Chapter 2 - Temperature and its Measurement

Abstract

This chapter describes the temperature scales and the types of sensors and their comparisons. Temperature is one of the most important parameters in process control. Accurate measurement of the temperature is not easy and to obtain accuracies better than 0.5°C great care is needed. Errors occur due to several sources such as the sensor non-linearities, temperature gradients, calibration errors, and poor thermal contact. There are many types of sensors to measure the temperature. Some sensors, such as the thermocouples, resistance temperature detectors (RTDs), and thermistors are the older classical sensors and they are used extensively due to their big advantages. The new generation of sensors, such as the integrated circuit sensors and radiation thermometry devices, are popular only for limited applications. RTDs are used in medium range temperatures, ranging from -200–+600°C. They offer high accuracy, typically ±0.2°C. RTDs can usually be used in most chemical and physical environments, but they are not as robust as thermocouples. The operation of RTDs requires external power. The selection of appropriate sensor is not always easy and it depends on factors such as the accuracy, the temperature range, speed of response, thermal coupling, the environment (chemical, electrical, or physical), and the cost. Environmental applications, electronics hobby market, and automotive industries generally use thermistors or integrated circuit sensors. RTDs are commonly used in lower temperature, and higher precision chemical industries.