Overview

Temperature is a widely used measurement. Galileo is credited with inventing the first ther-mometer in 1595. Over the years thermometer technology has evolved, and measuring principles are continuously improved upon. Today, highly accurate and reliable devices are available. This chapter provides some of the basic knowledge users need to select the proper temperature-measuring device. However, it is essential that the instrument selector take into consideration the users' experiences.

For process applications, a typical temperature measurement assembly consists of a thermowell, a temperature element, sometimes extension/connecting wires, and a temperature transmitter (local or remote). Temperature elements frequently include a spring-loaded mechanism to ensure that the element tip makes positive contact with the internal bottom of the well.

Temperature elements should be installed where good mixing is ensured, such as in pipe bends and in the liquid phase (if a vapor/liquid interface exists). The optimum immersion length for temperature elements varies with the application. If they are installed perpendicular to the line, then the tip of the element should be between one-half and one-third the pipe diameter. If they are installed in an elbow (the recommended option), with the tip pointing towards the flow, about one-quarter pipe diameter is sufficient since the flow is impinging on the tip of the temperature element.