Choosing A Thermocouple


CHOOSING A THERMOCOUPLE

 

Type Of Thermocouples

Noble Metals

Platinum 10% Rhodium Expensive, chemically stable, good mechanical properties, must be protected in use, not suitable for low temperature applications.
Platinum 13% Rhodium Expensive, chemically stable, good mechanical properties, may be used in oxidizing atmospheres, must be protected in use, not suitable for low temperature applications.
Platinum 20% Rhodium Very expensive, for use in temperature ranges beyond the capabilities of the above noble metals.

  

Base Metals

Iron-Constantan Low cost, suitable for oxidizing or reducing atmospheres. Subject to chemical attack.
Copper-Constantan Suited to ambient temperatures, similar to the model above.
Chromel-Alumel Higher temperature range (1000-2000°F). Can be used in an oxidizing atmosphere, is mechanically strong, and is resistant to chemical attack.
Chromel-Constantan Very stable, not to be used in reducing atmospheres, subject to chemical attack.

1.) Exposed Junction
The thermocouple wires are welded together and the insulation is sealed against penetration by liquid or gas. This type of thermocouple offers the least protection and is best suited to ambient atmospheres.

Thermocouple Bare Junction

2.) Grounded Junction
The thermocouple wires are welded together with (grounded to) the sheath. This provides more protection, and is used in cases where an exposed junction would not work due to harsh atmospheric conditions, or when longer life is required.

Thermocouple Grounded Junction

3.) Ungrounded Junction
The thermocouple wires are welded together and are insulated from the sheath. This provides the most protection for the thermocouple elements and is recommended for the longest life. Different types of sheath materials (protection tubes) offer greater protection depending on the conditions.

Thermocouple Ungrounded Junction