MEASUREMENT OF PRESSURES 159 



is known not to be generally valid it was found true for this type of 

 gauge. 



The failure of diaphragm gauges to respond in a simple way to 

 shock wave pressures is shown by the variation of maximum depression 

 with charge weight and distance. These variations are illustrated for 

 small charges by Fig. 5.6, in which central deformations as a function of 

 charge weight are plotted to logarithmic scales for 2 distances. These 

 and other results show that for charge weights up to 20 pounds, the 

 deformation of steel diaphragms is proportional to the 0.6 power of 

 weight and —1.2 power of distance. Other experiments with charges 

 of 100 pounds or more (22, 23), give a weight exponent of 0.49 and dis- 

 tance exponent of — 1.13. The differences of the exponents with range 

 of charge weights and their failure to correspond exactly with simple 

 shock wave parameters indicate the nonexistence of any simple corre- 

 spondence with specific shock wave properties. 



Despite the limitations of diaphragm gauges for direct measurement 

 of fundamental quantities, they have continued to be very useful in 

 explosive comparison. This usefulness is the result of comparative 

 simplicity and excellent reproducibility when properly used. In addi- 

 tion, their use also furnishes valuable information for study of struc- 

 tural damage, as already mentioned and discussed in more detail in 

 section 10.5. (Fig. 5.6, for example, shows the agreement with ex- 

 periment of a theory developed by Kirkwood.) 



5.4. Piezoelectric Gauges 



Although the various mechanical gauges described in the last section 

 make possible fairly detailed measurements of shock wave pressures, 

 they evidently cannot practically give a continuous record of pressure 

 versus time and may be quite inadequate for investigation of pressure 

 waves which possess any appreciable irregularities of structure. They 

 are also employed with increasing difficulty for very small scale meas- 

 urements. Perhaps their most serious limitation is their inability to 

 measure simply and directly the weaker but more sustained secondary 

 pressure impulses following the shock wave as a result of later motion 

 of the gas sphere. For such measurements, what is evidently wanted is 

 a pressure sensitive device which can be arranged to give a continuous 

 indication of pressure as a function of time. The natural type of de- 

 velopment is then some kind of electromechanical gauge or transducer 

 to convert pressure changes into a varying electrical signal which can 

 be translated into a visible indication by some type of oscillograph and 

 recorded photographically. 



The simplest and most direct pressure-sensitive electrical phenome- 

 non is that of piezoelectricity, the property exhibited by some classes of 

 crystals of developing electrical charge on certain crystalline faces in 



