1397 
IV. ANALYSIS OF RECORDS 
1. Photographic Records 
Representative piezoelectric pressure-time records are 
shown in Figs. 9-17 for all shooting conditions for which 
results are presented in this report. Immediately underneath 
each record is a time scale which was recorded simultaneously 
on the film. Two different timing methods were used. When 
possible a separate cathode ray tube was utilized since it 
simplified photographic problems. Otherwise the crater tube 
flasher provided timing dots. In both cases the timing 
signal came from the same source, a multivibrator which, 
operating from a standard 100 kilocycle crystal, gave a 
10 kilocycle output and 100 microsec markers. 
2. Pressure Measurements 
Pressure-time records obtained in this shooting series were 
measured to find values of peak pressure, P., and the duration, T, 
of the compression wave, Because these shots were shallow, the 
reflected rarefaction or tension wave arrived at the gauge a 
very short time after the initial shock front. Therefore, 
norual extrapolation procedure was not possible, and to derive 
a pressure Pp, which approximates extrapolated peak pressure, 
the following method was used. 
It was assumed first of all that the initial compression 
wave arrives at the gauge unaffected by surface reflections, 
The first few microseconds of the record, therefore, yield a 
value for apparent peak pressure, Py, that starts to decay at 
the normal free-water rate until the arrival of the tension wave. 
From work done at this TanOretory with pentolite in free water), 
a value may be obtained for 0/W!/3, where @ is the decay time— 
constant in microseconds and W the charge weight in pounds, If 
©1348 taken as 62,6 microsec for a 250 gm charge, at a value of 
W1/5/R equal to 0.216, the extrapolated peak pressure in free 
water is psi and the pressure-time curve is as shown in 
Fig. 2, plotted on a semi-log grid so that the exponential decay 
appears as a straight line. 
The solid line shows the pressure-time curve as it would 
be observed by a gauge of infinitesimal size, rising to true 
peak pressure, P,, then decaying exponentially, The gauges 
used in this study have a diameter of more than 1/4 in, 
including the waterproof wrapping. This finite diameter results 
in the recording of a finite rise time and therefore a lower 
apparent peak pressure, Pa. As seen in Table I, the average 
rise time for the gauges used in this study is about 8 microsee, 
shown by the dotted line in Fig. 2. Zero time is defined as the 
time at which the shock wave passes the center of the gauge. For 
these gauges the expected Pg is seen to be 3750 psi in free water 
= 
