Patent GB-592379-A (GB)
Title: Improvements in vacuum gauges
IPC: G01L21/02
Filed: 1945-05-15
Abstract: 592,379. Vacuum gauges. EDWARDS & CO. (LONDON), Ltd., W., TILLS, J. W., and POTTS, F. C. May 15,1945, No. 12138. [Class 106 (ii)] In a compression-type vacuum gauge, a standard volume of gas at the pressure to be measured is compressed partly by the displacing action of a solid member in direct contact with the gas and partly by the displacing action of an operating liquid. In the gauge shown, the solid member comprises a piston 21 movable in a liner pressed into a block 23 mounted on a base block 24 which has an axial bore or reservoir to receive loosely a portion of the piston having a reduced diameter. A cylindrical casing 22 is mounted, by means of a lower joint ring, upon the block 23 and is sealed at its upper end by a diaphragm 87 which is held in position in an upper joint ring by means of a cap 85, the latter being clamped down by several holding-down bars 18, the lower ends of which are secured in the block 23. Rotation of a hand wheel 83, mounted in the cap 85, moves a cranked arm 91 vertically and so, by means of multiplying levers, moves a coupling block 100 secured to a pipe 34 which has a header cup 35 and is a sliding fit in a bore in the piston 21. A bolt 93, engaged in the piston, slides freely in a bore in the coupling block and a tension spring (not shown) extends from the piston to the upper joint ring of the casing. Gas at the pressure to be determined communicates with the interior of the casing by means of a flanged connection 36 and bores or grooves in the block 23 and lower joint ring. A gauge glass 28 has its ends sealed into adaptors 40, 46 mounted in lugs 25 and 29 respectively welded to the block 23 and casing 22 and is in communication with the space surrounding the reduced end of the piston by means of holes bored in the lug 25, block 23 and liner. The upper adaptor 46 is in contact with a plunger 45 movable by means of a cam 81 which is bored eccentrically to receive a cranked guard 80, the lower end of which is secured in the lug 25. By rotating the cam 81 by means of a tool, the tension in the guard 80 is released and the gauge glass may be removed so that mercury may be introduced into the reservoir in the base block 24. Such liquid may be drained off by means of a screw plug. In use, the hand wheel 83 is rotated and a standard volume of gas at the pressure to be measured is trapped when the lower end of the pipe 34 dips into the mercury. The trapped gas is then compressed initially by the movement of the piston itself and then by the displacement of mercury from the reservoir by the reduced portion of the piston. The lower edge of the larger portion of the piston is chamfered and has a recess 21c to enable the compressed gas to escape into the gauge glass. The quantity of mercury and the position of a plug 47 in the upper end of the gauge glass are adjusted so that, when the piston is in its lowermost position, the level of mercury in the header cup 35 coincides with the lower end of the plug 47. A scale plate, against which the final volume of the gas may be read, is attached to the casing 22. In a modification, the lower joint ring is sealed to the block 23 by means of a lead or white-metal grummet having annular grooves in association with a grease cup, and, moreover, the gauge glass is surrounded by a protective tube held between the adaptors 40 and 46.