APCS the Signal Conditioning Specialists

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Select products that measure Temperature using popular industrial probes and produce a standard process signal output or an alarm state.

Temperature Measurement

RTD

The RTD is a coil of wire that changes resistance with temperature.

The Pt100 is the most commonly used RTD. It has an ice point resistance of 100Ω and is relatively linear, stable and accurate up to about 600°C.

The RTD is most commonly supplied with three connection wires wired into a measurement bridge and excited from a regulated direct current power supply. The change in resistance of the RTD causes a bridge output to change that is proportional to temperature.

4 Wire RTD Sensor

4 Wire RTD Sensor


About Thermistors

A thermistor is an electrical device made of a solid semiconductor with a high temperature coefficient of resistivity which would exhibit a linear voltage-current characteristic if its temperature were held constant. When a thermistor is used as a temperature sensing element, the relationship between resistance and temperature is used to generate a proportion signal.

This signal is typically non linear and generally does not posses good repeatability between sensors. Recent advancements in repeatability and linearity have been made but thermistors are still not a popular temperature element.

Thermistors are generally included in probes, motors and sensors as due to their larger resistance change with temperature making them more acceptable when used with a 2 Wire connection. Thermistors mostly used with alarms as the linearity problem is reduced with a one point measurement.

A linear temperature output can be obtained with the SL340 after creating a table to linearise the output.

Information

Thermocouple Input

The thermocouple is widely used in industrial applications due to rugged construction and wide temperature range of operation. The thermocouple is two dissimilar metals welded together.

Standard combinations of alloys have become industry standards. Selection of the type is driven by cost, melting point, stability, and output. Different types are best suited for different applications.

Voltage vs Temperature Popular TC Types

Voltage vs Temperature Popular TC Types

The thermocouple type must be supported by the measurement instrument. Each type has a different sensitivity and linearity.

  • The non-linear output means that the measuring transmitter should have a compensation table if measuring the temperature over a wide range.
  • Compensation cable is required when extending a thermocouple leads.
  • Noise problems can arise due to low output level. Long cable lengths should be separated from power cables.
  • Ground loop potentials and earth currents are a major problem so a transmitter with an isolated input should be used.
  • The measuring instrument must contain cold junction compensation.

Thermocouple Principle

If a temperature gradient is present in a conductor, the heat flow will create a movement of electrons and an electromotive force (emf) will be generated. The magnitude and direction will be dependent on the direction of the temperature gradient and the material of which the conductor is made.

The emf generated by a single conductor of uniform material is not measurable as the sum of the temperature gradients around the completed circuit will be zero. If there are two different materials in the circuit, each one generating a different voltage per degree temperature difference between its ends, then a measurable voltage will be produced. This is basically how a thermocouple works.

Thermocouple Principle

Thermocouple Principle

In a practical thermocouple circuit there are two junctions of the dissimilar metal conductors - the hot junction where we measure the temperature and the cold junction where we connect the measurement instrument.

The emf measured is the temperature difference between the hot and cold junctions.

The cold junction should be held at some fixed temperature (0°C) or the cold junction temperature is measured and the necessary correction is made to obtain the correct reading.