Journal of the Society of Motion Picture Engineers (1930-1949)

was chosen because the whole event could be shown on a single sheet of paper. This figure not only shows an enlargement of the original record, and the graph of pressure vs. time obtained from it, but also shows the method of plotting this curve. The record is of a small-volume hydraulic reservoir that is carefully filled with oil to exclude all gas pockets. The pressure is raised to 1000 psi as read on a dead -weight tester, and a quick-acting valve then releases the pressure to atmospheric. The most satisfactory way of reducing the data of the film to a curve is to have a two-man team. An experienced man can read points off the film as fast as his partner can plot them on a piece of graph paper. With this system an average curve can be plotted in approximately 15 min. This type of rapid plotting allows one to achieve an accuracy of within about 0.5%. If higheraccuracy plots are desired, a more elaborate technique must be employed requiring some careful measurements of the fringe spacing. The accuracy obtainable by this gauge is not a function of the mechanics of the diaphragm but is determined by the accuracy of the original calibration and the accuracy with which the film can be read. To date we have never had an application in which the highest possible accuracy was required. We, therefore, have no data on what ultimately might be obtained. Figure 7 is a blast-pressure curve taken with the gauge shown in Fig. 3. This shot was taken on an asphalt apron that was level and dust-free. The diaphragm of the gauge was mounted flush with the surface of the apron and at a 24-ft air-line distance from the explosive. The explosive was a 22.6-lb cylinder of Composition B suspended in the air by a small string and having its cylindrical axis at right angles to the line of measurement. It was detonated simultaneously in the center of each end of the cylinder. The record shows clearly three separate shocks arriving at the gauge. These shocks are probably due to the interreaction of wave fronts from the cylindrical and flat sections of the charge, respectively. The secondary shock fronts arc a function of the orientation of the cylindrical charge as well as the distance of the gauge from the point of explosion. Figure 8 is the first part of a pressuretime plot of the internal pressure developed by a large rifle such as the Navy uses on shipboard. This record is presented to emphasize two points: one, the extremely high pressures which the gauge is capable of measuring accurately; and two, to show how easily the gauge handles the time resolution of events that we normally think of as being very rapid. Reference 1. W. E. Buck and W. H. Barkas, "Dynamic pressure measurement by optical interference," Rev. Sci. Instr., 19: 678-684, Oct. 1948. 2. W. H. Barkas and W. E. Buck, "Interferometer gauge," U.S. Patent Office, No. 2,591,666, Apr. 8, 1952. 3. Robert C. Mack, "Magnetic fluid clutch of unique design," Automotive Ind., 98: 38, 1948. Discussion Morton Sultanoff (Terminal Ballistics Laboratory, Aberdeen Proving Ground, Md.}: By the appearance of the gauge, I would assume you are measuring the reaction of gauge material to the shock. How do you correct back to the actual shock from the response of the gauge material? Dr. Buck: The quartz diaphragm actually responds to the shock profile within the limitations of its frequency response which is 10,000 to 100,000 cycles per second depending upon the pressure range used. The diaphragm does not, however, follow the shock exactly, as the shock pressure rises in much less than one hundred thousandths of a second. A pressure-time curve plotted with points every tenth of a millisecond would not see an error between actual and measured values, but if these points were plotted W. E. Buck: Pressure Recording With Interferometer Camera 377