Manometers

A manometer is a device used to measure pressure by balancing the weight of a liquid column against the pressure difference to be measured. The pressure difference is inferred from the height difference of the liquid column, with ΔP = ρ g h for a fluid of density ρ. Manometers can measure gauge, absolute, or differential pressures and are commonly read in pascals, kilopascals, millimeters of mercury (mmHg), or inches of water.

The simplest form is a U-shaped tube partially filled with a liquid, often mercury or water. One leg is exposed to the test pressure, the other to atmospheric pressure or another reference pressure. The vertical separation of the liquid surfaces corresponds to the pressure difference. Inclined manometers place the tube at a small angle to increase sensitivity, so small changes yield larger height changes.

Differential manometers have two pressure ports and indicate the difference between them. They may use a single column or multiple reservoirs connected by tubes. Mercury-based designs have fallen out of favor in many settings due to toxicity; water, alcohol, or silicone oil are common alternatives. Accurate readings require bubble-free fluid, proper venting, and calibration for temperature effects.

Manometers have the advantages of simplicity, direct reading, and no power requirement, making them useful in laboratories, HVAC systems, and meteorology. Limitations include limited measurement ranges, slow response to rapid changes, and the need for a stable, vertical installation. Modern variants include digital manometers and pressure transducers that convert the fluid-column signal into electrical readings.