Resistance thermometer
Many RTD elements consist of a length of fine wire wrapped around a heat-resistant ceramic or glass core but other constructions are also used.
The RTD wire is a pure material, typically platinum (Pt), nickel (Ni), or copper (Cu).
[citation needed] Conversely, two widely recognized standards for industrial RTDs IEC 60751 and ASTM E-1137 specify α = 0.00385 Ω/(Ω·°C).
Although RTDs are considered to be linear in operation, it must be proven that they are accurate with regard to the temperatures with which they will actually be used (see details in Comparison calibration option).
[citation needed] The three main categories of RTD sensors are thin-film, wire-wound, and coiled elements.
Resistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatability in some cases than thermocouples.
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 exhibit a temperature coefficient of resistance 0.00385/°C (0.385%/°C) (European Fundamental Interval).
The advantages of platinum resistance thermometers include: 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.
[citation needed] Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slower response time.
The measuring point, and usually most of the leads, require a housing or protective sleeve, often made of a metal alloy that is chemically inert to the process being monitored.
The RTD construction design may be enhanced to handle shock and vibration by including compacted magnesium oxide (MgO) powder inside the sheath.
MgO is used due to its dielectric constant, rounded grain structure, high-temperature capability, and its chemical inertness.
It is only used when high accuracy is not required, as the resistance of the connecting wires is added to that of the sensor, leading to errors of measurement.
To increase accuracy further, any residual thermoelectric voltages generated by different wire types or screwed connections are eliminated by reversal of the direction of the 1 mA current and the leads to the DVM (digital voltmeter).
[citation needed] The highest-accuracy of all PRTs are the Ultra Precise Platinum Resistance Thermometers (UPRTs).
Larger-diameter platinum wire is used, which drives up the cost and results in a lower resistance for the probe (typically 25.5 Ω).
SPRTs commonly use reference-grade, high-purity smaller-diameter platinum wire, metal sheaths and ceramic type insulators.
Contemporary to the Seebeck effect, the discovery that resistivity in metals is dependent on the temperature was announced in 1821 by Sir Humphry Davy.
[10] The practical 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, suggesting platinum as a suitable element.
In 1871 Carl Wilhelm Siemens invented the Platinum Resistance Temperature Detector and presented a three-term interpolation formula.
A 1971 paper by Eriksson, Keuther, and Glatzel identified six noble metal alloys (63Pt37Rh, 37Pd63Rh, 26Pt74Ir, 10Pd90Ir, 34Pt66Au, 14Pd86Au) with approximately linear resistance temperature characteristics.
(The failures of the sensors falsely suggested that a fuel pump was critically overheating, and the engine was automatically shut down.)
