Total and Partial Vacuum Pressure Measurement

The identification and quantification (partial pressure measurement) of the gaseous components in vacuum systems is of increasing importance in vacuum technology. This is achieved by the widely applied method of residual gas analysis. The simple theory of this is stated and relevant examples illustrate the application. 

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Chapter 5 considers the methods available for the measurement of both total and partial pressure in vacuum systems. For total pressure, the methods of measurement are described and, importantly, the uncertainty associated with that measurement is discussed. This may influence the choice of gauge. Also in Chapter 5, residual gas analysers (RGAs) for vacuum partial pressure measurements are described. These devices are being used increasingly for diagnostic work on vacuum systems. 

This approach allows measurement of both total and partial pressures in the panel, thus providing useful information on the gas ratios, this being useful as an R D tool or to monitor and improve the manufacturing process. However, since this is a destructive and relatively time-consuming test it can only be used for one-off samples. A nondestructive technique to measure total pressure in a vacuum panel has been proposed recently, which is based on the use of a laser beam source coupled to a detection system. 

Classically, the properties which are usually defined and measured are those of the undisturbed or side-on wave as it propagates through the air. Figure 1 shows graphically some of these properties in an ideal wave. Prior to shock front arrival, the pressure is ambient pressure pQ. At arrival time ta, the pressure rises quite abruptly (discontinuously, in an ideal wave) to a peak value Ps + p0. The pressure then decays to ambient in total time ta + tpartial vacuum and eventually returns to pQ. The quantity Ps is usually termed the peak side-on overpressure, or merely the peak overpressure. The portion of the time history above initial ambient pressure is called the positive phase, of duration t[Pg.3]

The same pressure units are used for the partial pressure of the component of interest and for the total barometric pressure readings, so that the units divide out. Thus, the partial pressure system is also dimensionless (a pure ratio) so that in all important respects it is equivalent to the volume per unit volume system for specifying concentrations of a gas in a gas. This system is particularly useful for making up synthetic mixtures of gases on a vacuum line when the partial pressure of each component is known. The carefully measured mixture(s) are useful for such purposes as calibration of instrumentation. 

The gas is applied as a mixture to the retentate (high pressure) side of the membrane, the components of the mixture diffuse with different rates through the membrane under the action of a total pressure gradient and are removed at the permeate side by a sweep gas or by vacuum suction. Because the only segregative mechanisms in mesopores are Knudsen diffusion and surface diffusion/capillary condensation (see Table 9.1), viscous flow and continuum (bulk gas) diffusion should be absent in the separation layer. Only the transition state between Knudsen diffusion and continuum diffusion is allowed to some extent, but is not preferred because the selectivity is decreased. Nevertheless, continuum diffusion and viscous flow usually occur in the macroscopic pores of the support of the separation layer in asymmetric systems (see Fig. 9.2) and this can affect the separation factor. Furthermore the experimental set-up as shown in Fig. 9.11 can be used vmder isobaric conditions (only partial pressure differences are present) for the measurement of diffusivities in gas mixtures in so-called Wicke-Callenbach types of measurement. 

In almost every system, the Mettler Thermoanalyzer I was coupled to a quadrupole mass spectrometer, such as is illustrated in Figure 8.14 (37. 69i. The sample may be studied under vacuum 10"6 Tom or under higher pressures to 1 atm. The reaction chamber, R. is surrounded by the furnace and separated from the balance by a diffusion baffle. The evolved gases pass directly to the mass analyzer. F. which is connected to a recorder, J. through the mass spectrometer control panel. Total pressure is determined by an ionization gauge, S, which also permits the recording of the EGD curve (in this case, due to the pressure change in the system). The relation between measured total pressure and the ion current of the calibration gas permits calibration of the mass spectrometer in absolute partial pressure units or A/Torr. 

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