How to Select PTFE INSULATED THERMO COUPLE WIRES

Thermocouple Type selection

A variety of thermocouples are available suitable for different measuring applications. They are usually selected based on the temperature range and sensitivity needed.Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions and higher price. Garg makes only Type J, T and K thermocouples.Other selection criteria include the inertness of the thermocouple material, and whether or not it is magnetic. One characteristic of thermocouples made with magnetic material is that they undergo a step change when the magnetic material reaches its Curie point.

Type T (copper–constantan) thermocouples are suitable for the -200 to 350 °C temperature range measurements. They are often used as a differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.

Type J (iron–constantan) is less popular than type K due to its limited range (-40 to +750 °C). The main application is with old equipment that cannot accept modern thermocouples. The Curie point of the iron (770 °C) causes an abrupt change to the characteristic and it is this that provides the upper temperature limit. Type J thermocouples have a sensitivity of about 50 µV/°C.

Type K (chromel–alumel) is the most commonly used for general purpose thermocouple. They are available in the -200 °C to +1350 °C range. The type K was specified at a time when metallurgy was less advanced than it is today and, consequently, characteristics vary considerably between examples. Another potential problem arises in some situations since one of the constituent metals, nickel, is magnetic. Curie point occurs for this thermocouple at 354 °C. Sensitivity is approximately 41 µV/°C.

Construction Selection

For simple thermocouples two leads with planetary twisting are good. Sometimes another outer jacket is provided. If the application is rough and the probes might be damaged then an outer SS braid is also provided. For further protection an outer jacket on SS braid is sometimes used.

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