How is vacuum pressure measured?
The pressures measured in vacuum technology today cover a range from 2000 mbar to 10-12 mbar, i.e. over 15 orders of magnitude. The enormous dynamics involved here can be shown through an analogy analysis of vacuum pressure measurement and length measurement, as depicted in Table 3.1.
Measuring instruments designated as vacuum gauges are used for measurement in this broad pressure range. Since it is impossible for physical reasons to build a vacuum gauge which can carry out quantitative measurements in the entire vacuum range, a series of vacuum gauges are available, each of which has a characteristic measuring range that usually extends over several orders of magnitude (see Fig. 9.16a). In order to be able to allocate the largest possible measuring ranges to the individual types of vacuum gauges, one accepts the fact that the measurement uncertainty rises very rapidly, by up to 100 % in some cases, at the upper and lower range limits. Therefore, a distinction must be made between the measuring range as stated in the catalogue and the measuring range for “precise” measurement. The measuring ranges of the individual vacuum gauges are limited in the upper and lower range by physical effects.
Fundamentals of low-pressure measurement
Vacuum gauges are devices for measuring gas pressures from just above to well below atmospheric pressure (DIN 28 400, Part 3, 1992 issue). In many cases the pressure indication depends on the nature of the gas. Exact measurement of partial pressures of certain gases or vapors is carried out with the aid of partial pressure measuring instruments which operate on the mass spectrometer principle (see section on gas analysis and mass spectrometers).
Dependence of the pressure indication on the type of gas
A distinction must be made between the following vacuum gauges:
- Instruments that by definition measure the pressure as the force which acts on an area, the so-called direct or absolute vacuum gauges. According to the kinetic theory of gases, this force, which the particles exert through their impact on the wall, depends only on the number of gas molecules per unit volume (number density of molecules n) and their temperature, but not on their molar mass. The reading of the measuring instrument is independent of the type of gas. Such units include liquid-filled vacuum gauges and mechanical vacuum gauges.
- Instruments with indirect pressure measurement. In this case, the pressure is determined as a function of a pressure-dependent (or more accurately, density-dependent) property (thermal conductivity, ionization probability, electrical conductivity) of the gas. These properties are dependent on the molar mass as well as on the pressure. The pressure reading of the measuring instrument depends on the type of gas.
The scales of these pressure measuring instruments are always based on air or nitrogen as the test gas. For other gases or vapors correction factors, usually based on air or nitrogen, must be given (see Table 3.2). For precise pressure measurement with indirectly measuring vacuum gauges that determine the number density through the application of electrical energy (indirect pressure measurement), it is important to know the gas composition. In practice, the gas composition is known only as a rough approximation. In many cases, however, it is sufficient to know whether light or heavy molecules predominate in the gas mixture whose pressure is to be measured (e.g. hydrogen or pump fluid vapor molecules).
If the pressure of a gas essentially consisting of pump fluid molecules is measured with an ionization vacuum gauge, then the pressure reading (applying to air or N2), as shown in Table 3.2, is too high by a factor of about 10.
Measuring in the rough vacuum pressure range – Atmosphere to 1mbar
Measurement of pressures in the rough vacuum range can be carried out relatively precisely by means of vacuum gauges with direct pressure measurement. Measurement of lower pressures, <10-3 on the other hand, is almost always subject to a number of fundamental errors that limit the measuring accuracy right from the start. So that it is not comparable at all to the degree of accuracy usually achieved with indirect measuring instruments
To be able to make a meaningful statement about a pressure indicated by a vacuum gauge in rough vacuum, one first has to take into account at what location and in what way the measuring system is connected. In all pressure areas where laminar flows prevail (1013 > p > 10-1 mbar), note must be taken of pressure gradients caused by pumping. Immediately in front of the pump (as seen from the vessel), a lower pressure is created than in the vessel. Even components having a high conductance may create such a pressure gradient. Finally, the conductance of the connecting line between the vacuum system and the measuring system must not be too small because the line will otherwise be evacuated too slowly in the pressure region of laminar flow so that the indicated pressure is too high.
Measuring in the Medium vacuum pressure range - 1mbar to 10-3mbar
Measuring in medium vacuum requires either the use of a low full scale capacitance sensor (such as a CTR100 0.1Torr) or more usually a thermal conductivity gauge such as the THERMOVAC series gauges (such as the TTR91RN). Typically in this range you are starting to transition from laminar to molecular gas flow, and therefore you need to start considering where the gauge is positioned in order to get the best performance. Measurements in this range are typically +-15% when using a thermal conductivity gauge, so you can achieve reasonable levels of accuracy, but not as high as when you can use the direct gauges, detailed in rough vacuum.
Measuring in the high and ultrahigh vacuum pressure range 10-3mbar to <10-12
The situation is more complicated in the case of high and ultrahigh vacuum. According to the specific installation features, an excessively high pressure or, in the case of well-degassed measuring tubes, an excessively low pressure may be recorded due to outgassing of the walls of the vacuum gauge or inadequate degassing of the measuring system. In high and ultrahigh vacuum, pressure equalization between the vacuum system and the measuring tubes may take a long time. Special consideration must always be given to the influence of the measuring process itself on the pressure measurement. For example, in ionization vacuum gauges that work with a hot cathode, gas particles, especially those of the higher hydrocarbons, are thermally broken down. This alters the gas composition. Such effects play a role in connection with pressure measurement in the ultrahigh vacuum range. The same applies to gas clean-up in ionization vacuum gauges, in particular cold cathode gauges (of the order of 10-2 to 10-1 l/s). Contamination of the measuring system, interfering electrical and magnetic fields, insulation errors and inadmissibly high ambient temperatures falsify pressure measurement.
In order to measure pressure in the medium and high vacuum ranges with a measurement uncertainty of less than 50 %, the person conducting the experiment must proceed with extreme care. Pressure measurements that need to be accurate to a few percent require great effort and, in general, the deployment of special measuring instruments. This applies particularly to all pressure measurements in the ultrahigh vacuum range (p < 10-7 mbar).
How to choose a suitable vacuum gauge
The desired pressure range is not the only factor considered when selecting a suitable measuring instrument. The operating conditions under which the gauge works also play an important role. If measurements are to be carried out under difficult operating conditions, i.e. if there is a high risk of contamination, vibrations in the tubes cannot be ruled out, air bursts can be expected, etc., then the measuring instrument must be robust. In industrial facilities, Bourdon gauges, diaphragm vacuum gauges, thermal conductivity vacuum gauges, hot cathode ionization vacuum gauges and Penning vacuum gauges are used. Some of these measuring instruments are sensitive to adverse operating conditions. They should and can only be used successfully if the above mentioned sources of errors are excluded as far as possible and the operating instructions are followed.