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Resistance thermometers are a type of temperature sensor and are slowly replacing the use of thermocouples in many lower temperature industrial applications (below 600°C). Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ...
This article needs to be cleaned up to conform to a higher standard of quality. ...
In electronics, thermocouples are a widely used type of temperature sensor. ...
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 linearly with temperature. The Peltier-Seebeck effect, or thermoelectric effect, is the direct conversion of heat differentials to electric voltage and vice versa. ...
Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ...
The word linear comes from the Latin word linearis, which means created by lines. ...
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. General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Atomic mass 195. ...
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. The ohm (symbol: Ω) is the SI unit of electric resistance. ...
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 connections can eliminate connection lead resistance effects from measurements. In electricity, current refers to electric current, which is the flow of electric charge. ...
In any branch of science dealing with materials and their behaviour, strain is the geometrical expression of deformation caused by the action of stress on a physical body. ...
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
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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 value of 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. In mathematics, the absolute value (or modulus1) of a real number is its numerical value without regard to its sign. ...
Resistance thermometer wiring configurations
Two-wire configuration
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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
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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
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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 bridge method 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. A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. ...
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References Text and images used by permission of Peak Sensors Ltd:
Resistance Thermometer Information
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