If there is no temperature difference then there is no voltage. See Wikipedia about thermocouple for details.

That is why that in order to have absolute readings one need to employ "cold-junction compensation".