176 



MEASUREMENT OF PRESSURES 



open to considerable uncertainty in some cases, even if corrections were 

 made for these differences. Comparative measurements, being un- 

 affected by these errors if reproducible, could be made with more assur- 

 ance, but even in this case the errors were larger than in work with 

 larger charges and occasionally changes of gauge sensitivity were en- 

 countered. As a result of such difficulties, adequate small charge data 

 with hourglass gauges were usually obtainable only by statistical pro- 

 cedures using a number of gauges in a series of shots. Individual gauge 



Csl 



tu 



"^ 



UJ 



a: 



(/) 

 (/) 

 UJ 

 (T 



a. 



20 

 15 



10 

 8 



6 



10 



20 



30 



40 



50 



60 



TIME (jUSEC ) 



Fig. 5.13 Semilogarithmic plot of shock wave pressure against time to illus- 

 trate correction for gauge response (1.36 feet from a 0.55 pound spherical 

 50/50 pentolite charge). 



records also often showed peculiarities of spurious bumps and oscilla- 

 tions not attributable to the pressure wave itself. (It is to be noted, 

 however, that such irregularities may be real, and are more often en- 

 countered in small charge work, as discussed in section 7.6.) 



The difficulties with small hourglass gauges were one factor leading 

 to the development of the improved Type B gauge already described. 

 Two-pile gauges of this later design using 34 iii<^h diameter disks have 

 been fairly thoroughly tested in preliminary experiments with charges 

 from 25 grams to 250 grams weight (5) . These tests have indicated no 

 systematic errors in gauge-to-gauge sensitivity exceeding the standard 

 deviations of single gauge readings on successive shots. This latter 

 error was found to be of the order of five per cent for the peak pressure 

 of the shock wave, and is sufficiently small to make the use of Type B 

 gauges for small charge work very practical and attractive. A funda- 

 mental difficulty which must be realized in such use is the distortion of 



