Mechanical Engineering Principles

HYDROSTATICS 239

is clear that the sum of the pressure components,
i.e. the total force acting on the sidesA andC,
exceeds the sum of the pressure components acting
on endsBandD. The result is that sidesAand
Ctend to move outwards andB andD inwards
tending to form a circular cross-section. As the
pressure in the tube is increased the tube tends to
uncurl, or if the pressure is reduced the tube curls
up further. The movement of the free end of the
tube is, for practical purposes, proportional to the
pressure applied to the tube, this pressure, of course,
being the gauge pressure (i.e. the difference between
atmospheric pressure acting on the outside of the
tube and the applied pressure acting on the inside
of the tube). By using a link, a pivot and a toothed
segment as shown in Figure 21.9(a), the movement
can be converted into the rotation of a pointer over
a graduated calibrated scale.

Graduated

scale

Toothed

segment

Pointer

Link

Pivot

E

Bourdon
tube

Fluid pressure

to be measured

(a)

A

C

(b)

D Pressure B

Figure 21.9

The Bourdon tube pressure gauge is capable of
measuring high pressures up to 10^4 bar (i.e. 7600 m

of mercury) with the addition of special safety

features.

Weights

Test

gauge

Oil in cylinder Piston

A dead weight tester

Figure 21.10

A pressure gauge must be calibrated, and this

is done either by a manometer, for low pressures,

or by a piece of equipment called a‘dead weight

tester’. This tester consists of a piston operating in

an oil-filled cylinder of known bore, and carrying

accurately known weights as shown in Figure 21.10.

The gauge under test is attached to the tester and a

screwed piston or ram applies the required pressure,

until the weights are just lifted. While the gauge is

being read, the weights are turned to reduce friction

effects.

21.10 Vacuum gauges

Vacuum gauges are instruments for giving a visual

indication, by means of a pointer, of the amount

by which the pressure of a fluid applied to the

gauge is less than the pressure of the surrounding

atmosphere. Two examples of vacuum gauges are

the McLeod gauge and the Pirani gauge.

McLeod gauge

The McLeod gauge is normally regarded as a stan-

dard and is used to calibrate other forms of vacuum

gauges. The basic principle of this gauge is that it

takes a known volume of gas at a pressure so low

that it cannot be measured, then compresses the gas

in a known ratio until the pressure becomes large

enough to be measured by an ordinary manometer.

This device is used to measure low pressures, often

in the range 10−^6 to 1.0 mm of mercury. A disad-

vantage of the McLeod gauge is that it does not give

a continuous reading of pressure and is not suitable

for registering rapid variations in pressure.