Page 162 - Instrumentation Reference Book 3E

146 Measurement of vacuum

10.3.1.1 The Bourdoiz tube gazige differing accuracies are also often available, and
A conventional gauge of this type can be used to the price normally reflects this.
measure pressure down to lOOPa if carefully
made. Its construction is described in Chapter 9 10.2.2 Liquid manometers
on pressure measurement.
These gauges measure the pressure of gases and
vapors from atmospheric to about 1 Pa by balan-
10.2.1.2 The quartz spiral gauge cing the force exerted by the gas or vapor against
This gauge measures differential pressures over a the weight of a column of liquid, usually mercury,
range of 100 Pa from any chosen reference pres- water, or oil. These devices provide the simplest
sure. It is suitable for use with corrosive gases or possible means of pressure measurement.
vapors.
Construction The construction of various forms
of liquid manometer is described in Chapter 9.
Constriiction The sensitive element is a helix of
0.5mm diameter quartz tubing usually 20mm in
diameter, 30mm long, to which the vacuum is Operotion For measuring relative pressures, the
applied internally. The helix coils and uncoils in open manometer shown in Figure 10.l(a) may be
response to pressure changes, the motion being used. In this case the difference h in levels may be
measured by observing the movement of a light taken to express the vacziziin directly in inches of
spot reflected from a small mirror attached to its water, or by use of the formula p = hpg, where p is
lower end. The whole assembly is mounted in a the density of the liquid, and g is the acceleration
clear glass or quartz enclosure. which can be due to gravity, the vacuum may be expressed in SI
brought to any desired reference pressure. If this units. The measurement of absolute pressures
is zero, the gauge indicates absolute pressure. requires a vacuum reference which may be
obtained in several ways as shown in Figures
lO.l(b)-(d). A barometer tube is used
10.2.1.3 Diaphragm gauge in diagram (b), immersed in the same liquid pool,
This gauge measures pressures of gases and vapors in this case usually mercury, as the manometer
down to 10 Pa. Its construction is described in Chap- tube. A more compact form is provided by the
ter 9. closed manometer, shown in (c). This again uses
mercury, and the space in the closed limb is evacu-
ated. A useful version of the closed manometer
10.2.1.4 Capacitance iiianometess which can be used with oil is shown in (d), where
Capacitance manometers are among the most the tap may be opened when the apparatus is at
accurate and sensitive gauges available for the zero pressure. and closed immediately before tak-
measurement of medium vacuum (see Chapter 9 ing measurements. When oil is used in vacuum
for a description of their operation). Gauge heads measurement, difficulty will be experienced with
are available with different full-scale ranges and the liberation of dissolved gases, which must be
usually operate over 3 to 4 decades. Heads with removed slowly by slow reduction of the pressure.

VACUUM VACUUM

Figure 10.1 Liquid manometers.