An RTD (resistance temperature detector) is a temperature sensor that operates on the measurement principle that a material’s electrical resistance changes with temperature. The relationship between an RTD’s resistance and the surrounding temperature is highly predictable, allowing for accurate and consistent temperature measurement. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD’s resistance can be calculated, and the temperature can be determined.
Different materials used in the construction of RTDs offer a different relationship between resistance and temperature. Temperature sensitive materials used in the construction of RTDs include platinum, nickel, and copper; platinum being the most commonly used. Important characteristics of an RTD include the temperature coefficient of resistance (TCR), the nominal resistance at 0 degrees Celsius, and the tolerance classes.
In addition to different materials, RTDs are also offered in two major configurations: wire wound and thin film. Wire wound configurations feature either an inner coil RTD or an outer wound RTD. An inner coil construction consists of a resistive coil running through a hole in a ceramic insulator, whereas the outer wound construction involves the winding of the resistive material around a ceramic or glass cylinder, which is then insulated.
The thin film RTD construction features a thin layer of resistive material deposited onto a ceramic substrate through a process called deposition. A resistive meander is then etched onto the sensor, and laser trimming is used to achieve the appropriate nominal values of the sensor. The resistive material is then protected with a thin layer of glass, and lead wires are welded to pads on the sensor and covered with a glass dollop.
An RTD takes a measurement when a small DC current is supplied to the sensor. The current experiences the impedance of the resistor, and a voltage drop is experienced over the resistor. Depending on the nominal resistance of the RTD, different supply currents can be used. To reduce self-heating on the sensor the supply current should be kept low. In general, around 1mA or less of current is used.
An RTD can be connected in a two, three, or four-wire configuration. The two-wire configuration is the simplest and also the most error prone. In this setup, the RTD is connected by two wires to a Wheatstone bridge circuit and the output voltage is measured. The disadvantage of this circuit is that the two connecting lead wire resistances add directly two the RTD’s resistance and an error is incurred.
There are many options when considering contact temperature measurement, including thermocouples, thermistors, and RTDs (wire wound and thin film). While thermocouples can handle very high temperatures and thermistors are inexpensive, there are many advantages of RTDs.
Some of these advantages include their accuracy, precision, long-term stability, and good hysteresis characteristics. Even beyond these, there are advantages of thin film RTDs over wire wound, including smaller dimensions, better response times, vibration resistance, and relative inexpensiveness. New advancements has even made thin film technology just as accurate as wire wound at higher temperatures ranges.