Temperature transmitters are by far the most difficult to calibrate because of the difficulty in generating precise controlled temperatures. Calibration of pneumatic filled-bulb type temperature transmitters is normally a lengthy test bench procedure. Temperature calibrators for generating process temperatures are of three varieties: (1) electrically heated direct air, (2) hot oil bath, and (3) a gas fluidized bed using a thermally conductive powder. All these require a considerable amount of time (20-40 minutes avg.) to reach a steady state temperature. Electronic temperature transmitters which use resistance temperature devices (RTD) have an advantage in that the sensing element calibration needs only a single point check and the transmitter can be calibrated electronically using the known intrinsic properties of the sensor. Field checking of these transmitters is practical.
Thermocouple transmitter calibration requires an accurate millivolt source to simulate the thermocouple signal at the upper and lower limits of the temperature range of the transmitter being calibrated. However, since all thermocouple tables show millivolt readings based on some reference temperature, usually 32°F, some sort of reference temperature compensation must be done. Historically, a reference junction in an ice bath was used to provide ice point compensation. Also a reference junction at ambient temperature was often used in conjunction with a thermometer to read the ambient temperature and provide a reference millivoltage. Modern devices make temperature transmitter calibration much simpler because of built-in reference compensation. Some digital devices merely require the entering of the desired temperature and they provide the correct millivoltage to simulate a thermocouple with automatic reference compensation. Also, compact electronic modules are available for ice point compensation and thermocouple linearization. Some electronic calibrators supply the millivolt signal as well as power for two-wire transmitters and indicators. The millivolt source is set to the values which correspond to the upper and lower temperature limits of the range of the transmitter and the transmitter is adjusted so that it produces the desired zero-scale (3 psi, 4 ma, etc.) and full-scale (15 psi, 20 ma, etc.) outputs. When field checking electronic devices, all safety codes must be observed. Calibrators which are battery powered and intrinsically safe are recommended.