Level Transmitters

Types of Level Measurement

• Differential pressure (DP): Measures the hydrostatic pressure at the bottom of the tank. Pressure = liquid height x density x gravity. Simple, reliable, and works for most liquids. Needs to account for liquid density.
• Ultrasonic: Sends an ultrasonic pulse from the top of the tank and measures the time for the echo to return. Non-contact. Good for corrosive or dirty liquids. Affected by vapour, foam, and turbulence.
• Radar (guided wave and non-contact): Uses radar pulses instead of ultrasonic. Not affected by vapour or density changes. More expensive than ultrasonic but more reliable in difficult applications.
• Float and cable: A float on the liquid surface is connected by a cable to a measuring mechanism. Simple and direct. Mechanical, so subject to wear.
• Capacitance: Measures the capacitance between a probe and the tank wall. Liquid changes the dielectric, changing capacitance. Good for non-conductive liquids. Affected by coating and buildup.
• Hydrostatic pressure: A pressure transmitter at the bottom of the tank measures the pressure, which is proportional to liquid height. Same principle as DP but uses a single pressure measurement.

Boiler Level Measurement

Boiler drum level is one of the most critical measurements in a steam plant. Too low: tubes overheat and rupture. Too high: water carryover damages equipment. Two methods are used:

• Differential pressure: A DP cell measures the difference between the water column (reference leg) and the drum pressure. Accurate but affected by density changes (at different pressures, water density changes, affecting the calibration).
• Conductivity (boiler gauge glass): A series of electrodes at different heights detect water level by conductivity. Simple, direct, and reliable. Used for level indication and low-water alarm.

Common Problems

• Density changes: DP level measurement assumes constant liquid density. If temperature or composition changes, the reading drifts. Compensate with temperature measurement.
• Foam and turbulence: Ultrasonic and radar meters can give false readings if the liquid surface is foamy or turbulent. Use guided wave radar for better performance in these conditions.
• Coating and buildup: Capacitance and float-type meters can be affected by material buildup on the probe or float. Clean regularly.
• Vapour interference: Ultrasonic meters can be affected by vapour above the liquid. Radar is less affected.
• Reference leg temperature: For boiler DP level measurement, the reference leg (constant head) must be at a known temperature. If it cools, the water density changes, causing level error. Insulate the reference leg.