Manometers

Manometers are differential pressure sensors. A differential pressure sensor measures the difference between a pressure being applied to it and a reference pressure (often atmospheric pressure). The U-tube manometer consists of a clear glass or plastic tube shaped into the form of a ‘U’. The tube is partially filled with a liquid, such as water, alcohol, or mercury. The lower density of the liquid results in higher sensitivity of the manometer. A pressure difference across the tube causes the liquid to shift position. The change in position can be measured to give the pressure. It is best suited to static pressure measurement. It is difficult to use for small pressure changes and unsuitable for very large pressures.

Figure 18.3 (a) Manometer Measuring Pressure of a Fluid, (b) Manometer Measuring Pressure Difference Between Two Fluids, and (c) Inclined Tube Manometer

In Figure 18.3 (a), the pressure of the fluid can be measured as

 

P = ρgh₂

 

where ρ is liquid density in manometer.

In Figure 18.3 (b), the pressure difference between two fluids can be measured as

 

P₁ − P₂ = ρgz₂, if D >> d

 

In Figure 18.3 (c), the pressure of the fluid in inclined manometer can be measured as

 

P = ρgz₂ = ρgh sin θ