All metals produce a positive change in resistance in response to a positive change in temperature, which is the main function of an RTD. System error is minimised where the RTD’s resistance nominal value is large; this means using a metal wire with high resistivity.
|Resistivity of the metals used in RTDs|
|Metal||Symbol||Resistivity Ω·cm/f||Resistivity Ω·mm²/m||Resistivity siemens|
Due to their low resistivity, gold and silver are rarely used in RTDs.
Le tungstène a une résistivité relativement élevée, mais est réservé pour des applications à très hautes températures parce qu’il est extrêmement fragile et difficile à travailler.
Tungsten has a relatively high resistivity, but is reserved for applications at very high temperatures because it is extremely fragile and difficult to work. Its upper temperature limit is only around 120 °C.
The most common RTDs are made of platinum, nickel or nickel alloys. Wires in nickel alloy are economic and used in a restricted temperature range, but they are non-linear and tend to drift over time.
Platinum is the obvious choice for all measurements.
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