When you using a thermocouple, there are a number of ways to connect it to your control system or transmitter. The simplest ways are the thermocouple extension cable and thermocouple compensating cable. They might sound similar but there are some key differences that determine which one you should use.

Thermocouple cable types

Extension cable

Thermocouples often require specific cables called thermocouple extension cables or extension wires. Extension cables use the same material as the thermocouple, so they send the emission-motive force (EMF) accurately. X marks the spot, or in this case, the extension cable. So if you see a cable marked “JX”, for example, it is a cable for a type-J thermocouple.

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Surface thermocouple type J Thermocouple sensor with glass fibre insulated connecting cable
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J98AA08 083.0-7

The extension cable also prevents you from connecting the sensor directly to the control system, depending on the distance. However, you can find applications with extra-long cables to make this connection. You won’t find this issue in an RTD installation. RTDs need certain cables too, but they cost a lot less.

Compensating cable

Compensating cables use cheaper materials than thermocouples. They produce similar numbers to extension cables but in a more limited range. The letter C identifies compensating cables, so a cable marked “RCA” is a compensating cable for a type-R thermocouple.

Some manufacturers recommend using C cables only to connect the sensor to the transmitter. Beyond that, the cable is less accurate than an X cable. The compensating cable’s conventional insulating material, often PVC, also covers a smaller temperature range.

To know more about thermocouple tables, please ask our engineers!

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