Patent GB-190501909-A (GB)
Title: Improvements in Diffusimeters.
Inventor: SHORT HENRY WILLIAM
Filed: 1905-01-31
Granted: 1905-09-21
Abstract: 1909. Short, H. W. Jan. 31. Pressure gauges.-Relates to an instrument for indicating the percentage of foreign gas or vapour in air, and particularly for detecting and measuring gas-leakages from mains, the action of the instrument depending on diffusion of the gas through a permeable septum, formed preferably of unglazed tetra-cotta. into an enclosed chamber. The instrument is of cylindrical form, and is provided at the top with a dial 2 over which moves an index-band 3 carried by an arbor 4, and at the lower end with a metal nozzle 5 which can be screwed on or off the body of the instrument.. This extension serves to joint the instrument to the ground while keeping the septum 7 and the rest of the body clean, the nozzle being readily detached and cleaned after use. The septum is held in a seating 8 in the pressure chamber 9, while a bridge 10 integral with the chamber wall supports the centre of the septum and carries, a central tube 11 projecting through the septum. The lower end of this tube is closed by a valve 12 mounted on a bridge-piece 13 provided in the nozzle. so that, when the nozzle is screwed home, the tube 11 is closed by the valve, while to flush the chamber 9 with air the nozzle is partly unscrewed, thus opening the valve. The valve 11 may be slightly tapered so that it may be adapted, when the nozzle is removed, to receive a jointing- nipple to make connexion with, say, a gas-pipe if it is desired to use the instrument as a manometer, a second scale being provided on the dial in this case. The pressure chamber 9 is closed at the top by the usual flexible corrugated diaphragm 14, above which is the mechanism chamber 15. The standard 16, supporting the arbor 4, and the motion-transmitting mechanism are mounted on a base 17 carried by the projecting wall 18 of the pressure chamber. The dial 2 is carried by the cylindrical wall 19 of the chamber 15, which is united with the casing 18 by a screw joint 20 so that by rotating the casing 18 the index can be set to zero when desired. The joint 20 is made sufficiently long to allow of this being done with safety, while the lower edge 21 of the casing 19 is partly cut away to accommodate a stud 22, which is fixed in the cylinder 18 and acts as a stop. A convenient motion transmitting mechanism is that shown. The base 17 carries an axis 23 which extends transversely and nearly intersects the main axis x-x of the instrument. Mounted on this axis 23 is a lever with arms 24,25 ; a conical-ended strut 26 fits in cups 29, 30 on the diaphragm 14 and the arm 24 respectively, and the length of the arm 24 can be adjusted by a set-screw 28 so as to bring the strut 26 into alinement with the axis x-x ; the arm 25 is supported by a light spring 27. A flexible cord 31, preferably of silk, is attached to this arm and is wound obliquely upon and fastened to the arbor 4 so as to form a helix, the pitch of which becomes longer towards the upper end. Motion of the diaphragm 14 thus causes this thread to wind upon or unwind from the arbor, the rotation of which is resisted by a hair-spring 32 coiled round the arbor and fastened to it and to the support 16. To compensate for the decrease in the length of cord unwound from or wound on the arbor for a given angular movement of the latter as the angle of incidence of the cord on the arbor diminishes, a diminishing scale is employed on the dial, or the arbor is suitably tapered. When this varying scale is employed, the index 3 must be set initially so that unit deflexion at different parts of the scale corresponds to equal changes in pressure. For this purpose, the fulcrum 23 can be adjusted in height relatively to the diaphragm 14 by rotating the framework and parts 16, 17 .relatively to the lower part of the instrument, with which they are connected by the screw joint 33. The stud 22 may screw through a hole in the wall 18 and bear against the screwed flange 33 of the base 17 to lock the parts after this adjustment has been made. An outer wooden casing 34 is fitted between the nozzle 5 and the bezel 35 which carries the glass top 36. The dial 2 fits in this bezel, which is removably clamped by set-screws 38 to the cylinder 19 and has its edge milled so that it can be rotated for setting the index. The parts of the scale corresponding to low percentages of gas are openly graduated, and those corresponding to higher ones are more closely graduated, and to compensate for this change a steel strip 41 is mounted so as to meet the cap 29 when the diaphragm has expanded sufficiently, thereby reducing the movement of the diaphragm.