218 PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 



positions and the need for introduction of reflecting or diffusing surfaces 

 or backgrounds must be determined for the case of interest. For many 

 purposes a silhouette is desirable, which can be obtained, for example, 

 by a translucent diffusing screen in front of a source facing the camera. 

 In small scale experiments, light sources and cameras can often be 

 mounted in air above the water surface by use of suitable mirrors. 



In high-speed simple flash records especially, accurate synchroni- 

 zation of lighting with the explosion event is necessary. This can be 

 done in a number of ways, the more common involving changes in firing 

 current on detonation, use of multiple firing circuits with flash charges, 

 or action of the shock wave on a mechanical or electrical triggering de- 

 vice (see section 5.9). Controlled time delays can be obtained by elec- 

 trical delay circuits, or by use of explosive time delays from unequal 

 lengths of detonating fuse initiating the main and flash charge from a 

 single detonator cap. The velocity of detonation in primacord fuse 

 being 20.6 feet per millisecond, a delay of 4.0 microseconds is intro- 

 duced by a 1-inch path difference. This method is particularly simple 

 in field work, and with various modifications has been used for many 

 purposes.^ 



6.3. Pressure Measurements by Optical Methods 



A. Shock velocity measurements. The velocity of an advancing shock 

 front is directly related to the pressure difference across the front by 

 the Rankine-Hugoniot conditions if the equation of state for the fluid is 

 known. In the case of water, the departures of the shock velocity from 

 the limiting acoustic value are appreciable only for pressures very close 

 to the charge, the increase being 30 per cent for a pressure difference of 

 70,000 Ib./in.^ (see section 2.5). Pressure determinations from velocity 

 measurements are evidently of limited accuracy unless this excess veloc- 

 ity is appreciable, and methods based on velocity are therefore in- 

 herently best suited (in the case of water) to measurements close to the 

 charge. This region is one which is explored with difficulty using 

 mechanical or electrical gauges, and the optical technique is therefore a 

 valuable supplement to more direct pressure measurements. A draw- 

 back, as far as pressure determination is concerned, is the increasing 

 uncertainty as to the pressure-velocity relation at higher pressures, 

 owing to meagerness of data for determining the equation of state. 



Measurements have been made at Bruceton"^ using a rotating drum 

 camera and slit, the photographic arrangement being as shown in Fig. 

 6.2(a). Illumination was provided by the luminous shock waves in two 



* A difficulty in such applications is the fact that primacord, being an explosive, 

 develops a shock front which may interfere with the j^henomenon of interest if not 

 properly arranged. 



6 See DF Report Number 20 (35) . 



