2J^6 SHOCK WAVE MEASUREMENTS 



and duration of the shock wave. These increases suggest the possibihty 

 of alternative forms of expressing the comparison in terms of weight or 

 volume ratios, which in many cases give simpler representations of the 

 data. 



C. Weight and volume ratios. A weight ratio for a given measure- 

 ment of two explosives such as shock wave pressure may be defined as 

 the ratio of weights of the two explosives which gives the same peak 

 pressure (or other measure) at the same distance. On this basis, the 

 ratio for explosives more powerful than TNT is less than unity, i.e., less 

 w^eight is needed. In order to avoid this situation, the reciprocal of the 

 figure just defined is more often used and referred to as an equivalent 

 weight. Ordinarily, the comparison explosive is taken to be TNT and 

 the equivalent w^eight of an explosive in terms of TNT is therefore equal 

 to the weight of TNT in pounds required to give the same peak pressure 

 at a given distance, for example, as one pound of this explosive. Similar 

 considerations can be applied to volume of charge rather than weight 

 in order to obtain equivalent volumes in terms of TNT or some other 

 reference explosive. 



The usefulness of equivalent weights or volumes lies in the fact that 

 the values obtained, when different shock wave parameters or other 

 measures of explosives are considered, are often very nearly the same. 

 The reason for this lies in the fact that an explosive which develops a 

 greater peak pressure than the same weight of another explosive, TNT 

 say, often but not always has an increase in time scale of pressure in 

 roughly the same proportions. If this is true, then a suitably larger 

 weight of TNT will develop very nearly the same shock wave as the 

 more powerful explosive, and this equivalent weight will be the same 

 whether deduced from peak pressure, impulse, or energy measurements. 

 Under such circumstances an equivalent weight is a more concise state- 

 ment of experimental results than the totality of peak pressure and 

 other ratios for equal weights or volumes. 



The extent to which one may speak of the equivalent weight of an 

 explosive, and the magnitudes of values obtained for various explosives, 

 can only be shown by explicit comparisons of calculations based on 

 shock wave peak pressure, impulse, and energy flux density. With a 

 few exceptions, the differences between variously obtained values for a 

 given explosive are less than or comparable with the experimental un- 

 certainties of the ratio. It is therefore possible in many cases to speak 

 of an explosive as equivalent to, say, 1.3 times its weight of TNT with- 

 out serious ambiguity. Even in this example, however, one can run 

 into difficulty for the reason that what is an e(iuivalent weight at one 

 distance from the charge is not at another. This behavior must be the 

 result of different variations with distance of the shock wave para- 

 meters, and an example where this sort of thing occurs is for shock 



