The Optical Magic Lantern Journal (February 1892)

20 Tac Optical Magic Lantern Journal and Photographic Enlarger. necessary to give you acnrsory description of the construction and mode of mannficture of pressure gauges as practiced by my firm. A pressure yauge as used for this purpose consists essentially of a steel tube of elliptical section bent to the form of a semicircle, one end being screwed to a boas through which the gas under pressure is admitted to the tube, whilst the other end is free to move, being simply closed by means of a brass cap screwed on,to the tube. When pressure is admitted to such a tube, the section has a tendency tochange from the elliptical to the circular form, and, in consequence of this action, the curvature of the tube is reduced. The tubs, in other words, shows a | tendency to straighten, causing the free end of the tube to move away from the boss, and the degree of this movement indicates the amount of pressure which has been brought to bear on the tube. The movement is magnified by the aid of a toothed quadrant which is in gear with a pinion carrying a pointer, and the latter indicates the pressure on a graduated scale, the whole being mounted io the manner shown in the illustration. In order to ensure accuracy of indication, it is of primary importance that all the parts of a gauge be fitted np in the nicest possible manner, so as to obtain perfect f:eedom of movement in the various joints and bearings without the slightest play in the direction of movement. Any such play or back-lash would be magnified by the multiplying gear, and render considerable variations oa the scale of the dial. A slight hack-lash between the teeth of the quadrant and pinion is, of course, unavoidable, and in all well-made gauges thisis taken up by means of a fine spiral hair spring. The most important part of the gauge is the tube, -and the reliability of a gauge depends chiefly upon . the choice of the material for the tube, and the care bestowed upon its manufacture and testing. For indicating ordinary boiler pressures of, say, about one hundred pounds per square inch, tubes made of a copper alloy are Ordinarily employed; but, for the high pressures which have now hecome customary in the storaze of gases, ganges with steel tubes are nearly always used. Some gauges are fitted with ordinary drawn-steel tubes, but these are vastly inferior to tubes turned out 0‘ volid steel which has been specially selected and is suitable for this purpose. The tubes in all rauges manufactured by my firm are bored and turned out of a special bran] of Sheffield octagonal or pressed stecl, the tubes being male in a very large variety of shapes and strength, according to the amount of pressure they are required t register, worked. : For the particular purpose with which we are more immediately concerned, tubes are used which will safely stand a pressure of 360 atmospheres without taking the slightest ‘set,’ the diameter of the tubes before being flattened being about one-half of an inch, witha wall thickness of nearly one-sixtcenth of an inch. After the 1 tubes have been turned, they are polished inside and out, ' to remove every trace of tool marks, and they are then microscopically examined by light reflected from a mirror, any tube showing marks or scratches being rejected as unfis for use. The tubes are then carefully flattencd and bent at a moderate heat, and to perform these operations with entire satisfaction necessitates the greate:t expcrience and skill. Finally, the tubes are hardened and tempered, and a‘great deal also depends upon the manner in which these operations are performed The tubes are then again carefully examined, and, if proved satisfactory, they are subjected to a series of te-ts. For this purpose each tube is t2mporarily attached toa special testing apparatus. the free end being connected | with a mechanism which is identical in all its main features with the works of the gauge which the tube is in tended to fit, The tube is then submitted toa pressure of 360 atmospheres for one hour, or lonzer if possible, and careful note is taken, when the pressure is released, : whether the pointer of the facsimile gauge returns to zero. If not, the tube is rejected and destroyed, a strict rale being made never t> temper a tube twice, in case the first attempt should not have proved a failure. The bursting strength of a tube successfully completed in this manner will be between seven and eight tons per square inch, or exceeding 1,000 atmospheres; but these are by no means the strcngest tubes that-can be manufactured, and we have produced gauges which indicate regularly and successfully pressares up to 30 tons per square inch, or 4,500 atmospheres, being nearly 40 times the highest pressure employed in oxygen cylind2rs. The tube is next attached to the carrier boss, and the cap is screwed on to the free end of the tube, the metallic joint at both ends of the tube being made by means of a sharp-edge projection, which is turned on each end of the tube, and beds itself into the material of the boss and cap. The multiplying mechanism is next fitted up, the several parts being made beforehand in large quantities and by special machinery. The gauge is then secured in its case, whereupon it is again attached to a test-pump. The multiplying mechanism is now adjusted, so as to give the required range of movement to the pointer, and the dial having been temporarily inserted, the scale is marked out point for point by comparison with two large standard testgauges, which are periodically tested upon a dead-weight frictionless testing machine specially constructed for this purpose. Each dial is written by hand, and is sub equently inserted into its gauge and secured by means of three screws. The pointer is then fastened to the pinionspindle, the gauge is completed, and submitted to another final test upon the hydraulic test-pump. A gauge made in this manner will, if fairly used, permanently indicate on the dial any pressure with extreme accuracy, and may be kept under constant pressure without liability to deterioration. . There are, however, a large number of gauges in use which do not comply with these essential requirements, some specimens placed upon the market being, indeed, very inferior in construction and workm-nship ; and, as there is little to distinguish these gauges in external appearanc: from efficient instraments, it may be useful to indicate a few simple tests by which the grossest faults, at any rat», can be discovered by anyone. For this purpose it is only necessary to remove the brass rim and glass, : and to fix the gauge upon a gas cylinder charged to the and the conditions under which they are intended to be | full pressure of 12) atmospheres, Before opening the ! valve, lift the pointer over the pia against which it rests, when there is no pressure on the gauge. By means of a pencil, mark apon the dial the exact spot where the pointer settle:; then lift th+ pointer back. and open the valve. After leaving the gauze under pressure for, say, a quarter of an hour, or, better still, half an hour, close the valve, release the pres:ure, and, after again lifting the pointer over the rest pin, observe whether the pointer comes back to the exact position which it occupied befor:— if there is a perceptible variation the tube has given way, and this forms conclusive evidence that the gauge cannot be reliedupon. A more rigorous test consists in making the same experiment under the maximum pressure to which the gauge is marked, siy to 250 atmospheres, or even tO a pressure exveeding this, and any well made gauge will easily stand the test. Tne ahove test:, which any lanternist may perform, will suffice to bring t> light the worst defects. When the gauge is under pressure the pointer shoald be gently moved both ways, and it shouid be observed whether it {nvariably retarns to precisely the same indication. Any variation would be the result of back-lash, or sticking, and points to inferior workmanship.