MEASUREMENT OF PRESSURES 175 



at Woods Hole have for the most part been either the hourglass or Type 

 B construction. 



The majority of shock wave data on larger charges (50 to 700 pounds 

 of explosive) cited in Chapter 7 were obtained with such gauges. In the 

 work at Woods Hole, gauges were ordinarily used in pairs to provide two 

 records of pressure under given experimental conditions, and four such 

 pairs were routinely used in much of the work. The average deviation 

 between two gauges of a pair in measurements of shock wave pressures 

 was of the order of three-five per cent in routine work, and somewhat 

 better precision has been obtained under more favorable experimental 

 conditions. It has been possible in test work to distinguish between 

 explosives differing by as little as two per cent in shock wave prop- 

 erties. It is fair to say that piezoelectric gauges constitute a proved 

 and practical method for routine measurements on service weapons, both 

 for comparison of effectiveness and for investigation of fundamental 

 properties of shock waves. 



The basic tourmaline gauge designs described have also been used 

 to a considerable extent in investigation of secondary or bubble pulses. 

 The earlier hourglass construction did not prove very satisfactory for 

 this purpose because of difficulty in obtaining dependable gauges of 

 sufficient sensitivity, but the Type B gauge design has been found quite 

 satisfactory. The agreement between two gauges of a pair is not as 

 good as for shock wave measurements, but a more serious difficulty 

 limiting the accuracy of secondary pressure measurements, particularly 

 for large charges, is lack of reproducibility of successive shots under 

 presumably similar conditions. This question is more fully discussed 

 in section 9.5, but it may be said here that the frequently disappointing 

 accuracy of such measurements is only in part attributable to the instru- 

 mental technique. Even with the difficulties and errors of such meas- 

 urements, piezoelectric gauges have provided much valuable informa- 

 tion not otherwise easily obtainable, if at all, and their use has been 

 fully justified. 



In experimental investigations of underwater shock wave and other 

 phenomena, it is frequently very desirable to work on a model scale with 

 small charges, one pound or less of explosive, for reasons of simplicity 

 in experimental arrangements and economy of time and money. As 

 far as shock wave measurements are concerned, the main problem is one 

 of obtaining sufficiently small gauges (in order to minimize distortion) 

 of adequate and reliable sensitivity. Although the hourglass design of 

 gauge has been used to a considerable extent in such investigations, both 

 at the Taylor Model Basin and at Woods Hole, it never was wholly 

 satisfactory. The major difficulties encountered were systematic dif- 

 ferences in indicated pressures of supposedly similar gauges, sometimes 

 amounting to ten per cent or more, which made absolute measurements 



