156 MEASUREMENT OF PRESSURES 



With a sufficient number of piston-crusher units, a block diagram ap- 

 proximation to the actual pressure-time curves can be constructed, as 

 sketched in Fig. 5.4(a), and a smooth curve drawn through the blocks, 

 as indicated by the dashed curve, is taken to represent the actual pres- 

 sure-time curve. 



In Hilliar's experiments, as many as 10 sizes of multiple piston 

 gauges were used, either 3 or 6 pistons and crushers being provided in 

 gauges to give a better average. The pistons used were 3^ inch in 

 diameter and from % inch to 8 inches long; the copper crushers, small 

 cylinders 0.32 inch in diameter and 0.50 inch long, were found to give 

 very consistent deformations in calibration tests; the free travels of the 

 pistons varied from }4 inch to 2J^ inches. A typical gauge with three 

 units is shown in Fig. 5.3(b). The gauge was normally hung vertically 

 and the pistons prevented from dropping out of the three chambers A 

 (only one is shown in the drawing) by the washers B. The annular air 

 chamber D, open to the water at E and to the chambers A below the 

 eyebolt C, served to equalize hydrostatic pressure and reduce air com- 

 pression by the moving piston. The crushers were attached to the 

 pistons and crushed against the eyebolt. 



With the use of piston and crusher gauges, Hilliar was able by a 

 series of 109 shots in 1918-19 to determine the major features of shock 

 waves, verify the principles of similarity, and determine weight-distance 

 laws. A typical pressure-time record from Hilliar's report is shown in 

 Fig. 5.4(b) for 300 pounds of TNT at a distance of 50 feet. Although 

 the absolute pressures and durations obtained by Hilhar differ by ten- 

 thirty per cent from more recent results, his determination of so many 

 of the major features of shock waves with relatively simple equipment 

 and facilities was a remarkable accomplishment. 



Some of the limitations of Hilliar's techniques are fairly obvious: 

 determination of a curve by what is essentially a series of time averages 

 may smooth over or obliterate irregularities and details, and some care 

 is necessary to pick the proper piston combinations. It has also been 

 found in later work that good technique is something of an art and that 

 considerable care and experience is necessary in order to obtain repro- 

 ducible results. Nevertheless the method does determine the major 

 features of the shock wave pressure-time curve and has been employed 

 extensively up to the present time."* One interesting modification of 

 the Hilliar gauge has been made by Hartmann (45) , in which the work- 

 ing time is made sufficiently long, by increasing the piston inertia and 

 decreasing its exposed surface, that the integrated pressure is measured 

 over an interval longer than the shock wave duration; the device thus 



^ An improved design of Hilliar gauge has been developed by G. K. Hartmann 

 and the Taylor Model Basin. The design of this multiple piston gauge, its precision, 

 an analysis of errors, and results obtained are described in a comprehensive report by 

 Hartmann (45). 



