VISUAL AIDS for instruction in VACUUM TECHNOLOGY AND APPLICATIONS Module 2: Total Pressure Measurement in Vacuum Second Edition Section 5 Calibration of Vacuum Gauges

Calibration of Vacuum Gauges Introduction In this Chapter we shall briefly discuss calibration of vacuum gauges In previous chapters, we have seen that many vacuum gauges do not measure pressure directly, so calibration is necessary Many gauges give readings which are species dependent, so that calibration is necessary for each type of gas for which pressure is to be measured Calibration methods used involve the generation of a known pressure in a calibration volume into which the gauge to be calibrated is inserted

Calibration of Vacuum Gauges Introduction All calibrations must ultimately be referred back to a direct measurement of pressure traceable to national standards – usually a National Standard Barometer A Piston gauge, in which a known mass is used to balance the gas pressure, is a suitable absolute gauge for pressures above about 10-2 Pa

Calibration of Vacuum Gauges Static Expansion Method The static expansion method uses a series of expansions of a known volume of gas at a known pressure and temperature to generate a series of pressures These pressures can be calculated using the gas laws if the gas behaves like a noble gas, e.g. Argon or Nitrogen A system for doing this is shown here

Calibration of Vacuum Gauges Static Expansion Method Either V1 or V2 is the initial volume and V6 is the final volume where the gauge to be calibrated is attached By using various combinations of the expansion volumes V3, V4 and V5 the various required pressures may be obtained in V6 The volume ratios must be accurately known and the temperatures must be kept constant

Calibration of Vacuum Gauges Static Expansion Method With this technique, pressures from atmosphere to 10-4 Pa may be generated and gauges calibrated over this range with accuracies of a few percent

Calibration of Vacuum Gauges Dynamic Expansion Method This method relies on the pressure drop, p, across an orifice of known conductance, G, under steady state conditions being given by where Q is the quantity of gas flowing through the orifice

Calibration of Vacuum Gauges Dynamic Expansion Method In the equipment shown, the gas flow, Qpv, is measured by an accurate flowmeter The orifice is a hole in a thin plate at (1) and its conductance is calculated knowing the area If the pressure on the pump side of the orifice is very much less than that in the calibration chamber it can be ignored so p = pcal

Calibration of Vacuum Gauges Dynamic Expansion Method Calibrations can be carried out with such a system for pressures down to 10-7 Pa with uncertainties of a few per cent

Calibration of Vacuum Gauges Molecular Beam Method A rather less common method of gauge calibration uses a molecular beam of atoms effusing from a Knudsen cell to generate a known number of molecules entering the calibration chamber through a small orifice Since the pressure in the Knudsen cell must be measured by a calibrated reference gauge, this is not an absolute method of calibration but a reference method

Calibration of Vacuum Gauges Molecular Beam Method The main advantage of the method is that it can be used to calibrate gauges down to pressures below 10-7 Pa with uncertainties ~30% at the lower end of the scale

Calibration of Vacuum Gauges Ranges of Pressure Generators Here we can see the pressure ranges over which the various calibration methods may be used and the corresponding uncertainties in the pressures generated using each technique

Calibration of Vacuum Gauges Pressure Ranges of Standards In the field, most calibrations are carried out by using secondary standard gauges rather than the primary standards described earlier Here we see a comparison of the ranges of operation of some secondary standards compared to primary standards

Calibration of Vacuum Gauges Ranges of Common Gauges Here are shown useful measurement ranges for some commonly available vacuum gauges The low pressure extensions are only achievable with special versions of the gauges

VISUAL AIDS for instruction in VACUUM TECHNOLOGY AND APPLICATIONS Module 2: Total Pressure Measurement in Vacuum Second Edition End of Section 5 Calibration of Vacuum Gauges