Vacuum Measurement

the aggregate of methods for measuring the pressure of rarefied gases. (There is no general-purpose method of measuring a vacuum.) The measurement of pressure is based on various physical relationships that are directly or indirectly connected with the pressure or density of the gas. The unit of pressure in the International System of Units (SI) is the newton per square meter (N/m²). A conventional unit, the millimeter of mercury (mm Hg), is also used in vacuum technology; 1 mm Hg is equal to 133.322 N/m². Vacuums are measured with vacuum gauges, each of which has its own range of measurements (see Figure 1). Vacuum gauges are divided according to construction into hydrostatic, mechanical (deformation, membrane, and other types), compression (such as the MacLeod pressure gauge), thermal (thermocouple and thermoelectric), ionization (vacuum ionization gauges), magnetic, electrical discharge, viscosity, and radiometric vacuum gauges. These gauges measure total pressure.

Figure 1. Ranges of operating pressures of various types of vacuum gauges. (Limiting pressures are shown by dotted line.).

In evaluating a vacuum it is often necessary to measure the partial pressures of the components of a gas in addition to the total pressure. Several types of mass spectrometers and special gauges are used for this purpose. These special gauges for measuring partial pressure, unlike analytical mass spectrometers, do not have their own vacuum systems and are mounted directly on the container being evacuated. The range of measurement of partial pressures is 10³ to 10^-10 N/m² (10 to 10^-12 mm Hg).

In a hydrostatic vacuum gauge (see Figure 2) the gas presses on a liquid contained in a U-shaped tube. One of the arms contains gas at the pressure p_v being measured, and the other contains a gas under a known (reference) pressure p_k. If the density of the liquid is ρ, then the pressure difference in the arms stabilizes the liquid column at a height h:

p_v - p_k = gρh

where g is the acceleration of gravity; usually p_k≪p_o. The liquids used (usually mercury or vacuum oil) have a small partial vapor pressure at operating temperature and are chemically neutral in relation to the gas and the substance of the tube. There are hydrostatic vacuum gauges with closed, open, bell-shaped, and other types of arms. The drawbacks of hydrostatic vacuum gauges are the leakage of vapors from the liquid into the vacuum system and the small pressure measurement range, with a lower limit of 10^-1 N/m²(10^-3 mm Hg).

Figure 2. U-shaped hydrostatic vacuum gauge with open (a) and closed (b) arms.

In a mechanical vacuum gauge, the gas exerts pressure on a sensing element (a spiral tube, a bellows, or a membrane). For example, in a membrane gauge (see Figure 3), a membrane hermetically separates the vacuum system from the volume containing a constant reference pressure, usually 100 to 1,000 times less than the pressure being measured. The deformation of the membrane is transmitted to a pointer that moves along a scale. In order to increase the sensitivity when measuring low pressures, the membrane is joined to an electrical sensor. Mechanical gauges are usually used to measure pressures up to 10² N/m² (1 mm Hg).

Figure 3. Membrane vacuum gauge.

Compression vacuum gauges can measure lower pressures—to 10^-3 N/m² (10^-5 mm Hg). The operation of this kind of gauge is based on the Boyle-Mariotte law. The basic parts of the device are a vessel of volume V, two capillary tubes of identical diameter d (one of which is sealed off), and a tube connecting the device with the system in which the pressure is measured. A liquid (in most cases, mercury) is introduced from below, thus cutting off the gas at a measured pressure p in the volume V, and then compresses it to a pressure p₁≥ p in the lesser volume V₁ = πd²h/4 of the sealed-off capillary tube, where h is the height of the part of the tube not filled with liquid. The pressure p₁ is determined by the difference in the levels of the liquid columns in the closed and open capillary tubes. According to the Boyle-Mariotte law, p = p₁V₁/V, so that the pressure being measured can be determined if d and V are known.

The readings of hydrostatic, mechanical, and compression vacuum gauges do not depend on the nature of the gas.

Measurements of vacuums to 10^-2 N/m² (10^-4 mm Hg) may also be made by a thermal vacuum gauge. Its principle of operation is based on the pressure dependence of thermal conductivity in rarefied gases. The sensing element of the device is a sealed vessel with a wire heated by an electrical current. As the pressure in the system is changed, heat loss by the filament of the sensing element changes and, consequently, so does its temperature (at constant power). A distinction is made between thermocouple vacuum gauges (whose wire temperatures are measured by thermocouples joined to them) and thermoelectric resistance gauges (whose wire temperature is determined by the electrical resistance of the wire).

In a vacuum ionization gauge the gas is ionized by some constant source of ionizing radiation. The intensity of gas ionization depends on the pressure. In electron vacuum ionization gauges, ionization is caused by a stream of electrons. Usually this kind of vacuum gauge has three electrodes: a cathode and an anode, which establish an electrical field that excites the electrons and imparts to them the energy necessary for ionization; and the negative electrode, which collects the positive ions formed in the gas. The magnitude of the ion current in the collector circuit is a measure of the gas pressure. Vacuum ionization gauges can measure vacuums within a wide range (see Figure 1). The ultrahigh-vacuum ionization gauge, the so-called Ballard-Alpert tube, can measure pressure within wide limits. This gauge has a cathode (located outside the vessel) and a collector (a thin wire located inside the anode grid); it can measure pressures to 10^-8 N/m² (10^-10 mm Hg).

The Lafferty vacuum ionization gauge operates in a magnetic field. This lengthens the electron path in the operating space and ensures high ionization efficiency at very low electrical current. The lower measurement limit of such a gauge is 10^-11 N/m² (10^-13 mm Hg). A radioisotope vacuum ionization gauge (alphatron) is used to measure pressures to 10^-5 N/m₂ (10^-7 mm Hg). In this device, ionization of the gas is caused by alpha particles.

Magnetic electrical-discharge vacuum gauges use the dependence of an electrical discharge current in a magnetic field on the concentration (pressure) of a gas. These vacuum gauges can also measure ultrahigh vacuums to 10^-12 N/m²(10^-14 mm Hg). The gauge (see Figure 4), consists of a converter that has two plane-parallel cathode plates C and a circular anode A, whose plane is parallel to that of the plates, placed between them. The tube is placed in the field of a permanent magnet with an intensity H equal to 32 kiloamperes per m (400 oersteds). The direction of the field is perpendicular to the plates. A voltage U = 2-3 kilovolts across a resistance R = 1 megohm is applied between the electrodes. The magnitude of the discharge current serves as a measure of pressure and is measured by a galvanometer. The concerted action of the electrical and magnetic fields lengthens the trajectory of the electrons by many times and increases the probability of gas ionization. This leads to the production and support of an independent discharge at very low pressures. The first electrical-discharge vacuum gauges measured pressures to 10^-2 N/m² (10^-4 mm Hg); modern gauges of this type (including those produced in the USSR) measure up to 10^-12 N/m² (10^-14 mm Hg).

Figure 4. Diagram of a magnetic electrical-discharge vacuum gauge: (p) pressure, (N) and (S) north and south poles of a magnet, (A) anode, (C) cathode, (H) intensity of the magnetic field, (G) galvanometer.

Viscosity vacuum gauges are used in laboratory work for measuring pressures to 10^-4 N/m² (10^-6 mm Hg). The principle of their operation is based on the dependence of the viscosity of a rarefied gas on its pressure. There are damping and disk vacuum gauges. In the first type the measurement of pressure is the decay time of the free oscillations of a vibrator in the gas. In the second type, a rapidly revolving disk imparts a torque moment through the gas to another disk, which is suspended on a thin thread. The displacement angle of the second disk serves as a measure of the pressure.

The radiometric effect is used in radiometric vacuum gauges. Forces arise between two unequally heated plates placed in a rarefied gas, deflecting the plates proportionally to the pressure of the gas. The data produced by this type of vacuum gauge are virtually independent of the nature of the gas. The limit of measurement is 10^-5 N/m² (10^-7 mm Hg).