For the thermocouple probe to provide high accuracy information, the cold junction must be at a stable or measurable temperature. You should know that this reference junction is actually generally made up of two different metals.
When the distance between the measuring point and the device measurement is important, it may be necessary to add an extension. The material of this cable must be of a calibration identical to the type of thermocouple to avoid false measurements. One solution is to use the same material directly, which can be expensive, it is then a extension cable. It is also possible to use a compensation cable, made from different metals than the thermocouple.