Chapter 4 - RTD Temperature Sensors

Abstract

This chapter explores RTD temperature sensors. A resistance temperature detector (RTD) is a temperature sensing device whose resistance increases with temperature. RTDs are quite linear devices and have a typical temperature-resistance characteristic. RTDs operate on the principle that the electrical resistance of metals change with temperature. Although, in theory any kind of metal can be used for temperature sensing, in practice, metals with high melting points that can withstand the effects of corrosion, and those with high resistivities are chosen. The most commonly used RTDs are made of either nickel, platinum, or nickel alloys. Nickel sensors are used in cost sensitive applications such as consumer goods and they have a limited temperature range. RTDs have excellent accuracies over a wide temperature range and some RTDs have accuracies better than 0.001°C. Another advantage of RTDs is that they drift less than 0.1°C/year. RTDs are resistive devices and a current must pass through the device so that the voltage across the device can be measured. This current can cause the RTD to self-heat and consequently, it can introduce errors into the measurement. Self-heat can be minimized by using the smallest possible excitation current. The amount of self-heat also depends on where and how the sensor is used. An RTD can self-heat much quicker in still air than in a moving liquid.