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469. Applications of components for high-vacuum engineering
Classified Abstracts 468--475
238
investigated (for earlier report see Holden, Holland and Laurenson, J. Sei. Instrum., 36, 1959, 281), to find the effect of the flange surface finish on the adhesion of the aluminium joint and the performance of such seals in bakeable apparatus. Tests were made with short lengths of super-pure aluminium wire clamped between the ends of stainless steel blocks, machined and polished to give various degrees of finish. Finishes leaving surface asperities varying between 10 and 50 microinches average height were used. The joints would not cold weld with a force of nearly 9000 lb per inch of wire, but if baked at 250°C or more whilst clamped with a force of 5000 lb/in., a cemented joint was formed. The adhesion strength of such joints varied widely, up to 350 lb/in, being recorded ; the greater strengths were obtained with the smoother surfaces and the higher baking temperatures. It was also found preferable for the machining marks (if not polished out) to run perpendicularly to the wire. Vacuum seals having a total length of 9 ft were successfully used in a 28-1itre, stainless steel evaporation plant. The plant was exhausted by an oil diffusion pump and liquid nitrogen trap (overall speed 40 1./sec). After baking to 320 ° or 350°C for about 5 hr, an ultimate pressure (as shown on a BayardAlpert gauge) of 3.5 × 10 ~ torr was recorded, which could be further reduced to 7 × 10 -l° mm by filling the trap, and to 1 × 10 -1° m m by firing an auxiliary titanium getter. Suitable torques for tightening flange bolts are recommended. (Authors, modified) L. Elsworth, L. Holland and L. Laurenson, J. Sei. Instrum., 37 (12), Dec. 1960, 449-451.
dates the sluice plate, which is mounted on a central vertical spindle, and may be turned by an external handle (or by a motor). To close the valve, the sluice plate is positioned so as to block the 6-in. aperture in the main assembly, the seal being maintained by O-rings in grooves surrounding the openings in the large plates. To open the valve, the sluice plate is withdrawn into the space between the large plates, then occupying a position diametrically opposite the aperture. Thus, there is no baffling action. The sluice plate is lightly greased, but the grease is not exposed to the system when the valve is open. A torque of 35 lb wt. ft will operate the valve in 55 sec. M.D.A. P. H. Lissberger, J. Sci. Instrum., 38 (4), April 1961, 132.
26.
Automatic Protective and Control Equipment 26
472. Improved flow-sensitive switch. H. C. Burford, J. Sci. Instrum., 37 (12), Dec. 1960, 490. 26
473. A recording flowmeter using a radioactive float. D. W. Tiros, J. Sci. Instrum., 38 (4), April 1961, 145-149. 26
474. A level controller for liquid nitrogen. D. Miller and H. C. Evans, J. Sci. Instrum., 38 (4), April 1961, 162-163.
23:31
468. A versatile metal to dielectric seal. Great Britain. A bakeable vacuum window seal suitable for use as an optical window, or as a radio-frequency window on highpower microwave devices, is described. The dielectric disc fits into a window frame made from an outer stainless steel tube, and an inner tube (of stainless steel or copper) with an external groove so that a channel is formed between the two tubes. Hydraulic pressure applied via this channel (oil or gas may be used) deforms the inner tube of the frame so as to grip the disc, whose edge is radiused. The initial vacuum seal between metal and dielectric is made on the first baking ; in the case of a silica disc, silver plating on the inside of the frame bonds to the silica, ceramic discs are first metallized. During subsequent heating and cooling, the seal is maintained, in spite of the different expansion coefficients, by the fluid pressure. Suitable pressures for temperatures up to 450°C are indicated. Appropriate dimensions are suggested for 4-in. dia. windows. M.D.A. P. C. Conder, H. Foster and H. A. H. Boot, J. Sci. lnstrum., 38 (4), April 1961, 134-136.
24.
Valves 24
469. Applications of components for high-vacuum engineering. S. D~Shnel, ,lena Rev., (1), 1962, 39-45. 24:51
470. Flap valve for long narrow apertures. G. W. King, J. Sci. Instrum., 37 (11), Nov. 1960, 439. 24
471. Vacuum sluice of large aperture. Great Britain. This note describes a valve designed to isolate the pumping unit in a large kinetic pumping system. The valve is of flat construction, with a 6-in. dia. aperture towards one side of a circular assembly of more than twice this dia. This assembly is built up from two circular plates, one of which is machined to leave a shoulder near the periphery ; this shoulder seals to an O-ring in a matching groove in the other plate, so that a circular space, some 14 in. in dia., is enclosed between the plates when they are bolted together. This space accommo-
27.
L e a k Detectors and L e a k Detection 27 : 37
475. Lens mass-spectrometer leak detector. Great Britain. This paper describes an instrument which applies the principles of " electron optics " to the selection of hydrogen or helium ions. The ion source of the detector is connected to the system under test via an isolation valve and a liquid air trap, to protect the source from vapours which might yield hydrogen on decomposition. The source contains a filament and an anode which consists of a gauze box inside which the ions are p r o d u c e d ; the ions are then accelerated towards the (earthed) source chamber and emerge through an aperture on the axis of the spectrometer proper. This gives a substantially mono-energetic beam of ions, which are accelerated towards the collector system at the other end of the spectrometer, also on the axis, passing through the field of a conventional magnetic lens carrying d.c. For constant electric and magnetic fields, the focal length of such a lens is directly proportional to the ionic mass, and the source aperture and the collector are arranged to be in conjugate positions for ions of hydrogen (or helium) used as probe gas. Other, heavier, ions are less deflected by the magnetic field and are intercepted by a baffle located in the middle of the lens, on the axis. For hydrogen, an accelerating voltage of 300 V and a magnetic field of 2000 G are used. The spectrometer is connected, via a second liquid air trap, to a mercury diffusion pump. The electron emission from the filament in the ion source is stabilized by a method often used for ionization gauges, and a feedback d.c. amplifier is used to effectively multiply the ion current so as to operate a l - m A meter. At maximum sensitivity, full-scale deflexion was obtained for an ion current of 3 × 10 -la A, equivalent to a leak of about 5 × 10 10 tort 1./sec. The ion current obtained for a given leak is claimed to be 100 times higher than is usually obtained with a given leak; the useful sensitivity is limited by current due to the hydrogen background. This was found typically to be about 1 × 10 11A, stable to the order of ~ 1 × 10 13A, and can be backed off by a manual control. To keep the background down to this level, metal gaskets are used, as well as the liquid air traps in inlet and pumping lines. Some experimental results are given, showing the resolution of the instrument, and there is a description of a reference leak constructed by compressing
238
investigated (for earlier report see Holden, Holland and Laurenson, J. Sei. Instrum., 36, 1959, 281), to find the effect of the flange surface finish on the adhesion of the aluminium joint and the performance of such seals in bakeable apparatus. Tests were made with short lengths of super-pure aluminium wire clamped between the ends of stainless steel blocks, machined and polished to give various degrees of finish. Finishes leaving surface asperities varying between 10 and 50 microinches average height were used. The joints would not cold weld with a force of nearly 9000 lb per inch of wire, but if baked at 250°C or more whilst clamped with a force of 5000 lb/in., a cemented joint was formed. The adhesion strength of such joints varied widely, up to 350 lb/in, being recorded ; the greater strengths were obtained with the smoother surfaces and the higher baking temperatures. It was also found preferable for the machining marks (if not polished out) to run perpendicularly to the wire. Vacuum seals having a total length of 9 ft were successfully used in a 28-1itre, stainless steel evaporation plant. The plant was exhausted by an oil diffusion pump and liquid nitrogen trap (overall speed 40 1./sec). After baking to 320 ° or 350°C for about 5 hr, an ultimate pressure (as shown on a BayardAlpert gauge) of 3.5 × 10 ~ torr was recorded, which could be further reduced to 7 × 10 -l° mm by filling the trap, and to 1 × 10 -1° m m by firing an auxiliary titanium getter. Suitable torques for tightening flange bolts are recommended. (Authors, modified) L. Elsworth, L. Holland and L. Laurenson, J. Sei. Instrum., 37 (12), Dec. 1960, 449-451.
dates the sluice plate, which is mounted on a central vertical spindle, and may be turned by an external handle (or by a motor). To close the valve, the sluice plate is positioned so as to block the 6-in. aperture in the main assembly, the seal being maintained by O-rings in grooves surrounding the openings in the large plates. To open the valve, the sluice plate is withdrawn into the space between the large plates, then occupying a position diametrically opposite the aperture. Thus, there is no baffling action. The sluice plate is lightly greased, but the grease is not exposed to the system when the valve is open. A torque of 35 lb wt. ft will operate the valve in 55 sec. M.D.A. P. H. Lissberger, J. Sci. Instrum., 38 (4), April 1961, 132.
26.
Automatic Protective and Control Equipment 26
472. Improved flow-sensitive switch. H. C. Burford, J. Sci. Instrum., 37 (12), Dec. 1960, 490. 26
473. A recording flowmeter using a radioactive float. D. W. Tiros, J. Sci. Instrum., 38 (4), April 1961, 145-149. 26
474. A level controller for liquid nitrogen. D. Miller and H. C. Evans, J. Sci. Instrum., 38 (4), April 1961, 162-163.
23:31
468. A versatile metal to dielectric seal. Great Britain. A bakeable vacuum window seal suitable for use as an optical window, or as a radio-frequency window on highpower microwave devices, is described. The dielectric disc fits into a window frame made from an outer stainless steel tube, and an inner tube (of stainless steel or copper) with an external groove so that a channel is formed between the two tubes. Hydraulic pressure applied via this channel (oil or gas may be used) deforms the inner tube of the frame so as to grip the disc, whose edge is radiused. The initial vacuum seal between metal and dielectric is made on the first baking ; in the case of a silica disc, silver plating on the inside of the frame bonds to the silica, ceramic discs are first metallized. During subsequent heating and cooling, the seal is maintained, in spite of the different expansion coefficients, by the fluid pressure. Suitable pressures for temperatures up to 450°C are indicated. Appropriate dimensions are suggested for 4-in. dia. windows. M.D.A. P. C. Conder, H. Foster and H. A. H. Boot, J. Sci. lnstrum., 38 (4), April 1961, 134-136.
24.
Valves 24
469. Applications of components for high-vacuum engineering. S. D~Shnel, ,lena Rev., (1), 1962, 39-45. 24:51
470. Flap valve for long narrow apertures. G. W. King, J. Sci. Instrum., 37 (11), Nov. 1960, 439. 24
471. Vacuum sluice of large aperture. Great Britain. This note describes a valve designed to isolate the pumping unit in a large kinetic pumping system. The valve is of flat construction, with a 6-in. dia. aperture towards one side of a circular assembly of more than twice this dia. This assembly is built up from two circular plates, one of which is machined to leave a shoulder near the periphery ; this shoulder seals to an O-ring in a matching groove in the other plate, so that a circular space, some 14 in. in dia., is enclosed between the plates when they are bolted together. This space accommo-
27.
L e a k Detectors and L e a k Detection 27 : 37
475. Lens mass-spectrometer leak detector. Great Britain. This paper describes an instrument which applies the principles of " electron optics " to the selection of hydrogen or helium ions. The ion source of the detector is connected to the system under test via an isolation valve and a liquid air trap, to protect the source from vapours which might yield hydrogen on decomposition. The source contains a filament and an anode which consists of a gauze box inside which the ions are p r o d u c e d ; the ions are then accelerated towards the (earthed) source chamber and emerge through an aperture on the axis of the spectrometer proper. This gives a substantially mono-energetic beam of ions, which are accelerated towards the collector system at the other end of the spectrometer, also on the axis, passing through the field of a conventional magnetic lens carrying d.c. For constant electric and magnetic fields, the focal length of such a lens is directly proportional to the ionic mass, and the source aperture and the collector are arranged to be in conjugate positions for ions of hydrogen (or helium) used as probe gas. Other, heavier, ions are less deflected by the magnetic field and are intercepted by a baffle located in the middle of the lens, on the axis. For hydrogen, an accelerating voltage of 300 V and a magnetic field of 2000 G are used. The spectrometer is connected, via a second liquid air trap, to a mercury diffusion pump. The electron emission from the filament in the ion source is stabilized by a method often used for ionization gauges, and a feedback d.c. amplifier is used to effectively multiply the ion current so as to operate a l - m A meter. At maximum sensitivity, full-scale deflexion was obtained for an ion current of 3 × 10 -la A, equivalent to a leak of about 5 × 10 10 tort 1./sec. The ion current obtained for a given leak is claimed to be 100 times higher than is usually obtained with a given leak; the useful sensitivity is limited by current due to the hydrogen background. This was found typically to be about 1 × 10 11A, stable to the order of ~ 1 × 10 13A, and can be backed off by a manual control. To keep the background down to this level, metal gaskets are used, as well as the liquid air traps in inlet and pumping lines. Some experimental results are given, showing the resolution of the instrument, and there is a description of a reference leak constructed by compressing