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Resistance thermometers, also called resistance temperature detectors (RTDs), are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs). They are slowly replacing the use of thermocouples in many industrial applications below 600 °C. Fig. ...
Not to be confused with censure, censer, or censor. ...
Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ...
General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ...
In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. ...
Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). ...
General description There are two broad categories, "film" and "wire-wound" types. - Film thermometers have a layer of platinum on a substrate; the layer may be extremely thin, perhaps 1 micrometer. Advantages of this type are relatively low cost and fast response. Such devices have improved in performance although the different expansion rates of the substrate and platinum give "strain gauge" effects and stability problems.
- Wire-wound thermometers can have greater accuracy, especially for wide temperature ranges. The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimize strain and consequential drift.
The current international standard which specifies tolerance and the temperature to electrical resistance relationship for platinum resistance thermometers is IEC 751:1983. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 °C, and are called Pt-100 sensors ('Pt' is the symbol for platinum). The sensitivity of a standard 100 ohm sensor is a nominal 0.385 ohm/°C. RTDs with a sensitivity of 0.375 and 0.392 ohm/°C are also available. Plating is the general name surface-covering techniques in which a metal is deposited onto a conductive surface. ...
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Typical foil strain gauge. ...
The ohm (symbol: Ω) is the SI unit of electric resistance. ...
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 to generate a voltage, resistance thermometers use electrical resistance and require a small power source to operate. The resistance ideally varies 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 thermometers are usually made using platinum, because of its linear resistance-temperature relationship and its chemical inertness. 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 Standard atomic weight 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 100Ω at 0 °C. This is defined in BS EN 60751:1996. The American Fundamental Interval is 0.392 Ω/°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 resistive 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. ...
Look up strain in Wiktionary, the free dictionary. ...
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 660 °C. At temperatures above 660 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer. This is why laboratory standard thermometers replace the metal sheath with a glass construction. 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 impurities and boundary scattering and thus basically independent of temperature. As a result, the sensitivity of the RTD is essentially zero and therefore not useful.
- Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slower response time. However, thermistors have a smaller temperature range and stability.
Normal modes of vibration progression through a crystal. ...
Impurities are substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. ...
See: Sensitivity (electronics) Sensitivity (human) Sensitivity (tests) For sensitivity in finance, see beta coefficient This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ...
NTC thermistor, bead type, insulated wires Thermistor symbol A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. ...
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 to temperature change
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.
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 system.
Three-wire configuration
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In order to minimize 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 meters 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 a better alternative configuration four-wire Kelvin connection that should be used in all RTD's. It provides full cancellation of spurious effects and cable resistance of up to 15 Ω can be handled. Actually in four wire measurement the resistance error due to lead wire resistance is zero. A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. ...
Four-terminal sensing (4T sensing) is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage sensing electrodes to make more accurate measurements than traditional two-terminal (2T) sensing. ...
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History The application of the tendency of electrical conductors to increase their electrical resistance with rising temperature was first described by Sir William Siemens at the Bakerian Lecture of 1871 before the Royal Society of Great Britain. The necessary methods of construction were established by Callendar, Griffiths, Holborn and Wein between 1885 and 1900. In science and engineering, conductors, such as a electrical connector, are materials that readily conduct electric current through electrical conduction. ...
Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ...
Wilhelm Siemens Carl Wilhelm Siemens (en: Charles William Siemens) (April 4, 1823 – November 19, 1883) was a German engineer. ...
The Bakerian Lecture is a prize lecture of the Royal Society, the Societys most prestigious lecture on physical sciences. ...
The premises of The Royal Society in London (first four properties only). ...
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 +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: | Tolerance Class | Temperature Deviation | | Class B | t = ±(0.3+0.005|t|) | | Class A | t = ±(0.15+0.0025|t|) | | 1/3 DIN | t = ±1⁄3 × (0.3+0.005|t|) | | 1/5 DIN | t = ±1⁄5 × (0.3+0.005|t|) | | 1/10 DIN | 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 Ω then the basic values and tolerances also have to be multiplied by n. In mathematics, the absolute value (or modulus[1]) of a real number is its numerical value without regard to its sign. ...
Temperature to resistance equation The relation between temperature and resistance is given by the Callendar-Van Dusen equation, The Callendar-Van Dusen equation is an equation that describes the relationship between resistance (R) and temperature (t) of platinum resistance thermometers. ...
![R_T = R_0 left[ 1 + AT + BT^2 + cT^3 (T-100) right] ; (-200;{}^{circ}mathrm{C} < T < 0;{}^{circ}mathrm{C}),](http://upload.wikimedia.org/math/1/b/2/1b2b866a4b043bc4ced08de262cdbb8f.png) ![R_T = R_0 left[ 1 + AT + BT^2 right] ; (0;{}^{circ}mathrm{C} leq T < 850;{}^{circ}mathrm{C}).](http://upload.wikimedia.org/math/9/a/8/9a8ee16cacd873b4592af95df3dddfc1.png) Here, RT is 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 C coefficients are relatively small, the resistance changes almost linearly with the temperature.
References Text and images used by permission of Peak Sensors Ltd:
Resistance Thermometer Information
See also NTC thermistor, bead type, insulated wires Thermistor symbol A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. ...
Bi-metallic thermostat for buildings A thermostat is a device for regulating the temperature of a system so that the systems temperature is maintained near a desired setpoint temperature. ...
In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. ...
General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ...
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