The electrical resistance of a metal conductor increases as temperature increases. This is because the electrical conductivity of a metal relies on the movement of electrons through its crystal lattice. Due to thermal excitation, the vibration of electrons increases, which slows the electrons’ movement, thus causing the resistance to increase.
The temperature coefficient of an alloy is often very different from that of the constituent metals. Small traces of impurities can greatly change the temperature coefficients.
An RTD is a passive device, requiring a current to pass through to produce a measurable voltage. If the excitation current passing through an RTD is Iex, and the output voltage across the RTD is Vout, then the measured temperature T (in °C) can be obtained by.
Where R0 is the resistance of the RTD at 0°C; A and B (C = 0 in this case when T > 0°C) are the Callendar–Van Dusen Coefficients. The excitation current also causes the RTD to heat internally (self-heating), which can result in a measurement error. Self-heating is typically specified as the amount of power that will raise the RTD’s temperature by 1°C (in mW ⋅ °C−1). To minimize self-heating-caused error, the smallest possible excitation current (1 mA or less) should be used in measurement. The amount of self-heating also depends greatly on the medium in which the RTD is immersed. For example, an RTD can self-heat up to 100 times higher in still air than in moving water. In addition, the temperature across the RTD must be uniform. Otherwise, an error will appear, which is directly related to the difference in temperature between the leads of the RTD.
RTDs can be difficult to measure due to their relatively low resistance (e.g., 100 Ω), which changes only slightly with temperature (<0.4 Ω ⋅ °C−1). To accurately measure these small changes in resistance, special configurations should be used to minimize errors from lead wire resistance.