Patent GB-621668-A (GB)
Title: Improvements in or relating to variable resistances for electrical measuring apparatus
IPC: G01L11/00
Filed: 1939-05-22
Abstract: 621,668. Compression resistances; resistance casings. MULLER, J. May 22, 1939, No. 15200. Convention date, May 31, 1938. [Class 37] [Also in Group XXXVIII] Apparatus for the electrical measurement of pressures, temperatures, speeds, or deliveries of fluids in proportion to the pressure, comprises a column or pile of washers of conducting or semi-conducting material, inserted in an electric circuit including a measuring instrument, and compressed with its ends between a stationary stop and a movable stop on which acts the pressure to be measured, the stationary stop being adjustable and the movable stop being pressed by an adjustable spring against the corresponding end of the pile of washers. In the barometric gauge shown in Fig. 1, a metal cup 8 having a metal bottom 50 is screwthreaded at its open end and closed by a screwed cover 51 which forms an annular shoulder 52 and is provided with a central opening 53. The inner wall of the cup is lined with an insulating sheath 54 which contains a column of semi-conducting washers 55, for example of graphite. The lower washer rests on the metal bottom 50 of the cup 8 and on the upper washer bears a metal piston 56 extended by a metal rod 57 which passes with predetermined clearance through the opening 53, so that a short-circuit cannot be produced. An evacuated chamber 1 is formed by an outer cylinder 2, a bottom 4 and a top plate 3 with a central opening to which a tubular diaphragm 5 is hermetically connected. The cavity of the diaphragm 5 communicates with the open air through this opening. The lower end of the diaphragm 5 is closed by a plate 6 which carries a central tube 7, on the upper end of which the shoulder 52 rests so that the cup 8 is suspended within the tube. On the upper end of the tube 7 are screwed adjusting nuts 10, between which and the top plate 3 is interposed a compression spring 9 tending to compress the tubular bellows axially. Pressure of the external air acts on the upper face of the plate 6 and is partly counterbalanced by the spring 9. The cylinder 2 is located in a protecting casing 11 to which it is secured by bolts through the flange of the top plate 3. Columns 40 secured by nuts 41 carry a cover 13 having a central opening to take an adjusting screw 14 which is tightened on the end of the rod 57. An insulated terminal 15 is connected to the rod 57, the other terminal being connected to the casing. Between the casing 11 and the cover 13 is interposed an intermediate tube 12 provided with lateral openings. In a modification for use as a thermometer, the closed chamber formed by the parts 2, 3, 4, 6 and the bellows 5 is filled with air or gas, which expands with rise of temperature. A tubular extension is provided at the bottom of the casing so as to project into the interior of a space of which it is desired to measure the temperature. Fig. 3 shows a modification for measuring high pressures. The cylinder 2, Fig. 1, is omitted and the operating chamber is formed between the casing 11 and the bellows 5, the spring 21 being interposed between the lower face of the plate 6 and the bottom of the casing 11. A rod 20 extends downwardly from the plate 6 and slides tightly in the bore of a tubular member 22 by which the apparatus is screwed to a pressure vessel. For measuring low pressures, the rod 20 may be omitted and the pressure caused to act on the lower face of the plate 6. In the high pressure gauge shown in Fig. 4, the tubular bellows is replaced by a diaphragm 24 having its peripheral edge tightened against the casing 11, whilst the centre is secured to the tube 7. For measuring the speed of a stream of fluid, a converging and diverging pipe has the neck 26, Fig. 5, partly closed by a shock disc 27 provided with an axial rod 28 of which the end 29 bears against the rod 57. The cup 8 is supported by its annular shoulder 52 against a seating on a support 31 centred in the nozzle by arms 32. Initial compression of the column of semiconductive washers is adjusted by a compression spring 33. In a modification, Fig. 6, the arrangement is similar to that shown in Fig. 1 except that the plate 6 is connected rigidly by a rod 37 to a cylindro-conical shock body 38. The casing 11 is interposed in a measuring conduit so that the stream of fluid acts on the body 38. A cavity 43 communicating with the interior of the casing 11 by a passage 42 collects fluid leaking past the rod 37. The operating chamber communicates with the outer air through a passage 44 to prevent the formation of a vacuum in the operating chamber 1 under the action of suction applied to the passage 42.