(ii) 
519 
SES 
Non-periodic Irregularities. Effect of Air Cavities and Charge Case. 
Even in the case where the mounting of the gauge is non-resonant many of the p/t records 
still exhibit irregularities which must be ascribed to other causes. A large number 
of records have been obtained in which two different p.e. gauges have been used = the 
oscillograph record consisting of two peaks corresponding to the passage of the pressure 
pulse over the two gauges. A comparison of such pairs of records reveals many 
interesting features some of which we have mentioned already (e.g. variation of 7 and 
P with size of gauge). Apart from the influence of the gauge itself it is at once 
evident that many of the pecularities occurring in one record of a pair appear also in 
the other. A large amount of evidence of this nature makes it clear that the charge 
itself is responsible for a large proportion of these irregularities. For example, it 
has been found that the p/t records of 184 1b. charges are very variable, some records 
being comparatively smooth whilst others show very serious irregularities. Such 
irregularities and differences between individual -charges receive an explanation when 
it is remembered that the 184 1b. G.C. charge contains air cavities of considerable 
volume and somewhat irregular distribution, 24 1b. charges are also somewhat variable 
but not quite as Dad as the 18% Ibs. - they also contain some air but only a small 
volume. In order to eliminate entirely the possibility that such irregularities were 
cue to a particular gauge or oscillograph a number of records were made using two 
independent oscillographs and three p.e. gauges (strip and two circular (one Broadside, 
7 inches one edge 1 inch)). In al) cases the three p/t peaks recorded were practically 
identical (making allowance for any differences arising from the difference in the sizes 
of the gauges used). Many of the irregularities accurring in the records of T.N.T. 
poured filled charges are probably due also to the presence of air cavities in the 
explosive — about 30 or 40% of the records are, however, practically "smooth" Records 
a and b Sheet 5 are a typical example of a smooth record reproduced on 3 different °¢ 
gauges and with two different oscillographs. In this case the charge was 100 Ibs. 
block-filled T.N.T. 
As was anticipated, the general effect of increasing size of p.e. gauge (from 1 inch 
(7 inches broadside) to 7 inches edge on) was to smooth the recorded p/t curve. This 
effect vas strongly marked in the comparison of gauge records, the broadside-on gauge 
sometimes revealing very sharp pressure peaks which were smoothed ot almost entirely 
by the edge-on gauge. 
In certain of the earlier T.N.T. shots it was observed that the fall of pressure was 
irregular, but showed the same general features in all the records. it was then 
noticed that in these cases certain large diaphragm gauges containing air (cylinders 
6 inches to 12 inches diameter) were laid at a certain distance (about 14 feet further 
from the charge) from the p.e. gauge. These large air cavities constitute a "sink" of 
energy for the pressure pulse and the reflected tension wave neutralises a portion of 
the primary pressure passing over the p.e. gauge. Repeating the experiment, after 
removing the diaphragm gauges, records comparatively free from irregularity were obtained. 
!t appeared from such results as these that air cavities have a very powerful influence 
on the distribution of pressure at appreciable distances away. In order to test the 
point directly a 2-gauge 2-oscillograph comparison was made with an 18% lbs. G.C. charge 
placing an empty primer tin about 2 feet above the charge. The same irregularities 
appeared in the second oscillograph record. A similar experiment (with a 2 gallon petro} 
tin near the charge) usIng two gauges recording on one oscillograph gave the result shown 
in Sheet 5 record ‘g'. In both cases it will be seen that the air cavity introduces 
very definite irregularities into the form of the pressure pulse. The effect of the 
two gallon petrol tin Is very pronounced and occurs at an instant coinciding with the 
difference of path traversed by the primary wave to the gauge and the wave which is 
reflected from the empty tin. The earlier irregularities on the p/t curve of the 
circular gauge which do not appear on the ‘strip’ record are probably due to the fact 
that the second record commences before the first is finished = this statement applies 
to many of the records where pairs of gauges were used a few feet apart. 
IN sccves 
