A thermocouple is a set of two wires welded together at one end, where a junction is formed between the two wires. Because each wire is composed of a different type of metal, each wire reacts differently to the temperature of the junction. Because of this difference, a voltage is generated. The voltage varies consistently with changes in temperature, and so there are devices, such as digital thermometers, that measure the voltage and convert it to a temperature reading. [Doug Hewitt, eHow Contributor]
There's lots of calibration procedures on the web. Here's one http://www.kostic.niu.edu/tc_cal.html
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