Resistance Thermometr

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Resistance thermometer

Resistance thermometers, also called resistancetemperature detectors (RTDs), are temperaturesensors

that exploit the predictable change in electricalresistance of some materials with changing temperature.As they are almost invariably made ofplatinum, they areoften called platinum resistance thermometers (PRTs).They are slowly replacing the use of thermocouples in

many industrial applications below 600 C.General description

There are two broad categories, "film" and "wire-wound"types.

Film thermometers have a layer of platinum on asubstrate; the layer may be extremely thin, perhaps 1micrometer. Advantages of this type are relatively lowcost and fast response. Such devices have improved inperformance although the different expansion rates of

the substrate and platinum give "strain gauge" effectsand stability problems.
http://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Thermocouplehttp://en.wikipedia.org/wiki/Celsiushttp://en.wikipedia.org/wiki/Platinghttp://en.wikipedia.org/wiki/Metrehttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Metrehttp://en.wikipedia.org/wiki/Platinghttp://en.wikipedia.org/wiki/Celsiushttp://en.wikipedia.org/wiki/Thermocouplehttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Temperature


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How do resistance thermometers work?

Resistance thermometers are constructed in a number offorms and offer greater stability, accuracy and repeatability

in some cases than thermocouples. While thermocouplesuse the Seebeck effect to generate a voltage, resistancethermometers use electrical resistance and require a smallpower source to operate. The resistance ideally varieslinearly with temperature.

Resistance thermometers are usually made using platinum,because of its linear resistance-temperature relationshipand its chemical inertness. The platinum detecting wire

needs to be kept free of contamination to remain stable. Aplatinum wire or film is supported on a former in such a waythat it gets minimal differential expansion or other strainsfrom 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
http://en.wikipedia.org/wiki/Seebeck_effecthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Linearhttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Ohm_(unit)http://en.wikipedia.org/wiki/Ohm_(unit)http://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Linearhttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Seebeck_effect


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Advantages and limitations

Advantages of platinum resistance thermometers:

High accuracy

Low drift Wide operating range

Suitability for precision applications

Limitations:

RTDs in industrial applications are rarely used above 660C. At temperatures above 660 C it becomes increasinglydifficult to prevent the platinum from becomingcontaminated by impurities from the metal sheath of thethermometer. This is why laboratory standard

thermometers replace the metal sheath with a glassconstruction. At very low temperatures, say below -270 C(or 3 K), due to the fact that there are very few phonons,the resistance of a RTD is mainly determined by impuritiesand boundary scattering and thus basically independent

of temperature. As a result, the sensitivity of the RTD is
http://en.wikipedia.org/wiki/Phononhttp://en.wikipedia.org/wiki/Impuritieshttp://en.wikipedia.org/w/index.php?title=Boundary_scattering&action=edithttp://en.wikipedia.org/wiki/Sensitivityhttp://en.wikipedia.org/wiki/Sensitivityhttp://en.wikipedia.org/w/index.php?title=Boundary_scattering&action=edithttp://en.wikipedia.org/w/index.php?title=Boundary_scattering&action=edithttp://en.wikipedia.org/w/index.php?title=Boundary_scattering&action=edithttp://en.wikipedia.org/wiki/Impuritieshttp://en.wikipedia.org/wiki/Phonon


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Resistance thermometer elements

Resistance thermometer elements are available in a

number of forms. The most common are:

Wire wound in a ceramic insulator - wire spiral within

sealed ceramic cylinder, works with temperatures to 850

C

Wire encapsulated in glass - wire around glass core with

glass fused homogenously around, resists vibration, more

protection to the detecting wire but smaller usable range

Thin film - platinum film on ceramic substrate, small and

inexpensive to mass produce, fast response totemperature change


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Resistance thermometer construction

These elements nearly always require insulated leads attached. At low

temperatures PVC, silicon rubber or PTFE insulators are common to 250C.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.


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Resistance thermometer wiring configurations

Two-wire configuration The simplest resistance thermometer configuration

uses two wires. It is only used when high accuracyis 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 system.

Three-wire configuration In order to minimize the effects of the leadresistances 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 meters of cable.
http://en.wikipedia.org/wiki/Image:Threewire.gif


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Resistance thermometer wiring configurations

Four-wire configuration The four wire resistance thermometerconfiguration 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 aboveWheatstone bridge method uses a little

more copper wire and is not a perfect

solution. Below is a better alternative

configuration four-wire Kelvin

connection that should be used in all

RTDs. It provides full cancellation ofspurious effects and cable resistance of

up to 15 can be handled. Actually in

four wire measurement the resistance

error due to lead wire resistance is

zero.
http://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/w/index.php?title=Kelvin_connection&action=edithttp://en.wikipedia.org/w/index.php?title=Kelvin_connection&action=edithttp://en.wikipedia.org/w/index.php?title=Kelvin_connection&action=edithttp://en.wikipedia.org/w/index.php?title=Kelvin_connection&action=edithttp://en.wikipedia.org/w/index.php?title=Kelvin_connection&action=edithttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Wheatstone_bridge


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History

The application of the tendency ofelectrical conductors to

increase their electrical resistance with rising temperature

was first described by Sir William Siemens at the BakerianLecture of 1871 before the Royal Society ofGreat Britain.

The necessary methods of construction were established

by Callendar, Griffiths, Holborn and Wein between 1885

and 1900.

Standard resistance thermometer data

Temperature sensors are usually supplied with thin-film

elements. These are rated in accordance with BS EN

60751:1996 as:

Tolerance class Valid range

Tolerance class B 70 to +500C

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 850C. Sensor

tolerances are calculated as:

Tolerance Class Temperature DeviationClass B t= (0.3+0.005|t|)

Class A t= (0.15+0.0025|t|)

1/3 DIN t= 13 (0.3+0.005|t|)

1/3 DIN t= 15 (0.3+0.005|t|)

1/10 DIN t= 110

(0.3+0.005|t|)

where |t| = absolute value of temperature inC. Where elements have a resistance ofn x

100 then the basic values and tolerances

also have to be multiplied by n.
http://en.wikipedia.org/wiki/Electrical_conductorhttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Carl_Wilhelm_Siemenshttp://en.wikipedia.org/wiki/Bakerian_Lecturehttp://en.wikipedia.org/wiki/Bakerian_Lecturehttp://en.wikipedia.org/wiki/Royal_Societyhttp://en.wikipedia.org/wiki/Great_Britainhttp://en.wikipedia.org/wiki/Absolute_valuehttp://en.wikipedia.org/wiki/Absolute_valuehttp://en.wikipedia.org/wiki/Great_Britainhttp://en.wikipedia.org/wiki/Great_Britainhttp://en.wikipedia.org/wiki/Great_Britainhttp://en.wikipedia.org/wiki/Royal_Societyhttp://en.wikipedia.org/wiki/Royal_Societyhttp://en.wikipedia.org/wiki/Royal_Societyhttp://en.wikipedia.org/wiki/Bakerian_Lecturehttp://en.wikipedia.org/wiki/Bakerian_Lecturehttp://en.wikipedia.org/wiki/Bakerian_Lecturehttp://en.wikipedia.org/wiki/Carl_Wilhelm_Siemenshttp://en.wikipedia.org/wiki/Carl_Wilhelm_Siemenshttp://en.wikipedia.org/wiki/Carl_Wilhelm_Siemenshttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_conductorhttp://en.wikipedia.org/wiki/Electrical_conductorhttp://en.wikipedia.org/wiki/Electrical_conductor


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Temperature to resistance equation

The relation between temperature and resistance is given

by the Callendar-Van Dusen equation, Here, R

Tis the resistance at temperature T, R0 is the

resistance at 0 C, and the constants (for an

alpha=0.00385 platinum RTD) are

Since the B and Ccoefficients are relatively small, theresistance changes almost linearly with the temperature.
http://en.wikipedia.org/wiki/Callendar-Van_Dusen_equationhttp://en.wikipedia.org/wiki/Callendar-Van_Dusen_equationhttp://en.wikipedia.org/wiki/Callendar-Van_Dusen_equationhttp://en.wikipedia.org/wiki/Callendar-Van_Dusen_equation

Resistance Thermometr

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