Review of pressure measurement techniques for ultrahigh vacua

This review includes a brief listing of the physical principles used in pressure measuring devices and a detailed discussion of selected topics associated with ionization gauges. Ionization gauges are currently the only devices available for measuring pressures in the extreme ultrahigh vacuum region. This is not because of their high sensitivity, but rather because of development of techniques for measuring small currents conveniently. Twenty-five years ago, it was not possible to measure the lowest pressures attainable. Today the situation is reversed; the lowest total pressures currently produced not only can be measured, but the composition of the residual gases can be analyzed as well. Notwithstanding their great popularity, ionization gauges suffer from many sources of error. Despite these difficulties, some of which are discussed in this paper, the total pressure ionization gauge can, when properly designed and operated, give pressure readings that are accurate to better than an order of magnitude and perhaps even a few percent under ideal conditions when carefully calibrated and the gas composition is known. The low pressure limit of present ionization gauges not using electron multipliers is determined by the low space charge densities at which they must be operated to avoid instabilities associated with high random energy electrons. Lower temperature electron emitters will be required in measuring pressures below 10−13 Torr to avoid outgassing problems.