Mechanical Engineering Principles

238 MECHANICAL ENGINEERING PRINCIPLES

Measuring

scale

Gas

A

Mercury

BA B

h 1

(a) (b)

(c) (d)

Gas

ABAB

h 2 h

PA PB

Figure 21.7

to move as shown in Figure 21.7(b), such that the
difference in height ish 1. The measuring scale can
be calibrated to give the gauge pressure of the gas
ash 1 mm of mercury.
If limbAis connected to a container of gas whose
pressure is below atmospheric then the levels of
mercury will move as shown in Figure 21.7(c), such
that their pressure difference ish 2 mm of mercury.
It is also possible merely to compare two pres-
sures, say,PA andPB, using a U-tube manome-
ter. Figure 21.7(d) shows such an arrangement with
(PB−PA) equivalent tohmm of mercury. One
application of this differential pressure-measuring
device is in determining the velocity of fluid flow
in pipes (see Chapter 22).
For the measurement of lower pressures, water
or paraffin may be used instead of mercury in the
U-tube to give larger values ofhand thus greater
sensitivity.

Inclined manometers

For the measurement of very low pressures,
greater sensitivity is achieved by using an inclined

manometer, a typical arrangement of which is shown

in Figure 21.8. With the inclined manometer the

liquid used is water and the scale attached to the

inclined tube is calibrated in terms of the vertical

heighth. Thus when a vessel containing gas under

pressure is connected to the reservoir, movement

of the liquid levels of the manometer occurs. Since

small-bore tubing is used the movement of the liquid

in the reservoir is very small compared with the

movement in the inclined tube and is thus neglected.

Hence the scale on the manometer is usually used

in the range 0.2 mbar to 2 mbar.

Gas under pressure

h

Reservoir

Water

Scale

Inclined manometer

Figure 21.8

The pressure of a gas that a manometer is capable

of measuring is naturally limited by the length of

tube used. Most manometer tubes are less than

2 m in length and this restricts measurement to a

maximum pressure of about 2.5 bar (or 250 kPa)

when mercury is used.

21.9 The Bourdon pressure gauge

Pressures many times greater than atmospheric

can be measured by the Bourdon pressure gauge,

which is the most extensively used of all pressure-

indicating instruments. It is a robust instrument. Its

main component is a piece of metal tube (called the

Bourdon tube), usually made of phosphor bronze or

alloy steel, of oval or elliptical cross-section, sealed

at one end and bent into an arc. In some forms the

tube is bent into a spiral for greater sensitivity. A

typical arrangement is shown in Figure 21.9(a). One

end,E, of the Bourdon tube is fixed and the fluid

whose pressure is to be measured is connected to this

end. The pressure acts at right angles to the metal

tube wall as shown in the cross-section of the tube

in Figure 21.9(b). Because of its elliptical shape it