Thermocouple circuits often have several connection points along their length. These may be from the thermocouple to lead wire, lead wire to connector, or at other points dictated by necessary interruptions in the circuit such as through vessel or furnace walls. Occasionally a connection may be inadvertently cross-connected so that the negative wire is connected to the positive. This will result in an obviously erroneous signal at the readout instrument due to an incorrect polarity.
When this occurs it is sometimes corrected by switching the connection at the instrument rather than locating the point at which the original cross connection occurs. That is, the positive and negative legs are cross-connected at two points in the loop. If this occurs, the readout instrument will see a signal of the correct polarity but it will be in error. The magnitude of the error will depend upon the temperature differential between the two points at which the connections are crossed. Such errors can greatly exceed those normally seen by inherent errors in thermocouple wire and lead wire relative to the nominal curve.
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