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
![insulated thermocouple hot junction](../images/thermocouples/insulated-thermocouple-junction.png)
Hot junction insulated from sheath. Gives floating output with typical insulation resistance for mineral insulated thermocouples in excess of 100 megohms.
Grounded Thermocouple Junction
![grounded thermocouple hot junction](../images/thermocouples/grounded-thermocouple-junction.png)
The thermocouple junction is welded to the sheath tip giving earthed output and faster response to temperature changes.
Exposed Thermocouple Junction
![exposed thermocouple hot junction](../images/thermocouples/exposed-thermocouple-junction.png)
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.