Glass Single Limb Manometer With Connection Size 1/4 inch BSP are used for precision measurement of pressure, vacuum, differentials pressure flow and liquid level measurement. It works on the principle of hydrostatic equilibrium and is used for measuring the pressure (static pressure) exerted by a still liquid or gas. Hydrostatic equilibrium states that the pressure at any point in a fluid at rest is equal, and its value is just the weight of the overlying fluid.