The table is an example of system accuracy for a process with a critical temperature of 250°F (121°C) using a transmitter with ±0.18°F (0.10°C) accuracy. The matched sensor and transmitter accuracy of ±0.198°F (0.11°C) is more than three times better than a standard grade A sensor with ±0.63°F (0.35°C) accuracy and seven times better than the grade B sensor. This differential gets larger as the temperature increases or decreases from 0°F (0°C).

One method that eliminates up to 85 percent of the interchangeability error in a temperature measurement system is to use a platinum resistance thermometer (PRT) sensor in conjunction with a transmitter that has matching capability.

Transmitters with matching capability allow a specific resistance (R) vs. temperature (T) relationship to be entered into the transmitter software, according to the temperature measurement experts at Burns Engineering, Minnetonka, Minn. In the case of an analog transmitter, the potentiometers for zero and span are adjusted to match the unique PRT resistance at the endpoints of the range. This method will nearly eliminate the interchangeability error, but errors due to calibration and some external influence effects still will be present.

When the matched calibration option is specified, there is no need to select a PRT with tight interchangeability because the actual R vs. T relationship, which is determined by a comparison calibration, is used to calibrate the transmitter. In that case, a less expensive grade B sensor can specified. The table shows the improvement in accuracy that can be achieved with this method. For more information, contact Burns Engineering at