resistance thermometer

Other Definitions
resistance thermometer (dict)

Resistance thermometers are a type of temperature sensor and are slowly replacing the use of thermocouples in many lower temperature industrial applications (below 600C).

How do resistance thermometers work?

Resistance thermometers come in a number of construction forms and offer greater stability, accuracy and repeatability in some cases than thermocouples. Where thermocouples use the Seebeck effect, resistance thermometers use resistance and require a small power source to operate. The resistance tends to vary linearly 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 created and 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 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 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 strains on the resistance thermometer in its application. Lead wire resistance should be considered and adopting three and four wire connection strategies can result in eliminating 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

Resistance Thermometer Construction These elements will 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 along with offering convenient process attachment features.

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+0.002|t|)
1/3 Class B change in t=+/- 1/3 x (0.3+0.005|t|)
1/5 Class B change in t=+/- 1/5 x (0.3+0.005|t|)
1/10 Class B change in t=+/- 1/10 x (0.3+0.005|t|)

Where |t| = absolute 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

Two Wire Resistance Thermometer 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

Three Wire Resistance Thermometer 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

Four Wire Resistance Thermometer 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.

References

Text and images used by permission of Peak Sensors Ltd: Resistance Thermometer Information

Resistance Thermometer