Incorrectly wired compensating cable
Incorrect wiring of thermocouple cables can be further complicated because thermocouples often use compensating cable. It is possible that the compensating cable is incorrectly wired to the thermocouple. It may appear correctly wired at the instrument but not correct between the thermocouple and compensating cable. To avoid this, make sure -ve and +ve legs match correctly along the full length of the thermocouple, to the compensating cable, to the instrument.
Using the wrong compensating type
Each compensating cable is also tied to a specific thermocouple type. To ensure accuracy you must only use compensating cable of the same type e.g. a type K thermocouple could only be extended with type k compensating cable. Sometimes other types of cable such as general instrument cable, or copper cable is used to extend thermocouples and this also creates errors.
Using compensating cable in high temperature environments
The final quirk with compensating cable is that it should not be used in high temperature environments. Thermocouple compensating cable is rated for a lower temperature than thermocouple cable. Try to use compensating cable in ambient temperatures to reduce errors.