A resistance temperature detector is a passive circuit element whose resistance increases as temperature increases. They are generally constructed using platinum, copper, or nickel, and one major advantage of RTDs is that they support a wide span of temperature, ranging from –200°C to +850°C. The accuracy limits of an RTD are defined by the class, or grade, of the RTD. The characteristics of platinum, copper, or nickel determine the linear approximation of resistance versus temperature within the 0°C to 100°C temperature range. The platinum RTD is known for its strong linearity and repeatability characteristic.

DIN/IEC 60751 is considered the worldwide standard for platinum RTDs. For a PT100 RTD, the standard requires the sensing element to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C between 0°C and 100°C.

Equation 1 and Equation 2 define the resistance-to-temperature relation for temperature ranges above and below 0°C.

with:

• A = 3.9083 × 10^–3
• B = –5.775 × 10^–7
• C = –4.183 × 10^–12

RTDs are typically designed with three common configurations: two-wire, three-wire, and four-wire. In the two-wire configuration, as shown in Figure 1-1 below, the RTD is connected with two wires to either end of the RTD. In this configuration, the lead wire resistances cannot be separated from the RTD resistance, adding an error that cannot be separated from the RTD measurement. Two-wire RTDs yield the least accurate RTD measurements and are used when accuracy is not critical or when lead lengths are short. Two-wire RTDs are the least expensive RTD configuration.

In the three-wire configuration, as shown in Figure 1-2, the RTD is connected to a single lead wire on one end and two lead wires on the opposite end. Using different circuit topologies and measurements, lead resistance effects can effectively be cancelled, reducing the error in three-wire RTD measurements. Compensation for lead wire resistance assumes that the lead resistances match.

In the four-wire configuration, as shown in Figure 1-3, two lead wires are connected to either end of the RTD. In this configuration, the RTD resistance may be measured with a four-wire resistive measurement with superior accuracy. The RTD excitation is driven through one lead on either end, while the RTD resistance is measured with the other lead on either end. In this measurement, the RTD resistance is sensed without error contributed from the lead wire reacting with the sensor excitation. Four-wire RTDs yield the most accurate measurements, but are the most expensive RTD configuration.

There are four tolerance classes specified in DIN/IEC751:

These tolerance classes also represent the interchangeability of a detector. Should a detector become damaged, good interchangeability assures that the replacement sensor delivers the same readings under the same conditions as the predecessor. Another important criterion for selecting a temperature sensor is the long-term stability. Great stability produces little output signal drift over time, thus reducing the frequency of costly calibrations. Depending on the application requirement, today’s RTDs can provide long-term drifts from as little as 0.003°C/year up to 0.01 and 0.05°C/year.

RTDs are considered to be amongst the most accurate temperature sensors available. In addition to high accuracy, they offer excellent stability, repeatability, and high immunity to electrical noise. However, for each class, the accuracy varies as the temperature changes. Figure 1-4 shows the accuracy of different classes of RTDs.