Electrical Resistance Thermometer

HOW DO RESISTANCE THERMOMETERS WORK?

Resistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatability in some cases than thermocouples. While thermocouples use the Seebeck Effect , resistance thermometers use Electrical Resistance and require a small power source to operate. The resistance tends to vary Linear ly with temperature.

Resistance thermocouples are usually made using Platinum , due to its stability with temperature. The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration.

Commercial platinum grades are produced which exhibit a change of resistance of 0.385 Ohms /°C (European Fundamental Interval) The sensor is usually made to have a resistance of 100 Ω at 0 °C. This is defined in BS EN 60751:1996. The American Fundamental Interval is 0.392 Ω/°C.

Resistance thermometers require a small Current to be passed through in order to determine the resistance. This can cause self-heating, and manufacturers' limits should always be followed along with Heat Path considerations in design. Care should also be taken to avoid any Strain s on the resistance thermometer in its application.
Lead wire resistance should be considered, and adopting three and four wire connections can eliminate connection lead resistance effects from measurements.

RESISTANCE THERMOMETER ELEMENTS

Resistance thermometers elements are available in a number of forms. The most common are:

Wire-wound in a ceramic insulator - works with temperatures to 850 °C

Wires encapsulated in glass - resists vibration, offers the most protection to the detecting wire, and is inexpensive to mass-produce.

RESISTANCE THERMOMETER CONSTRUCTION

These elements nearly always require insulated leads attached. At low temperatures PVC, silicon rubber or PTFE insulators are common to 250 °C. Above this, glass fibre or ceramic are used. The measuring point and usually most of the leads require a housing or protection sleeve. This is often a metal alloy which is inert to a particular process.
Often more consideration goes in to selecting and designing protection sheaths than sensors as this is the layer that must withstand chemical or physical attack and offer convenient process attachment points.

STANDARD RESISTANCE THERMOMETER DATA

Temperature sensors are usually supplied with thin-film elements. These are rated as:

Resistance thermometer elements can be supplied which function up to 850 °C. Sensor tolerances are calculated as:

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Electrical Resistance Thermometer

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