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It's also important to understand that since the thermocouple produces a dc millivoltage, the two wires that are used as its terminals have polarity. This means that one must be able to determine which wire will produce the positive voltage and which will have negative polarity . Fig. 1 shows a complete list of 11 of the most common thermocouples and the polarity of each of the two wires. To make the polarity easier to identify, a standard color code has been established by the American National Standards Institute (ANSI) and it has been adopted by manufacturers. (It should be noted that the British, German, Japanese, and French have each adopted their own individual color codes, which are shown in the table.)
Above: Fig. 1 Thermocouple identification table showing
the type of thermocouple by classification, the type of material that
each thermocouple is made of, and the color code for the positive and negative
wires. Also see this link for Wire
Color Codes and Limits of Error.
In some applications where the thermocouple is mounted a long distance from the controller or display, it's not practical or cost-effective to use thermocouple wire to make these long runs. In these applications specialized extension wire is used because it's less expensive and easier to install since it's not as stiff as thermocouple wire. Extension wire is specifically designed so that it can transmit the millivoltage over a long distance and not be affected by temperature changes or voltage drops. Another important feature of extension wire is that it can be connected to thermocouple wire without making another junction that must have compensation.
EXERCISE
One is asked to identify the type and determine the polarity of a thermocouple that is suspected to not be functioning correctly. The thermocouple is the temperature sensor for a piece of older equipment where a technician is working. Since this piece of equipment is not new, the documentation for the sensor is missing and the blueprint is severely faded so that one can't read it, but one is able to determine that the machine was made in New York in 1987. One has determined that the thermocouple wire is white and red.
SOLUTION
Since one has determined the thermocouple wire is white and red, one may use the table in Fig. 1 and determine that this is a J-type thermocouple. Since one has determined this machine was built in the United States, one may assume that the thermocouple colors use the AN51 code. If your preliminary test indicates that this is not a J-type thermocouple, one may need to assume that this thermocouple may be following one of the foreign color codes.
A second test can help one determine if this is actually a J-type thermocouple and that it's following the ANSI color code. Since the table shows that the J-type thermocouple is made of iron and constantan, and the iron wire is the white wire, one may use a magnet to detect the iron wire. If the magnet shows that the white wire is magnetic, one may accurately say this is a J-type thermocouple. If the red wire is magnetic, this may further disprove your initial assumption and lead one to make further tests to determine which country this thermocouple comes from.
If one has determined this is a J-type thermocouple, one may use the table to determine that the temperature range for this thermocouple is 0°C-750°C and it will produce 0-42 mV over this temperature range.
