100205-1610 EST
Larry:
First read the section from Principle of Operation from
http://en.wikipedia.org/wiki/Thermocouple
Principle of operation
Main article: Seebeck effect
In 1821, the German?Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and is between 1 and 70 microvolts per degree Celsius (?V/?C) for standard metal combinations.
It should be noted that there are a lot of minor variations in material composition, uniformity of the material, and the effects of temperature.
You can theoretically view the voltage from a thermocouple as the difference of the voltage generated along one wire (material) compared to the voltage along the other wire as the output voltage of the open circuit thermocouple resulting from the temperature gradient along the two wires. In the real world you can not measure this without introducing more dissimilar junctions. With the two wires being of different materials with a different voltage change from a given temperature change there will be a generated voltage from the temperature difference.
Next, assume the two thermal couple wires are not copper and both have a thermal electric output with copper that is non-zero. At one end the thermocouple wire is connected together to form the measurement junction. The other end is connected together with a copper wire. The copper wire and its two junctions are all at the same temperature. Because there is no temperature gradient along the copper wire there is no voltage generated along the copper wire. Therefore, this is the equivalent of the thermocouple wires being connected together instead of coupling thru the copper wire.
Make the copper wire a long loop. From a temperature measurement perspective the temperature difference measured is that of the probe point temperature to the temperature at the point where the copper loop and the thermocouple wires connect together. This reference temperature is not the end point of the copper loop. I am assuming the end point of the copper loop is the location of the measuring instrument and also the location of the absolute temperature reference.
Thus, one wants to have the uniform homogeneous thermocouple extend to the location where the reference temperature is defined. Basically the location of the measuring instrument.
It would be possible to make the thermocouple to copper termination some place out in a field and locate some kind of absolute temperature measurement element, such as a thermistor, at that copper junction location and bring back its signal to provide the zero reference in the instrument. But why do that instead of bringing the thermocouple wire back to the measuring instrument?
For those asking the question about thermocouple extension wire. The wire composition needs to be the same as the thermocouple wire, but insulation would not have to meet the requirements of the probe and therefore the extension wire could be a lower cost.
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