614 
44 
duration of such explosions imposes requirements upon the pickup, amplifier, 
Sweep generator, and cathode-ray tube which are much more stringent than for 
explosions of larger charges. 
PRESSURES AT A SURFACE OF AN UNDERWATER STRUCTURE 
A modified form of the tourmaline gage has been used to obtain 
pressure-time records at a surface. The rubber insulation is molded about 
the crystal and also in a cavity sunk in a brass plate. The latter is bolted 
to the surface of an underwater structure; see Figure 6 on page 9. Thus far 
only preliminary comparisons have been made between this surface type of gage 
and the usual open-water type. 
The brass plate was bolted to the center of a piece of plywood 2 
feet square by 1.5 inch thick. The plywood was supported only by a rope so 
that it was free to move in the water. A 22-gram charge of tetryl wes placed 
20 inches from the surface gage so that the line joining the gage and the 
charge was perpendicular to the plane of the plywood. An open-water type of 
tourmaline gage, also placed 20 inches from the charge, served as a compari- 
son pickup. 
Figures 2/a and 2/b show a typical pair of comparison records. Two 
interesting differences always appear in these comparisons. The surface gage 
shows a higher peak pressure than is observed in open water; the ratio of 
these peak pressures is somewhat less than 2. The time histories of the two 
records are strikingly different. In Figure 2/7b the pressure indicated by 
the surface gage has dropped to zero in a time interval during which the pres- 
sure indicated by the open-water gage has decayed only to approximately 1/3 
of its peak value. ; 
ee 
3340 Ib/in@ 
5950 Ibyin@ 
100 microseconds anaes 
Figure 27a - Explosion Pressures in Open Water Figure 27b - Explosion Pressures on a Surface 
of an Underwater Structure 
100 microseconds abe aad 
Figure 27 - Comparison of Explosion Pressures on a Surface 
of an Underwater Structure and in Open Water 
The record in Figure 27a was obtained with the usual type of open-water gage; that in Figure 27b 
with a surface-type of tourmaline gage sunk into a brass plate. The return to zero 
pressure is much more rapid on the surface than in open water. 
