SEVERAL different types of manometers have been used for recording the pressures developed during the course of mine explosions, says the United States Bureau of Mines, Department of Commerce. It is importmlt that the results obtained with the different manometers be compared. Accordingly, the Pittsburgh Experiment Station of the Bureau of Mines undertook to determine the relative accuracy or degree of agreement of several commonly used types of manometers. The final comparison included the British Coal Dust (B.C.D.), Crosby, Illinois, Diederichs, and the Bureau of Mines instruments. The comparisons were made by connecting the manometers to a spherical explosion bomb in which mixtures of methane and air or hydrogen and air were exploded. The manometers were connected to the bomb in pairs, the Bureau of Mines instrument being common to all comparisons.
Although analysis and interpretation of the experimental data are not yet complete, it appears as a result of the intercomparisons that in general the Bureau of Mines manometer indicates a higher maximum pressure than any of the other instruments tested. This was not true in all tests with the B.C.D. instrument but was uniformly true for all tests with all the other manometers. The Bureau of Mines instrument attained maximum pressure in a shorter time after ignition of the gas than did any of the other manometers except the B.C.D. For the latter, the time to maximum pressure was sometimes greater and sometimes less than for the Bureau of Mines instrument.
The Bureau of Mines instrument showed more favorable characteristics as regards elastic hysteresis than did the other instruments. In the test using hydrogen-air mixtures, pronounced vibrations appear in the time-pressure records. These are believed to be vibrations of the diaphragms caused by the suddenness of the explosions, rather than by stationary waves set up in the explosion bomb, although the latter appears as a possibility. Initially the vibrations were of approximately the same frequency as that of the experimentally determined free-period vibrations of the respective instruments; but the frequency invariably decreased as the pressure decreased and the vii)ration died away. The frequency finally reached about one-half the original frequency.