Mechanical Engineering Principles

236 MECHANICAL ENGINEERING PRINCIPLES

Atomospheric

pressure

Vacuum

Mercury

Barometric

hegiht,h

Trough

(a)

Atomospheric

pressure

Vacuum

Mercury

h

(b)

Figure 21.3

high exerts a pressure equal to the standard value of
atmospheric pressure.
There are thus several ways in which atmospheric
pressure can be expressed:
Standard atmospheric pressure

=101325 Pa or 101.325 kPa

=101325 N/m^2 or 101.325 kN/m^2

= 1 .01325 bars or 1013.25 mbars

=760 mm of mercury

Another arrangement of a typical barometer is
shown in Figure 21.3(b) where a U-tube is used
instead of an inverted tube and trough, the principle
being similar.
If, instead of mercury, water was used as the
liquid in a barometer, then the barometric height
hat standard atmospheric pressure would be 13.6
times more than for mercury, i.e. about 10.4 m high,

which is not very practicable. This is because the

relative density of mercury is 13.6.

Types of barometer

TheFortin barometeris an example of a mercury

barometer that enables barometric heights to be mea-

sured to a high degree of accuracy (in the order of

one-tenth of a millimetre or less). Its construction

is merely a more sophisticated arrangement of the

inverted tube and trough shown in Figure 21.3(a),

with the addition of a vernier scale to measure

the barometric height with great accuracy. A dis-

advantage of this type of barometer is that it is not

portable.

A Fortin barometer is shown in Figure 21.4. Mer-

cury is contained in a leather bag at the base of the

mercury reservoir, and height,H, of the mercury

in the reservoir can be adjusted using the screw at

the base of the barometer to depress or release the

leather bag. To measure the atmospheric pressure

the screw is adjusted until the pointer atH is just

touching the surface of the mercury and the height

of the mercury column is then read using the main

and vernier scales. The measurement of atmospheric

pressure using a Fortin barometer is achieved much

more accurately than by using a simple barometer.

Main

Vernier scale

scale

Barometric

height in

millimetres

of mercury

H

Mercury

reservoir

Figure 21.4