1406 
B. Constant Charge Depth and Varying Gauge Depths. Because 
the deviation from predicted pressure and duration values is 
greatest at very shallow charge depths, there remains the 
possibility that part of the deviation, at least, is due to 
venting effects rather than to surface reflection phenomena. To 
shed some light on these effects, further shooting was done with 
the charge maintained 4 in, beneath the surface and with several 
different gauge positions. It would have been advisable to 
obtain records from gauges placed directly underneatu the charge, 
The mechanical difficulties involved prevented such a rig at 
this time, and instead gauges were mounted off to one side of 
the charge and 29 in. below it, as shown in position B in Fig. 1B. 
Records thus obtained are compared, in Table III, with those from 
gauges at the 4 in. depth. As is noted in the figure the point of 
surface in evidence for that part of the reflected wave which strikes 
the deeper gauge is much closer to the charge than the similar 
point for the shallower gauge. Therefore, the angle of incidence 
and the pressure are both considerably higher than for the 4 in, 
gauge position. 
In both cases the gauge pressures are lower than similitude 
values, but for the 33 in. gauge depth the measured duration is 
not significantly higher than the theoretically predicted value. 
This is particularly interesting in view of the fact that Table II 
indicates that, for changing angles and constant pressure, the 
duration starts to deviate from the theory at larger angles 
than does the pressure. 
TABLE IV 
COMPARISON OF RESULTS FOR GAUGE DEPTHS OF 4 AND 33 IN. WITH 
CHARGE DEPTH 4 IN., AND CHARGE-TO-GAUGE DISTANCE 45 IN. IN BOTH CASES 
wV3/p = 0.216 at gauge (similitude Pm = 4300 psi) 
x Pg [Average P,| Pp |Ratio Pp/Py 
(in.)1(°)] (psd) (psi) 
o ro 
(psi) (%) (microsec) (4%) 
4 |10.1] 9,700}2820 18.4} 3040]; 0.718 20.7 15.9| 1.77 
33 146.4|75,900)3150 9.4] 3370 
It may be noted that since the duration time of 90 microsec was 
ample, normal extrapolation techniaues could be applied. Hxtrapolated 
peak pressure was 3300 psi, differing from derived peak pressure 
by only 1.2%, and thus affording a check on the procedure out- 
lined in the analysis section. Shock-wave time constants could 
also be obtained from the extrapolation plots, and the average 
value obtained for this group of shots was 59.1 microseconds 
98.1 6.5 1.07 
-l4- 
