Page 13 - Instrumentation and Measurement

There are several ways in which atmospheric pressure can be expressed:
Standard atmospheric pressure = 101 325 Pa or 101.325 kPa.

= 101 325 N/m2 or 101.325 kN/m2
= 1.01325 bars or 1013.25 mbars.
Many of the devices used to monitor fluid pressure in industrial processes involve the monitoring of
the elastic deformation of diaphragms, bellows, and tubes. The following are some common
examples of such sensors.
2.2.1 Manometers
Manometers are good examples of pressure measuring instruments, though they are not as common
as they used to be because of the development of new, smaller, more rugged, and easier to use
pressure sensors.
U–tube manometers consist of U-shaped glass tubes partially filled with a liquid. When there are
equal pressures on both sides, the liquid levels will correspond to the zero point on a scale as shown
in Fig. 2.1a. The scale is graduated in pressure units. When a pressure is applied to one side of the
U-tube that is higher than on the other side, as shown in Fig. 2.1b, the liquid rises higher in the lower
pressure side, so that the difference in the heights of the two columns of liquid compensates for the
difference in pressure. The pressure difference is given by
PR - PL =  x difference in height of the liquid in the columns

Figure 2.1 Simple U-tube manometers with (a) no differential pressure and (b) higher pressure on
the left side.
where g is the specific weight of the liquid in the manometer.

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