B2 - Schoonover, Inc.

Basic Terms of Vacuum Metrology
Vacuum Gauges
Basic Terms of Vacuum Metrology
Today, the total range of measurable vacuum pressure extends from
atmospheric pressure (about 1000 mbar, 750 Torr) down to 10 -12 mbar
(Torr) Ð over 15 decades. The instruments used for measuring pressure
within this wide range are called vacuum gauges. It is impossible to create
a single vacuum sensor capable of performing quantitative measurements
throughout the entire pressure range. Therefore, a variety of different
vacuum gauges are available, each with its own characteristic
measurement range, commonly extending over several decades.
Note the difference between direct and indirect pressure measurements.
With direct pressure measurements, vacuum gauge readings are
independent of gas type. Vacuum gauges, in which pressure is
determined directly by recording the force acting on the surface of a
diaphragm, are common.
With indirect pressure measurements, pressure is determined as a function
of a pressure-dependent property of the gas (i.e. thermal conductivity or
resulting ion current). These properties not only depend on pressure, but
also on the molar mass of the gases. Therefore, vacuum gauge presssure
readings relying on indirect pressure measurements are gas composition
dependent. These readings usually relate to air or nitrogen as the
measurement gas. Appropriate correction factors must be applied for the
measurement of other gases or vapors.
Vacuum Gauges with Pressure Readings
Independent of the Type of Gas
Capacitance Diaphragm Vacuum Gauges
A capacitance diaphragm gauge has two chambers. One is connected to
the vacuum to be measured; the other holds a certain reference vacuum.
The chambers are separated through a metal coated ceramic or thin metal
membrane. Together with a parallel electrode this membrane functions as a
condensator. When the pressure in one chamber is different from the
pressure in the other chamber the membrane bends and thus changes the
capacitance. The change is registered and converted into a pressure signal,
a voltage proportional to the pressure. This method makes gas type
independent absolute pressure measurement possible. Absolute capacitance
gauges can accurately measure pressures from 10 -5 mbar (Torr) to well above
atmospheric pressure using capacitance gauges having diaphragms of
different thickness. 1)
INFICON improvements in materials composition provide more stable and
reliable measurements over an extended period of operation. At the same
time, corrosion resistance is greatly enhanced by using ceramic instead
of metal.
Vacuum Gauges with Pressure Readings
Depending on the Type of Gas
Thermal Conductivity Vacuum Gauges (Pirani)
The energy transfer from a hot wire by a gas can be used to measure the
pressure. The heat is transferred into the gas by molecular collisions with
the wire, i.e. by heat conduction and the rate at which the heat is
transferred depends on the thermal conductivity of the gas.
The heat loss from a wire (typically 5 µm to 20 µm in diameter) can be
determined indirectly with a Wheatstone bridge circuit which both heats the
wire and measures its resistance and therefore its temperature.
A thin metal wire is suspended with at least one side electrically insulated
in the gauge head and is exposed to the gas. Tungsten, nickel, iridium or
platinum may be used for the wire. The wire is electrically heated and the
heat transfer is electronically measured. There are three common operating
methods: constant temperature method, constant voltage bridge, and the
constant current bridge. All these methods indirectly measure the
temperature of the wire by its resistance.
This measurement principle uses the thermal conductivity of gases for
pressure measurements from 10 -4 mbar (Torr) to atmospheric pressure.
INFICON improvements in temperature compensation provide stable
pressure readings in spite of large temperature changes, especially when
measuring low pressures.
Ionization Gauges
When the pressure in a vacuum system is below about 10 -4 mbar (Torr),
direct methods of measurement of the pressure by means such as the
deflection of a diaphragm or measurement of bulk gas properties such as
thermal conductivity are no longer readily applicable. Hence, it is necessary
to resort to methods which essentially count the number of gas molecules
present i.e., it is the number density not the pressure which is measured.
One of the most convenient methods to measure the number density is to
use some technique to ionize the gas molecules and then collect the ions.
The resulting ion current is directly related to pressure and a calibration can
be performed. The probability of ionizing a gas molecule will depend on a
variety of factors and hence, the ionization gauge will have different
sensitivity values for different gas species.
B6
1) For p < 1 mbar and T Gauge ­ T Vacuum the linearity of a gauge with a controlled temperature is
influenced by the thermal transpiration (gas type dependent) at the maximum in the same order
of magnitude as the zero point stability. See K. F. Poulter, et al., Vacuum 33, 331 (1983);
W. Jitschin and P. Ršhl, J. Vac. Sci. Technol. A, Vol. 5, No. 3, 1987.
B6.3