Resistance Temperature Devices | Sensor & Industrial Instrumentation - Electronics and Communication Engineering (ECE) PDF Download

Resistance Temperature Devices

RTDs are either a metal film deposited on a form or are wire-wound resistors, which are then sealed in a glass-ceramic composite material [2]. Figure 10.4 shows a three-wire RTD encased in a stainless steel sheath for protection. The coil is wound to be noninductive. The space between the element and the case is filled with a ceramic power for good thermal conduction. The element has three leads, so that correction can be made for voltage drops in the lead wires. The electrical resistance of pure metals is positive, increasing linearly with temperature. Table 10.6 gives the temperature coefficient of resistance of some common metals used in resistance thermometers. Platinum is the first choice, followed by nickel. These devices are accurate—temperature changes of a fraction of a degree can be measured. The RTD can be used to measure temperatures from −300° to +1,400°F (−170° to +780°C). The response time is typically between 0.5 and 5 seconds.Figure 10.4 Cross section of a typical three-wire RTD.

Table 10.6 Temperature Coefficient of Resistance of Some Common Metals:

In a resistance thermometer, the variation of resistance with temperature is given by:

where  is the resistance at temperature T₂, and R_T is the resistance at temperature T₁.