28 ENGINE TESTS. 



CALIBRATION OF INSTRUMENTS. 



For a satisfactory comparison of the steam-pipe gauge with 

 the initial pressure shown by the diagram, the best plan is to 

 compare the gauge and the indicator without changing them 

 from their working positions. This can be done at the same 

 time that the leakage tests are in progress, as, for example, 

 when testing the piston and exhaust valves, the fly-wheel being 

 blocked, and the throttle valve and admission valve set wide 

 open. By taking the reading of the steam gauge and that of 

 the indicator at the same time (the latter being done by open- 

 ing the indicator-cock, then drawing a short line on the blank 

 card which has been applied for the purpose), not only will the 

 error of the gauge itself be allowed for, but also the error pro- 

 duced by the head of water contained in the gauge pipe, if any 

 such error exists. This comparison alone is sufficient to estab- 

 lish the difference in pressure between that in the main pipe 

 (or in the boiler to which the gauge is attached) and the 

 initial pressure in the cylinder of the working-engine, whether 

 the gauge, or indicator, or both, are in themselves correct or in 

 error. The gauge is then calibrated by reference to a standard, 

 and the accuracy of the indicator is established at the particular 

 pressure used. This single calibration is considered in many 

 cases sufficient for determining the correct scale of the indicator 

 in question. 



The most satisfactory method of determining the correctness 

 of the gauge, is to remove it from its place, and attach it to a 

 dead weight testing-apparatus, of the form sold by the steam- 

 gauge manufacturers, in which the pressure is produced by 

 sealed weights resting upon the top of a vertical plunger of 

 known area, the pressure being transmitted to the gauge 

 through the medium of oil or glycerine. The convenience of 

 this method and the portability of the apparatus, together with 

 its extreme reliability, place it ahead of all other systems for 

 calibrating gauges. Having made the calibration, the indica- 

 tion of the gauge in its working position must be corrected for 

 the head of water in the supply-pipe of the gauge, if any exists, 

 whether it be to increase the indication or to reduce it. 



