How to Measure Pressure using a U-tube Manometer

There are three categories of pressure measurement, namely absolute pressure, gauge pressure and differential pressure. The absolute pressure is the differences between the pressure at a particular point in a fluid and the absolute zero of pressure, that is, a complete vacuum. When the pressure measuring device measures the difference between the unknown pressure and local atmospheric pressure, the measurement is known as gauge pressure. On the other hand, when the pressure measuring device measures the difference between two unknown pressures, neither of which is atmospheric pressure, then the measurement is known as the differential pressure. The simplest pressure measurement method involves balancing the unknown pressure against the pressure produced by a column of liquid of known density using an instrument such as the U-tube manometer.

U-tube manometers consist of ‘U’ shaped glass tubes partially filled with a liquid as shown below:

Figure 1.0: Simple U-tube manometers, with (a) no differential pressure and (b) higher pressure on the left side.

When there are equal pressures on both sides, the liquid levels will corresponds to zero point on a scale as shown in figure 1.0 (a) above. The scale is graduated in pressure units. When a higher pressure is applied to one side of the U-tube as illustrated in figure 1.0 (b) above, the liquid rises higher in the lower pressure side, so that the difference in height of the two columns of liquid compensates for the difference in pressure. The pressure difference is given by:

P (gauge pressure) = P_R – P_L = γ x difference in height of the liquid in the columns

Where γ is the specific weight of the liquid in the manometer; Gauge pressure because the atmospheric pressure is superimposed on each manometer leg measurement as demonstrated below:

Related articles:

John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.