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Resistance thermometer

Resistance thermometers are a type of temperaturesensorand are slowly replacing the use of thermocouplesin many lower temperature industrial applications (below 600°C).

Inhaltsverzeichnis

1 How do resistance thermometers work?
2 Resistance thermometer elements
3 Resistance thermometer construction
4 Standard resistance thermometer data
5 Resistance thermometer wiring configurations
- 5.1 Two-wire configuration
- 5.2 Three-wire configuration
- 5.3 Four-wire configuration
6 References
7 See also

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 resistanceand require a small power source to operate. The resistance tends to vary linearlywith 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 ohms at 0 °C. This is defined in BS EN 60751:1996. The American Fundamental Interval is 0.392 ohms/°C.

Resistance thermometers require a small currentto be passed through in order to determine the resistance. This can cause self-heating, and manufacturers' limits should always be followed along with heat pathconsiderations in design. Care should also be taken to avoid any strainson 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

Image:Rtdconstruction.gif

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:

Tolerance Class	Rating over the range
Tolerance class B	-70 to +500 °C
Tolerance class A (1/2B)	-30 to +350 °C
Tolerance class 1/3B	0 to +100 °C

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

Class B	change in t = ±(0.3+0.005\|t\|)
Class A	change in t = ±(0.15+0.0025\|t\|)
1/3 Class B	change in t = ±¹⁄₃ × (0.3+0.005\|t\|)
1/5 Class B	change in t = ±¹⁄₅ × (0.3+0.005\|t\|)
1/10 Class B	change in t = ±¹⁄₁₀ × (0.3+0.005\|t\|)

where |t| = absolute valueof temperature in °C. Where elements have a resistance of n x 100 ohms then the basic values and tolerances also have to be multiplied by n.

Resistance thermometer wiring configurations

Two-wire configuration

Image:Twowire.gif

The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required as the resistance of the connecting wires is always included with that of the sensor leading to errors in the signal. Using this configuration you will be able to use 100 metres of cable. This applies equally to balanced bridge and fixed bridge systems. The values of the lead resistance can only be determined in a separate measurement without the resistance thermometer sensor and therefore a continuous correction during the temperature measurement is not possible.

Three-wire configuration

Image:Threewire.gif

In order to minimise the effects of the lead resistances a three wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms, there is a lead resistance in each arm of the bridge and therefore the lead resistance is cancelled out. High quality connection cables should be used for this type of configuration because an assumption is made that the two lead resistances are the same. This configuration allows for up to 600 metres of cable.

Four-wire configuration

Image:Fourwire.gif

The four wire resistance thermometer configuration even further increases the accuracy and reliability of the resistance being measured. In the diagram above a standard two terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above Wheatstone bridgemethod uses a little more copper wire and is not a perfect solution. Below is an better alternative configuration that should be used in all RTD's. It provides full cancellation of spurious effects and cable resistance of up to 15 ohms can be handled.

Image:4wirebetter.gif

References

Text and images used by permission of Peak Sensors Ltd:

Resistance ThermometerInformation