Page 160 - Instrumentation Reference Book 3E

IO Measurement of vacuum

D. J. PACEY

10.1 Introduction Many technological applications of vacuum
use the long free paths, or low molecular inci-
10.1.1 Systems of measurement dence rates that vacuum makes available. These
require pressures that are only a very small frac-
The measurement of vacuum is the measurement tion of atmospheric, where the force exerted by
of the range of pressures below atmospheric. the gas is too small to be measured, making abso-
Pressure is defined as force divided by area. lute gauges unusable. In such cases non-absolute
The American National Standard unit is the gauges are used which measure pressure indir-
pound/inch2 or psi, and the SI unit is the ectly by measuring a pressure-dependent physical
newton/ineter2(Nm-2) or pascal (Pa). Pressure property of the gas, such as thermal conductivity,
may also be stated in terms of the height of a ionizability, or viscosity. These gauges always
column of a suitable liquid, such as mercury or require calibration against an absolute gauge,
water, that the pressure will support. The relation for each gas that is to be measured. Commercial
between pressure units currently in use is shown gauges are usually calibrated by the manufacturer
in Table 10.1. using dry air, and will give true readings only
In engineering, it has long been customary to when dry air is present. In practice it is difficult
take atmospheric pressure as the reference, and to to be certain of the composition of the gases in
express pressures below this as "pounds per square vacuum apparatus, thereby causing errors. This
inch of vacuum," or "inches of vacuum" when using problem is overcome in the following way. When
a specified liquid. The continual changes in atmos-
pheric pressure, however, will lead to inaccuracy a gauge using variation of thermal conductivity
indicates a pressure of lO-'Pa, this would be
unless they are allowed for. It is preferable to use recorded as an equivalent dry air pressure of
zero pressure as the reference, and to measure pres- lo-' Pa. This means that the thermal conductiv-
sure above this. Pressures expressed in this way are
called absolute pressures. ity of the unknown gases present in the vacuum
apparatus has the same value as that of air at
lo-' Pa, and not that the pressure is lo-' Pa.
10.1.2 Methods of measurement
Since pressure is defined to be force/area, its meas- 10.1.3 Choice of non-absolute gauges
urement involves directly or indirectly the meas- Since the gauge referred to above measures ther-
urement of the force exerted upon a known area. mal conductivity, it is particularly useful for use
A gauge that does this is called an absolute gauge, on vacuum apparatus used for making vacuum
and allows the pressure to be obtained from a flasks, or in which low-temperature experiments
reading and known physical quantities associated are carried out, and in which thermal conductiv-
with the gauge, such as areas, lengths, sometimes ity plays an important part. Similarly an ioniza-
temperatures, elastic constants, etc. The pressure tion gauge would be suitable in the case of
when obtained is independent of the composition apparatus used for making radio valves and cath-
of the gas or vapor which is present. ode ray tubes in which the ionizability of the gases

Table 10.1 Relation between pressure units
Nlin'(Pa) torr mb atrn
Nim' (Pa) 1 7.50 x 10-3 10-2 9.87 x
torr 133.3 1 1.333 1.316 x
mb 100 0.750 1 9.87 x 10-4
atm 1.013 x lo5 760 1.013 x lo3 1