Patent GB-693283-A (GB)
Title: Improvements in or relating to vacuum gauges
IPC: G01L7/16
Filed: 1950-02-13
Abstract: 693,283. Pressure-gauges. GRANT, C. R. A., and PRESSURE GAUGES, Ltd. Feb. 13, 1951 [Feb. 13,1950], No. 3638/50. Class 106 (ii). A vacuum gauge usable also as a pressure . gauge comprises a cylinder 11, a piston 13 slidable in the cylinder in a fluid-tight manner and a plunger 14 of considerably smaller diameter than the piston slidable parallel to the cylinder axis in a fluid-tight manner in the upper end wall of the cylinder 11 and at its lower end capable of contacting the piston 13 whilst its upper end is adapted to receive weights to counterbalance the force acting on the plunger 14 when the upper end of the cylinder 11 is connected either to a source of vacuum or a source of high positive pressure and the lower end is connected to the atmosphere, the vacuum acting to cause the piston 13 to rise to contact and tend to lift the plunger 14 and the pressure acting solely on the plunger 14 to cause it to tend to rise. As shown, Fig. 1, the cylinder 11 is fixed to a base 10 by screws 12 and the plunger 14 is located in its upper end wall by a guide 16 screwed down to tighten a sealing gland 15. At the upper end of the plunger 14 a pan 17 receives the weights and at its lower end it is hollow and serrated to co-operate with serrations in the upper surface of the piston 13 whereby the piston may be shaken by rotating the plunger to avoid friction errors. Screw-threaded connecting ports 21, 22 give access to the upper and lower ends of the cylinder 11, the operative connection being made to one while a plug 23 with a small bleed hole 24 is screwed into the one left open to atmosphere to damp down violent movements of the piston. In use, a vacuum gauge to be calibrated is connected to a pipe through which a source of variable vacuum communicates with the port 21 while the plug 23 is screwed into the port 22. Weights corresponding to a certain vacuum, taking account of the weight of the piston, plunger and pan, are placed in the pan 17 and the vacuum varied until there is no tendency for movement whereupon the position of the needle of the gauge being tested is marked appropriately and the process repeated for other weights. A highpressure gauge to be tested is connected in the same way, but allowance is made only for the weight of the plunger and pan as the pressure urges the piston to the bottom of the cylinder. In use as a low positive pressure gauge, Fig. 5, the plug 23 is screwed into port 21 and connection 29 is taken from port 22 to a source of variable positive pressure, the low-pressure gauge 31 being connected to the connection 29. The low pressure lifts the piston 13 to contact the plunger 14 and acts over the piston crosssectional area against the total weight of piston, plunger, pan and weights. In order to use one standard set of weights for all three purposes, the cross-sectional areas of plunger and piston have to be appropriately determined. Special sets of weights may be determined for giving the measure of pressure in inches of mercury. If one mode of pressure measurement only is to be used, piston and plunger cross sections may be determined so that one set of weights can be used to measure pressure in pounds per square inch and vacuum in inches of mercury.