i. 
2. 
549 
A MEASUREMENT OF THE PRESSURE CLOSE 
___ TO AN EXPLOSIVE UNDER WATER 
G. I. Taylor and R. M. Davies 
The Engineering Laborator Cambridge. 
June 1944 
* * * * ™ * * 
Summary 
This Report describes an experiment in which an electrical modification of the Hopkinson 
pressure bar was used to measure the pressure developed by a charge of 1.765 1b. T.N.T., primed 
with 0,05 1b. C.E., and exploded under water with the pressure end of the bar at a distance of 
7.79 inches from the centre of the charge. 
The record obtained in the experiment can be interpreted as giving an intial pressure of 
about 17 tons/sq. in, which remains constant for about 0,08 to 0,10 milliseconds; for the conditions 
of the experiment, the calculations of Penney and Dasgupta give an initial pressure 6f about 20 tons/ 
sq. in., dropping to half-value in about 0 07 milliseconds, The agreement between the theory and 
our experiment is not good, it must, however, be remembered that the theoretical result is fora 
spherical detonation wave, whereas the experiment was carried out with a cylindrical charge; at the 
same time, difficulties arise in calculating the pressure from the oscillogram on account of the 
disturbing effects of dispersion in the pressure bar and of the norma) pressure acting on its 
cylindrical surface when the shock wave is propagated outwards. 
Introduction. 
In two recent official reports we have described an electrical modification of the Hopkinson 
pressure bar which can be used to measure high transient pressures; the apparatus has been adapted 
to measure the pressure near a charge exploded under water and the present report gives the results 
of an experiment carried out with this apparatus in Portsmouth dockyard in February, 1942. The 
oscil logram which was obtained in the course of the experiment showed a number of unexpected features, 
some of which might have been caused by the dispersion of the elastic waves'in the pressure bar; the 
publication of the results of the experiment was therefore postponed until the theoretical study of 
the propagation of elastic waves in bars had been completed, 
Description of the Apparatus, 
The general arrangement of the pressure bar, etc., is shown schematically in Figure 1. The 
pressure bar was made froma bar of tool steel, 14 inches diameter and 22 feet long; the duration 
of the longest pulse which can be recorded by a bar of this length without overlapping due to reflected 
pulses is about 24 mibliseconds, 
The bar was supported vertically in a girder frame attached to a crane, so that in the 
experiment, the bar was immersed to a depth of about 15 feet; the total depth of water was about 
30 feet. 
It is Important to support the bar with rubber or some similar material, so as to insulate 
the bar as far as elastic waves @re concerned, For this purpose, two horizontal steel shelves, 
about 3 feet apart in a vertical direction, were provided in the girder frame; a hole 12 inches 
diameter was drilled in the lower shelf and a hole 2a inches diameter in the upper shelf. a short 
length of rubber hose-pipe of suitable diameter was clamped to the bar by a "jubilee* wormdrive hose 
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