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Selecting the Right Temperature Sensor for Your Application

Thermocouples

Thermocouples are two wire devices made up from dissimilar metal welded together which when heated produce a small millivolt potential. The output depends upon the materials of construction with a whole range of different materials for differing applications and temperature ranges, which is way beyond the scope of this article. For example a general low cost thermocouple would be a type K which is manufactured from Nickel and Chromium and produces an output of 41μV/oC being a magnetic material can cause some problems with linearity at temperatures above 350ºC this is again outside the scope of this article. For higher temperature applications Type B, R or S can be used up to 1600ºC these thermocouples are considerably more expensive being manufactured from Platinum / Rhodium with an output of 10μVºC.

The disadvantage of thermocouples is they cannot be connected to standard copper cable as another junction of dissimilar metals would be made in the connecting head which would also produce a millivolt signal and hence an error. So a cable with the same characteristics as the thermocouple must be used to connect back to the temperature controller this is called compensating cable. The connection at the instrument terminals with the comp cable can produce a small millivolt potential which needs to be compensated for this is often referred to as the cold junction temperature.

PRT Theory

The temperature instrument or transmitter supplies a low voltage to the platinum resistance sensor which causes a current to flow; making an electrical circuit.

By ohms law the voltage drop in the circuit and hence the current flow is proportional to the resistance in the circuit. As the temperature increases the resistance of the PT100 increases:- this is a positive temperature coefficient. The problem with the two wire configuration is the instrument reads the resistance of the connecting cable as well as the temperature sensor.

There are some simple ways to circumnavigate this problem has listed below in the connection details for the 3 & 4 wire systems.

Connections for two wire instruments

Red Wire

White Wire

In this circuit the resistance is 3 + 100 + 3 Ω = 106 Ω

Connections for three wire instruments

Red Wire

Red Wire

White Wire

In this circuit the temperature instrument measures the resistance between the red and white wires, it also measures the resistance between the two red wires.

The temperature controller will subtract the resistance between the red wires from the resistance between the red and white wires to compensate for the resistance in the cable. The instrument assumes that the resistance in all the wires are equal to each other.

Connections for four wire instruments pyrometer

Red Wire

Red Wire

White Wire

White Wire

The four wire connections are usually connected to the four arms of a wheatstone bridge type circuit so that the resistances cancel each other out.

In our opinion the cost of installation of a long cable run is greater than installing a 4-20mA two wire temperature transmitter.

A two wire 4-20mA temperature transmitter offers precision signal transfer over an effective distance up to 1000 metres away. We would suggest the TT100 4-20mA Temperature Transmitter paired with a BC7635 Controller as a suitable alternative.

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