Editing Resistance thermometer (section)

==Resistance/temperature relationship of metals==
[[File:SPRT Glass Capsule - RTD.jpg|thumb|SPRT glass capsule – RTD]]
Common RTD sensing elements for biomedical application constructed of [[platinum]] (Pt), [[nickel]] (Ni), or [[copper]] (Cu) have a [[repeatability|repeatable]],{{Efn|Meaning that the same measurement remains unchanged for the same temperature under influence of the surroundings.}} resistance versus temperature relationship (''R'' vs ''T'') and [[operating temperature]] range. The ''R'' vs ''T'' relationship is defined as the amount of resistance change of the sensor per degree of temperature change.<ref name=sensortech /> The relative change in resistance ([[temperature coefficient]] of resistance) varies only slightly over the useful range of the sensor.{{Citation needed|date=May 2022}}

[[Platinum]] was proposed by [[Sir William Siemens]] as an element for a resistance temperature detector at the [[Bakerian lecture]] in 1871:<ref>{{cite journal |last=Siemens |first=William |date=1871 |title=On the Increase of Electrical Resistance in Conductors with Rise of Temperature, and Its Application to the Measure of Ordinary and Furnace Temperatures; Also on a Simple Method of Measuring Electrical Resistances |url=https://archive.org/stream/philtrans09056316/09056316_djvu.txt |journal=The Bakerian Lecture |publisher=[[Royal Society]] |access-date=May 14, 2014}}
</ref> it is a [[noble metal]] and has the most stable resistance–temperature relationship over the largest temperature range. [[Nickel]] elements have a limited temperature range because the temperature coefficient of resistance changes at temperatures over 300&nbsp;°C (572&nbsp;°F). [[Copper]] has a very linear resistance–temperature relationship; however, copper oxidizes at moderate temperatures and cannot be used over 150&nbsp;°C (302&nbsp;°F).{{Citation needed|date=May 2022}}

The significant characteristic of metals used as resistive elements is the linear approximation of the resistance versus temperature relationship between 0 and 100&nbsp;°C. This temperature coefficient of resistance is denoted by α and is usually given in units of [[ohm|Ω]]/(Ω·°C):{{Citation needed|date=May 2022}}
: <math>\alpha = \frac{R_{100} - R_0}{100~^\circ\text{C} \cdot R_0},</math>
where
: <math>R_0</math> is the resistance of the sensor at 0 °C,
: <math>R_{100}</math> is the resistance of the sensor at 100 °C.

Pure [[platinum]] has α = 0.003925 Ω/(Ω·°C) in the 0 to 100&nbsp;°C range and is used in the construction of laboratory-grade RTDs. {{Citation needed|date=May 2022}} Conversely, two widely recognized standards for industrial RTDs IEC 60751 and ASTM E-1137 specify α = 0.00385 Ω/(Ω·°C). Before these standards were widely adopted, several different α values were used. It is still possible to find older probes that are made with platinum that have α = 0.003916 Ω/(Ω·°C) and 0.003902 Ω/(Ω·°C).{{Citation needed|date=May 2022}}

These different α values for platinum are achieved by [[dopant|doping]] – carefully introducing impurities, which become embedded in the lattice structure of the platinum and result in a different ''R'' vs. ''T'' curve and hence α value.{{citation needed|date=February 2016}}