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Developments of the Knudsen type gauge
Conference on Fundamental Problems of Low Pressure Measurements In addition, the same temperature compensation maintains the rapid response, which this gauge is known to give at low pressures, over the whole pressure range. J English, B Fletcher and W Steckelmacher, Edwards High Vacuum
International Ltd, Crawley, Sussex Developments of the Knudsen type gauge Since Knudsen calculated the forces exerted on a lightweight vane due to bombardment of gaseous molecules rebounding from a heated surface, there have been many attempts to devise a pressure gauge making use of this effect. Most systems depending upon the suspension of a light-weight vane suffered from the disadvantages that (1) they were susceptible to external shocks and noises unless they were isolated by elaborate mechanical filters; (2) they were slow acting because the more delicate the suspension the longer the time constant of the movement becomes; (3) the systems could not be outgassed easily; (4) at low pressures of 10 -7 torr approximately the pressure of radiation was comparable with the pressures due to molecular bombardment. Recently two systems have been devised which overcome some of these objections. The first system uses a thin aluminium ribbon which is made to vibrate due to intermittent bombardment by the energetic molecules rebounding from a heated strip plane parallel to the ribbon. A quartz strip on the end of a lever vibrates between the hot strip and the thin ribbon and interrupts the bombardment without interrupting the infra-red radiation. The vibration of the ribbon is therefore solely due to molecular bombardment. The amplitude is proportional to the pressure and is measured electronically to the lower limit of 10-3 torr. At the higher pressures the ribbon is made to vibrate by putting it in a magnetic field and passing a.c. through it. The amplitude of vibration of the A1 ribbon is viscosity controlled and is proportional to pressure. Linear calibration is obtained from 10+3 to 10 -3 torr. The second gauge discards mechanical support and uses a divergent magnetic field to support a diamagnetic graphite disc where it can be acted upon by molecular bombardment. Below 10 -7 torr the radiation effect is minimised by allowing the disc to rotate in a copper tube at --196°C with axial slots. Some of the slots are fitted with radiation transparent germanium and are so sited to counteract the effect of radiation through open slots through which molecular bombardment takes place. The rate of rotation is about 1 revolution/s at a pressure of 10 -9 torr so that it takes an appreciable time for a measurement to be made visually. It is possible that a photocell may be used to detect the rotations in which case the change in rotation speed may be measured electronically. The gauge is remarkably free from external noise and it can be easily degassed before inserting in the magnetic field. The magnetic field required is about 15 kg and hence the magnet is rather bulky. Problems of accommodation coefficient do not arise in the ribbon gauge because at pressures of 10 -3 tort it is probable that at least a monolayer of gas is present on all surfaces in the gauge, but in the disc gauge accommodation coefficients may have to be considered. N W Robinson, Mullard Research Laboratories, Redhill
pressure measuring element, this type of crystal requires to be operated at low amplitude of vibration to avoid its disintegration. It is therefore convenient to stabilize the crystal amplitude by the use of d.c. negative feedback, and to use the feedback voltage as an index of the pressure. This arrangement closely resembles the R E V A manometer due to Becker, of which it may be regarded as the piezoelectric analogue, having similar operating characteristics, indicating pressures in the range 10 -1 torr to 1000 torr. Attempts to increase the sensitivity of the quartz crystal gauge to enable the measurement of pressures as low as those indicated by the Langmuir gauge by refinement of the electronics have been discontinued, since the geometry of the crystals used is unfavourable. D J Pacey, Brunel College, London (to be published in Vacuum)
Pressure measurement by adsorption of a molecular beam on a resonating quartz crystal The effusion rate of gas or vapour through an orifice is used to measure the pressure in the source volume. This rate is determined by the adsorption of a part of the effusing gas on a piezoelectric quartz crystal. The crystal is held at resonance frequency by an oscillator. This frequency is linearly dependant on the crystal's mass. Adsorption of gas on the crystal causes the resonance frequency to decrease. ?he rate of change of the resonance frequency plus the known geor, etrical arrangement of the crystal vis-/~-vis the orifice yield the gas c •vapour pressure in the source volume. L L Levenson, Cenm d'Etudes Nucldaires de Saclay, Gif-sur- Yvette Recent developments in the field of continuous indicating vibration friction gauges Applying modern electronic control and measuring techniques new technical constructions of the friction type vacuum gauges have been developed during the past two decades, offering the possibility to measure pressures continuously between about 10 -5 m m Hg and several atmospheres with the same gauge. In the first part of this paper a historical survey of the development of these friction type vacuum gauges is given. The second part is dedicated to the problem of finding an adequate analytical expression for the calibration curves which cover a pressure range of about 1:10 8. H W Drawin, Groupe de Recherche de l'Association EURATOMCEA, Fontenay-aux-Roses (to be published in Vacuum)
The quartz oscillator gauge The use of a vibrating piezoelectric crystal for the purpose of pressure measurement was suggested by the quartz fibre gauge of Langmuir and its several developed forms. These are attractive on account of their simplicity, and particularly for their complete freedom from pumping activity. An instrument incorporating an oscillating quartz crystal has already been described, in which the amplitude of the crystal vibrations at a frequency of 200 kc/s under conditions of constant power input, is measured electrically and is used to measure pressures in the range 10 -1 torr and 760 torr. Several designs further have been evolved including the simple transistorised version in which a coupling circuit enables the power input to the crystal to be reduced to a low level. Investigations carried out with the original design of gauge showed the rate of energy loss from the crystal to be proportional to pressure over the whole range from 10 -1 tort to 760 tort but that its dependence upon the molecular weight of the gas was not in agreement with theory. This was traced to the presence of standing waves within the gauge envelope and the difficulty was overcome by mounting the crystal in a padded ceil. Further observations using a crystal bar vibrating in the flexural mode at a frequency of 2.2 kc/s in which the standing wave pattern is absent showed the rate of energy loss to be in agreement with theory, without the use of a padded cell. When used as a 27
International Ltd, Crawley, Sussex Developments of the Knudsen type gauge Since Knudsen calculated the forces exerted on a lightweight vane due to bombardment of gaseous molecules rebounding from a heated surface, there have been many attempts to devise a pressure gauge making use of this effect. Most systems depending upon the suspension of a light-weight vane suffered from the disadvantages that (1) they were susceptible to external shocks and noises unless they were isolated by elaborate mechanical filters; (2) they were slow acting because the more delicate the suspension the longer the time constant of the movement becomes; (3) the systems could not be outgassed easily; (4) at low pressures of 10 -7 torr approximately the pressure of radiation was comparable with the pressures due to molecular bombardment. Recently two systems have been devised which overcome some of these objections. The first system uses a thin aluminium ribbon which is made to vibrate due to intermittent bombardment by the energetic molecules rebounding from a heated strip plane parallel to the ribbon. A quartz strip on the end of a lever vibrates between the hot strip and the thin ribbon and interrupts the bombardment without interrupting the infra-red radiation. The vibration of the ribbon is therefore solely due to molecular bombardment. The amplitude is proportional to the pressure and is measured electronically to the lower limit of 10-3 torr. At the higher pressures the ribbon is made to vibrate by putting it in a magnetic field and passing a.c. through it. The amplitude of vibration of the A1 ribbon is viscosity controlled and is proportional to pressure. Linear calibration is obtained from 10+3 to 10 -3 torr. The second gauge discards mechanical support and uses a divergent magnetic field to support a diamagnetic graphite disc where it can be acted upon by molecular bombardment. Below 10 -7 torr the radiation effect is minimised by allowing the disc to rotate in a copper tube at --196°C with axial slots. Some of the slots are fitted with radiation transparent germanium and are so sited to counteract the effect of radiation through open slots through which molecular bombardment takes place. The rate of rotation is about 1 revolution/s at a pressure of 10 -9 torr so that it takes an appreciable time for a measurement to be made visually. It is possible that a photocell may be used to detect the rotations in which case the change in rotation speed may be measured electronically. The gauge is remarkably free from external noise and it can be easily degassed before inserting in the magnetic field. The magnetic field required is about 15 kg and hence the magnet is rather bulky. Problems of accommodation coefficient do not arise in the ribbon gauge because at pressures of 10 -3 tort it is probable that at least a monolayer of gas is present on all surfaces in the gauge, but in the disc gauge accommodation coefficients may have to be considered. N W Robinson, Mullard Research Laboratories, Redhill
pressure measuring element, this type of crystal requires to be operated at low amplitude of vibration to avoid its disintegration. It is therefore convenient to stabilize the crystal amplitude by the use of d.c. negative feedback, and to use the feedback voltage as an index of the pressure. This arrangement closely resembles the R E V A manometer due to Becker, of which it may be regarded as the piezoelectric analogue, having similar operating characteristics, indicating pressures in the range 10 -1 torr to 1000 torr. Attempts to increase the sensitivity of the quartz crystal gauge to enable the measurement of pressures as low as those indicated by the Langmuir gauge by refinement of the electronics have been discontinued, since the geometry of the crystals used is unfavourable. D J Pacey, Brunel College, London (to be published in Vacuum)
Pressure measurement by adsorption of a molecular beam on a resonating quartz crystal The effusion rate of gas or vapour through an orifice is used to measure the pressure in the source volume. This rate is determined by the adsorption of a part of the effusing gas on a piezoelectric quartz crystal. The crystal is held at resonance frequency by an oscillator. This frequency is linearly dependant on the crystal's mass. Adsorption of gas on the crystal causes the resonance frequency to decrease. ?he rate of change of the resonance frequency plus the known geor, etrical arrangement of the crystal vis-/~-vis the orifice yield the gas c •vapour pressure in the source volume. L L Levenson, Cenm d'Etudes Nucldaires de Saclay, Gif-sur- Yvette Recent developments in the field of continuous indicating vibration friction gauges Applying modern electronic control and measuring techniques new technical constructions of the friction type vacuum gauges have been developed during the past two decades, offering the possibility to measure pressures continuously between about 10 -5 m m Hg and several atmospheres with the same gauge. In the first part of this paper a historical survey of the development of these friction type vacuum gauges is given. The second part is dedicated to the problem of finding an adequate analytical expression for the calibration curves which cover a pressure range of about 1:10 8. H W Drawin, Groupe de Recherche de l'Association EURATOMCEA, Fontenay-aux-Roses (to be published in Vacuum)
The quartz oscillator gauge The use of a vibrating piezoelectric crystal for the purpose of pressure measurement was suggested by the quartz fibre gauge of Langmuir and its several developed forms. These are attractive on account of their simplicity, and particularly for their complete freedom from pumping activity. An instrument incorporating an oscillating quartz crystal has already been described, in which the amplitude of the crystal vibrations at a frequency of 200 kc/s under conditions of constant power input, is measured electrically and is used to measure pressures in the range 10 -1 torr and 760 torr. Several designs further have been evolved including the simple transistorised version in which a coupling circuit enables the power input to the crystal to be reduced to a low level. Investigations carried out with the original design of gauge showed the rate of energy loss from the crystal to be proportional to pressure over the whole range from 10 -1 tort to 760 tort but that its dependence upon the molecular weight of the gas was not in agreement with theory. This was traced to the presence of standing waves within the gauge envelope and the difficulty was overcome by mounting the crystal in a padded ceil. Further observations using a crystal bar vibrating in the flexural mode at a frequency of 2.2 kc/s in which the standing wave pattern is absent showed the rate of energy loss to be in agreement with theory, without the use of a padded cell. When used as a 27