Page 161 - Instrumentation Reference Book 3E

Absolute gauges 145
is important. In general, it is desirable to match, as even under ideal conditions, the accuracy is rarely
far as possible, the physical processes in the gauge better than 2~20 percent, and in a carelessly oper-
with those in the vacuum apparatus. ated ionization gauge worse than 150 percent.
Representative values for the mid-range accuracy
of various gauges are given in Table 10.2 at the
10.1.4 Accuracy of measurement
end of this chapter along with other useful
Having chosen a suitable gauge, it is necessary to information.
ensure that the pressure in the gauge head is the
same as that in the vacuum apparatus. First. the
gauge head is connected at a point as close as 10.2 Absolute gauges
possible to the point where the pressure is to
be measured and by the shortest and widest 10.2.1 Mechanical gauges
tube available. Second, sufficient time must be
allowed for pressure equilibrium to be obtained. These gauges measure the pressure of gases and
This is particularly important when the pressure vapors by making use of the mechanical deform-
is below 1O-I Pa, and when ionization gauges, ation of tubes or diaphragms when exposed to
which interact strongly with the vacuum appar- a pressure difference. If one side of the sensitive
atus, are used. Times of several minutes are often element is exposed to a good vacuum, the gauge
required. When non-absolute gauges are used, is absolute.

Table 10.2 Properties of gauges
Gauge Pressure Accuracy Cost' Principal Principal
range (Pa) *% advantages limitations
Bourdon tube 105-102 10 A Simple. Robust. Poor accuracy below 100 Pa.
Quartz spiiral 105-10 10 B Reads differential Rather fragile.
pressures.
Diaphra,m 1 05-1 0 5 B Good general-purpose Zero setting varies
gauge.
Spinning rotor 10-io4 <5 F Sensitive and accurate Long response time
Liquid maiiiometers 10'1 02 5-10 A Simple. Direct reading. Vapor may contaminate
vacuum.
McLeod lo5-5 x SO-' 5-10 C Wide pressure range. Intermittent. Measures
Used for calibration. GAS pressures only.
Thermocouple 1 03-1 0-I 20 B Simple. Robust. Response not instantaneous.
Inexpensive. Reading depends on
gas species.
Pirani 103-10-2 10 C Robust Zero variation due to
filament contamination.
Reading depends on
gas species.
Thermistor 103-10-2 10 C Fast response. Low Reading depends on
current consumption gas species.
Discharge tube 103-10' 20 B Very simple. Robust. Limited pressure range.
Reading depends on
gas species.
Penning 1-10-5 10-20 C Sensitive. Simple. Large pumping effect.
Robust. Hysteresis.
Reading depends on
gas species.
Hot-cathode ion 10'10-6 10-30 E Sensitive linear Filament easily damaged.
scale. Instantaneous Needs skillful use.
response. Reading depends on
gas species.
Bayard-Albert 1-10-8 10-30 D As above, w-ith better As above.
low-pressure Reading depends on
performance. gas species.
Capacitance 105-1 0-3 <5 E Sensitive and accurate
nanometers
*Scale of costs (f): A 0-50; B 50-200; C 200400; D 400-600: E 1000-1.000: F 7000-10,000.