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VACUUM TECHNOLOGY

VACUUM TECHNOLOGY 7.3

column of liquid in the barometer would be observed to change. A reproduction of the barometers
made in the 1600s is shown on the following website: http://www.barometers.com/torricel.htm. The
measurement of pressure by this method is accurate only to approximately +/− 1 torr.
U-Tube Manometer. Developed following the liquid barometer, the U-tube manometer allows one
to measure pressure in two vessels or between a vessel and atmospheric pressure. The difference in
the heights of liquid in the U tube’s two columns provides a direct reading of the pressure difference
in the two sides of the U tube. Measurement of pressure by this method is accurate only to approx-
imately +/− 1 torr.

McLeod Gauge. The McLeod gauge was developed to extend the range of pressures that could be
measured using liquid-displacement gauges. In this device, a known volume of gas is isolated from
the vacuum vessel under study by tipping the McLeod gauge such that liquid mercury traps the gas
in a bulb of known volume. As the McLeod gauge is turned upright, the trapped gas is compressed
to a known amount and the pressure of the compressed gas is measured by comparing the levels of
mercury in the tubes of the device and applying Boyle’s law (P × V = P × V ). Pressure readings
1 1 2 2
−7
using a McLeod gauge can be accurate to pressures as low as 10 torr.
Bourdon-Tube Gauge. This gauge design uses the elastic deformation of a thin-walled metal tube to
sense pressure and display on a circular dial. The sensing element is not unlike a popular party favor
that unravels as one exhales into the mouthpiece. The interior of the Bourdon tube is connected to the
interior volume of a vacuum vessel. As the pressure in the vessel increases, the Bourdon tube responds
by elastically deforming. This motion is translated into the rotation of a dial indicator on the gauge face.
Diaphragm Gauge. This gauge design utilizes the elastic deformation of a diaphragm to measure
pressure. As the pressure inside the gauge tube is reduced, the diaphragm is elastically deformed and
this deformation is translated via an electrical signal or mechanical mechanism to a pressure read-
ing. Gauges of this type can typically read pressure from atmospheric down to 1 torr.

7.2.2 Capacitance Manometers

The capacitance manometer gauge contains a diaphragm that is elastically deformed as pressure in
the gauge tube changes. This diaphragm is one electrode of a capacitor, and as the diaphragm is dis-
placed relative to a fixed electrode, the capacitance of the assembly changes, yielding an electrical
signal that can be used to infer pressure. These gauges are referred to as being gas species insensi-
tive. Capacitance manometers are typically manufactured to accurately measure pressure across a
span of three to four decades below the rated pressure. For example, a 1000-torr capacitance
manometer can typically read pressures as low as 0.5 torr with an accuracy of +/− 0.25 percent.
Additional accuracy can be obtained if the gauge is equipped for temperature stabilization.

7.2.3 Thermal Transfer Gauges
The property of thermal conductivity of a gas is utilized in this family of gauges to infer the pres-
sure of the gas being measured. The thermal conductivity of a gas is a function of its molecular
weight and other characteristics, therefore pressure gauges based on thermal conductivity must be
calibrated for the gas of interest. These gauges are referred to as being gas species sensitive.

Thermocouple Gauge. This gauge uses a thermocouple junction to measure the temperature of a wire
heated by an applied constant electric current that is exposed to the environment of the vacuum vessel
interior. As pressure is reduced in the thermocouple gauge tube interior, fewer gas molecules per unit
time are available to remove heat from the thermocouple gauge heated filament. The temperature rise in
the filament of the gauge tube is sensed by the thermocouple. This signal is sent to the gauge controller
that computes the corresponding pressure based on an algorithm in the controller for a specified gas.

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