Mechanical Engineering Principles

HYDROSTATICS 237

Sealed

vessel S

Pointer

Pivot

Scale

Atmospheric

Pressure

Figure 21.5

A portable type often used is theaneroid barom-
eter. Such a barometer consists basically of a cir-
cular, hollow, sealed vessel,S, usually made from
thin flexible metal. The air pressure in the vessel
is reduced to nearly zero before sealing, so that
a change in atmospheric pressure will cause the
shape of the vessel to expand or contract. These
small changes can be magnified by means of a lever
and be made to move a pointer over a calibrated
scale. Figure 21.5 shows a typical arrangement of
an aneroid barometer. The scale is usually circu-
lar and calibrated in millimetres of mercury. These
instruments require frequent calibration.

21.7 Absolute and gauge pressure

A barometer measures the true or absolute pres-
sure of the atmosphere. The term absolute pressure
means the pressure above that of an absolute vac-
uum (which is zero pressure), as stated earlier. In
Figure 21.6 a pressure scale is shown with the line
ABrepresenting absolute zero pressure (i.e. a vac-
uum) and lineCDrepresenting atmospheric pres-
sure. With most practical pressure-measuring instru-
ments the part of the instrument that is subjected
to the pressure being measured is also subjected
to atmospheric pressure. Thus practical instruments
actually determine the difference between the pres-
sure being measured and atmospheric pressure. The
pressure that the instrument is measuring is then
termed the gauge pressure. In Figure 21.6, the line
EFrepresents an absolute pressure which has a value
greater than atmospheric pressure, i.e. the ‘gauge’
pressure is positive.

Pressure

scale

E

C

AB

D

Positive gaugeF

pressure Atmospheric pressure

Negative gauge

pressure

Absolute

pressure

Absolute

pressure

GH

Absolute zero pressure

(complete vacuum)

Figure 21.6

Thus,absolute pressure

=gauge pressure+atmospheric pressure.

Hence a gauge pressure of, say, 60 kPa recorded

on an indicating instrument when the atmospheric

pressure is 101 kPa is equivalent to an absolute

pressure of 60 kPa+101 kPa, or 161 kPa.

Pressure-measuring indicating instruments are ref-

erred to generally aspressure gauges(which acts

as a reminder that they measure ‘gauge’ pressure).

It is possible, of course, for the pressure indi-

cated on a pressure gauge to be below atmospheric

pressure, i.e. the gauge pressure is negative. Such a

gauge pressure is often referred to as a vacuum, even

though it does not necessarily represent a complete

vacuum at absolute zero pressure. Such a pressure

is shown by the lineGHin Figure 21.6. An indicat-

ing instrument used for measuring such pressures is

called avacuum gauge.

A vacuum gauge indication of, say, 0.4 bar means

that the pressure is 0.4 bar less than atmospheric

pressure. If atmospheric pressure is 1 bar, then the

absolute pressure is 1− 0 .4or0.6bar.

21.8 The manometer

A manometer is a device for measuring or compar-

ing fluid pressures, and is the simplest method of

indicating such pressures.

U-tube manometer

A U-tube manometer consists of a glass tube

bent into a U shape and containing a liquid such

as mercury. A U-tube manometer is shown in

Figure 21.7(a). If limbAis connected to a container

of gas whose pressure is above atmospheric, then the

pressure of the gas will cause the levels of mercury