To calibrate a HART instrument consistent with its application, it is very helpful to understand the functional structure of a typical HART transmitter. HART instruments consist of three distinct sections (see Figure ).
Proper HART calibration may involve either or both sensor trim and output trim. Adjusting range values (LRV and URV) without a calibrator is not calibration.
Sensor trim involves applying a known process variable (temperature or pressure for example) to the input of the transmitter and adjusting the Input Section so the PV agrees with the input applied.
Output trim involves commanding the transmitter to output both a 4 and 20 mA signal. These signals are then measured with a precise mA measurement tool. The Output Section is then adjusted to agree with the measurements.
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