615 
45 
4 
The approximate doubling of the peak pressure is to be expected be- 
cause of the reflection of the pressure wave that takes place at the surface 
of the brass plate. Professor Kennard of the Taylor Model Basin staff has 
offered the following explanation for the rapid fall of the pressure to zero 
which is indicated by the surface gage. The pressure wave impinges upon the 
brass plate and causes it to move initially as if it were backed by air, be- 
cause of the small acoustic impedance of the plywood. The brass plate under- 
goes very large accelerations and after a certain time t, cavitation occurs 
in the water. If we assume that cavitation sets in at zero pressure, and 
that the form of the shock wave is given by eink then the time t, at which 
the record should indicate zero pressure is (26)* 
1 
2—l 
1 
= — In x 
a 
where 
pe 
am 
pe is the specific acoustic impedance of the water; m is the mass 
per unit area of the brass plate. 
a was determined from the open-water record given by Gage 280 in 
Figure 27a. It is approximately 28,300 seconds * 
0.075 pound per square inch. This gives 
if t is in seconds; mis 
t, = 35.3 microseconds 
Direct measurement on the curve in Figure 27b gives 35 microseconds for the 
time required to reach zero pressure. 
USE OF THE TOURMALINE GAGE IN STUDYING A SLOWER PRESSURE CHANGE 
The duration of an explosion of 100 grams of tetryl is about 50 
microseconds. Recently the TMB tourmaline gage has been used to study pres- 
sure changes which are a thousand times as slow as this, i.e., about 50 to 
100 milliseconds in duration. 
The phenomenon under investigation was the load on an experimental 
6-inch, 47-caliber, high-angle turret due to the recoil of the guns when they 
were fired. The shock of recoil is absorbed by a hydraulic brake or buffer. 
A 1/3-scale model of the turret at the Philadelphia Navy Yard was used for 
this test. The load due to recoil was simulated by the impact of a 2000- 
pound car rolling down an inclined track, and striking the piston of the 
buffer (28) (29). 
* See also Reference (27), which deals with the explosive load on underwater structures as modified 
by bulk cavitation. 
