Sketch arrangement: A differential pressure (DP) transmitter is connected across the boiler drum/ water column. Two impulse lines run from the boiler: one from the steam space (top) and one from the water space (bottom) of the drum/ water column. The steam-side line is connected to a reference (wet) leg - a vertical pipe that is kept full of water (condensate) to a constant head - and the water-side line connects to the water space. The DP transmitter measures the difference between the constant reference head (steam leg) and the variable water head (water leg). As the boiler water level falls, the water-leg head decreases, so the differential pressure increases; as the level rises, the differential decreases. The transmitter converts this DP into a standard signal (e.g. 4-20 mA) proportional to water level, which is fed to the level controller. The controller compares the measured level with the set-point and operates the feed-water control valve (or feed pump) to maintain the level by adding or reducing feed water. A three-element control (level, steam flow, feed flow) is often used for better response.

1. Isolate the transmitter from the boiler (close the isolating valves on both impulse lines) and vent/ drain the lines safely.
2. Remove the old transmitter and fit the new unit, ensuring the correct orientation and that the impulse connections are correct (high side to the reference/ steam leg, low side to the water leg, or as per the maker's marking).
3. Zero/ span calibration: With the transmitter isolated and both sides at atmospheric pressure (or the reference leg filled to the correct head), apply the known reference conditions. Using a hand pump/ dead-weight tester or a calibrated pressure source, apply the DP values corresponding to the minimum and maximum water level (e.g. 0% and 100% level) and adjust the zero and span (range) adjustments on the transmitter so the output (4-20 mA) reads the correct level at each point.
4. Check linearity at intermediate points (e.g. 25, 50, 75%) and adjust if necessary.
5. Re-connect the impulse lines, open the isolating valves, and verify the transmitter reads the actual boiler water level (compare with the gauge glass/ sight glass).
6. Verify the controller receives the correct signal and that the feed valve responds; carry out a functional test of the level control loop.
7. Record the calibration on the calibration sheet and set the alarm/ trip set-points as required.