Notice: This page contains information for the legacy Phidget21 Library. Phidget21 does not support VINT Phidgets, and will not support any new Phidgets. Phidget21 will be maintained until 2020. We recommend that new projects be developed against the Phidget22 Library.
Cold Junction Compensation and Self-heating
Thermocouples consist of two junctions, one where the thermocouple meets the Phidget and one where the two wires are welded together at the sensing end of the device. In order to get accurate readings from the thermocouple one of these temperatures must be a known quantity. It doesn't make much sense to know the temperature at the end you are trying to sense at! Therefore it stands to reason that we need to know the temperature at the end where the thermocouple connects to the Phidget. This is why there is an ambient temperature sensor built onto the board.
An important thing to note though is that the ambient temp sensor measures the temperature of the air around the board and not specifically the junction. Generally you can assume they are nearly one and the same value, however as the electronics get hot due to being powered up there can be some small error introduced. This is exacerbated by having the board in an enclosed space where normal airflow is restricted thereby increasing the effect of self-heating. As a result we recommend that the board be left in as open and well ventilated/cooled a place as possible to minimize this error source.