Ken Jefferies reports on sophisticated sensors for precision thermometry
The use of a platinum resistance thermometer probe in industrial and scientific applications will normally result in good, accurate temperature sensing without the need for special cables (unlike thermocouples). To achieve the best possible accuracy and reliability, care is needed with installation of the probe, connections, and choice of the host instrumentation.
Choice of wire-wound or flat film sensing resistor
The resistance element used in the probe is produced in one of two forms, either wire-wound or flat film. Metal film resistors consist of a platinum layer on a ceramic substrate; the coil of a wire wound version is fused into ceramic or glass.
With regard to wire-wound resistors, various methods of detector construction are employed to meet the requirements of differing applications. The unsupported ‘bird cage’ construction is used for temperature standards, and the partially supported construction is used where a compromise is acceptable between primary standards and use in industrial applications. Other constructional methods include the totally supported construction, which can normally withstand vibration levels to 100g.
Meanwhile, metal film Pt resistors take the form of a thin (1 micron) film of platinum on a ceramic substrate. The film is laser trimmed to have a precise Ro value and then encapsulated in glass for protection.
Terminating the resistance thermometer is a key issue to be considered. Fundamentally, every sensing resistor is a two-wire device. When terminating the resistor with extension wires during probe construction, a decision must be made as to whether a two-, three- or four-wire arrangement is required for measurement purposes.
In the sensing resistor, the electrical resistance varies with temperature. Temperature is measured indirectly by reading the voltage drop across the sensing resistor in the presence of a constant current flowing through it using Ohm's Law: V = R.I
The connection between the thermometer assembly and the instrumentation is made with standard electrical cable with copper conductors in two, three or four core construction. The cabling introduces electrical resistance that is placed in series with the resistance thermometer. The two resistances are therefore cumulative and could be interpreted as an increased temperature if the lead resistance is not allowed for.
Instruments using three or four wire configurations reduce or eliminate lead resistance effects. However, the wiring configuration (two-, three- or four-wire) of the thermometer must be compatible with the input to the associated instrument.
High accuracy options (tolerance classes)
Assuming a three- or four-wire connection, and the use of a class B sensing resistor, a standard thermometer assembly will provide an accuracy of around 0.5°C between 0°C and 100°C. Considerable improvement on this figure can be achieved by various means including the use of closer tolerance sensors.
The principle of operation is the non-linear variation of the electrical resistance of the resistor with temperature. The commonly internationally used Pt100 has a resistance of 100.00 Ohms at 0°C and 138.51Ohms at 100°C according to IEC751.
The use of virtually pure rare metal (platinum) in the sensing resistor endows this type of temperature sensor with both high accuracy and long term stability; good quality instrumentation is therefore essential to realize these virtues.
It is important to remember that the overall accuracy of any measuring system (e.g. sensor, instrument, interconnection, application, etc) is compromised by the sum of the uncertainties in that system.
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Ken Jefferies is technical consultant to Labfacility.
