Pressure Transmitters
Pressure transmitters are the most common instrument in process plants. Learn the types, how they work, calibration, and common failure modes.
Types of Pressure Transmitters
- Gauge pressure: Measures pressure relative to atmospheric pressure. The most common type. Reading is zero at atmospheric pressure.
- Absolute pressure: Measures pressure relative to a perfect vacuum. Used for vacuum applications and where atmospheric pressure varies significantly.
- Differential pressure: Measures the difference between two pressure points. Used for flow measurement (orifice plates), level measurement, and filter condition monitoring.
How They Work
Most modern pressure transmitters use a piezoresistive or capacitance sensing element. Pressure deflects a thin diaphragm, and the deflection is converted to an electrical signal. The transmitter conditions this signal and outputs a standard signal: 4-20 mA, 0-10V, or a digital protocol (HART, Profibus, Foundation Fieldbus).
The 4-20 mA signal is the industry standard. 4 mA represents 0% of range, 20 mA represents 100%. A reading below 4 mA or above 20 mA indicates a fault (broken wire, sensor failure).
Calibration
Pressure transmitters drift over time. Annual calibration is recommended for most applications. Calibration involves applying known pressures (using a deadweight tester or calibrated hand pump) and adjusting the transmitter's zero and span to match.
Five-point calibration (0, 25, 50, 75, 100% of range) in both ascending and descending directions checks for linearity and hysteresis. Record the results and compare to the manufacturer's specification.
Calibration interval
Annual calibration is standard for most process pressure transmitters. Critical measurements (safety-related, custody transfer, high-accuracy) may require 6-monthly calibration. Non-critical transmitters may extend to 2-yearly. Base the interval on the manufacturer's recommendation, regulatory requirements, and historical drift data.
Common Problems
- Zero drift: The transmitter reads non-zero when pressure is zero. Caused by temperature changes, vibration, or ageing. Corrected by zero adjustment.
- Plugged impulse line: The line connecting the process to the transmitter becomes blocked with scale or sediment. The transmitter freezes at the last reading. Clean or rod the impulse line.
- Moisture in electronics: Water enters the transmitter housing, corroding the electronics. Causes erratic readings or complete failure. Ensure proper sealing and conduit drainage.
- Overpressure damage: Exposure to pressure above the transmitter's rated range damages the sensing element. Install a pressure relief or snubber for pulsating pressures.
- Signal interference: Electromagnetic interference from VFDs or radio transmitters corrupts the 4-20 mA signal. Use shielded twisted pair cable and proper grounding.
Tip: Keep a calibration record for every transmitter. Record the as-found and as-left readings, the date, and the technician. Trending the as-found data shows which transmitters drift most, allowing you to optimise calibration intervals. PlantLogica's compliance module can track calibration dates and send reminders.