What is a Hot Junction?
Thermocouple Hot Junction
A hot junction usually refers to the meauring point of a thermocouple temperature sensor. This is the point at which the dissimilar metals from which the thermocouple is made (e.g. Chomel and Alumel for a Type K thermocouple) are joined together to form a junction.
The hot junction can be exposed, insulated or grounded to the metal sheath of the thermocouple probe as shown below:
Insulated Thermocouple Junction
Hot junction insulated from sheath. Gives floating output with typical insulation resistance for mineral insulated thermocouples in excess of 100 megohms.
Grounded Thermocouple Junction
The thermocouple junction is welded to the sheath tip giving earthed output and faster response to temperature changes.
Exposed Thermocouple Junction
Hot junction exposed giving the fastest response, mainly for the measurement of air temperature in ducts. Mineral insulated thermocouples with an exposed junction are restricted to a maximum operating temperature of 600°C.