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THERMOCOUPLES

NFPA TIE ROD PNEUMATIC

AND HYDRAULIC CYLINDERS

LINEAR VALVE ACTUATORS

RTD’S 101

Metal


All metals produce a postive change in resistance for a positive change in temperature. This is the principal function of an RTD. System error is greatly reduced when the nominal value of the RTD resistance is high; this implies that a metallic wire has a high resistivity.


RTD Metals Resistivity
Metal Symbol Resistivity
cm/f
Resistivity
mm2/m
Resistivity
siemens
Gold Au 13 2,1612e-2 4,6272e+7
Silver Ag 8,8 1,4629e-2 6,8356e+7
Copper Cu 9,26 1,5394e-2 6,496e+7
Platinum Pt 59 9,8083e-2-2 1,0195e+7
Tungsten W 30 4,9873e-2 2,0051e+7
Nickel Ni 36 5,9847e-2 1,6709e+7

Due to their low resistivity, gold and silver are rearely used as RTD elements. Tungsten has a relatively high resistivity but is mainly used in applications that require an extremely high temperature measurement although the wire is very fragile and RTD construction difficult. Copper is used as an alternative to the popular platinum due to it’s linearity and more economical pricing. It’s low resistivity requires a longer length of wire than a standard platinum RTD and is limited to temperature measurement up to approx. 120°C. The most common RTDs are constructed with platinum, nickel, ar a nickel alloy blend. Nickel alloy wires are economical but are limited to a specific temperature range, are not linear, and have tendancy to drift over time. For precise temperature measurement, platinum is the obvious choice.