SHOCK WAVE MEASUREMENTS 21^7 



waves very close to the charge. More powerful explosives which de- 

 velop higher initial pressures may be expected to produce shock waves 

 which are dissipated more rapidly in their initial stages, the shock wave 

 may well be relatively stronger near the charge, and the equivalent 

 weight is therefore greater. 



In a fe\v exceptional cases, there is no single figure suitable as even 

 an approximate equivalent weight, and the advantage of the concept is 

 to some extent nullified for this or similar cases. Even in these circum- 

 stances, use of equivalent weights for specific parameters can be de- 

 fended on the ground that it gives a somewhat fairer picture than do 

 ratios of shock wave parameters at equal distances. This is particularly 

 true for peak pressure: a 15 per cent increase in peak pressure obtained 

 by using a more powerful explosive may seem relatively unimportant, 

 but it is not if a 50 per cent greater weight of the inferior explosive 

 would otherwise be required. This difference in viewpoint results from 

 the fact that the peak pressure varies only as the 0.33-0.4 power of 

 charge weight, and corresponding but smaller differences result for im- 

 pulse, which varies with weight as l^o-6-o.7_ 



D. Other measures for explosive comparison. Shock wave ratios or 

 equivalences are not the only relative measures of explosives which can 

 be used in their comparison. Another useful type of ratio is a distance 

 ratio : the ratio of distances from the charge at which equal volumes, or 

 weights, of explosive produce the same value of peak pressure, impulse, 

 or other parameter of the explosive. The distance ratios have as a 

 corollary volume ratios, which are the ratios of volumes of water around 

 the charge in which the parameters of interest exceed a specified value. 



In addition to the various shock wave parameters, as measured by 

 piezoelectric or mechanical pressure gauges, more empirical measures 

 can be used, such as deformations of diaphragms or plates at known dis- 

 tances from the explosion. These empirical values are usually related 

 to properties of the shock wave in rather complicated ways (see section 

 10.5), and are therefore to be related to specific shock wave parameters 

 with caution. They are, however, often highly reproducible and com- 

 paratively easily obtained results and so have been widely used for the 

 purpose of empirical comparison. From the present point of view of 

 shock wave comparison, the difficulties in interpretation of such data 

 make their detailed consideration here out of place, but it would be mis- 

 leading not to mention the importance of such methods in practical 

 testing. The results obtained, particularly if equivalent weights are 

 determined, usually give values which are closely consistent with results 

 obtained from more directly determined shock wave parameters. This 

 is of course to be expected, and apparently anomalous results can usu- 

 ally be understood in terms of differences in character of the shock 

 waves. 



