<P> Common RTD sensing elements constructed of platinum, copper or nickel have a repeatable 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 . The relative change in resistance (temperature coefficient of resistance) varies only slightly over the useful range of the sensor . </P> <P> Platinum was proposed by Sir William Siemens as an element for a resistance temperature detector at the Bakerian lecture in 1871: 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 amount of change in resistance per degree of change in temperature becomes very non-linear at temperatures over 572 ° F (300 ° C). Copper has a very linear resistance--temperature relationship; however, copper oxidizes at moderate temperatures and cannot be used over 302 ° F (150 ° C). </P> <P> Platinum is the best metal for RTDs due to its very linear resistance--temperature relationship, highly repeatable over a wide temperature range . The unique properties of platinum make it the material of choice for temperature standards over the range of − 272.5 ° C to 961.78 ° C. It is used in the sensors that define the International Temperature Standard, ITS - 90 . Platinum is chosen also because of its chemical inertness . </P> <P> The significant characteristic of metals used as resistive elements is the linear approximation of the resistance versus temperature relationship between 0 and 100 ° C . This temperature coefficient of resistance is denoted by α and is usually given in units of Ω / (Ω ° C): </P>

Explain why platinum rod is used in resistance thermometer
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