Pressure transmitter accuracy

This paper discusses the key causes of calibration drift in pressure transmitters and procedures for calibrating pressure transmitters to ensure their accuracy. Calibrating pressure transmitters involves adjusting the potentiometers in the sensor that controls the zero (lowest pressure at which a transmitter is calibrated) and span (the range of pressure the transmitter is to indicate) of the transmitter. The initial or bench calibration of pressure transmitters involves using a constant pressure source such as a deadweight tester. Once the transmitters are installed, temperature, pressure, humidity, vibration, maintenance activities, and normal aging can degrade their accuracy. Transmitter accuracy can also be degraded by transmitter sensing lines, when the water in a sensing line reference leg boils off, when non-condensable gases in the reference leg dissolve, and when voids, blockages, freezing, or leakage occur in sensing lines. On-line calibration techniques enable plants to avoid these accuracy problems by monitoring the output of an individual transmitter.