MEASUREMENT OF PRESSURES 173 



ness of ys inch to ij/s inches have been successfully used, the gauge 

 constants ranging from 2.5 (for 34 inch doublet) to 90 (for l3^ inch 8 

 pile) MM coulombs per Ib./in.^ 



A detailed discussion of the performance characteristics of the Type 

 B gauge is given in part (D), but it may be said here that the design ful- 

 fills the major requirements of a satisfactory gauge for recording ex- 

 plosion pressures in the majority of cases. Although it has not yet been 

 used as extensively as earlier designs, sufficient field work and tests have 

 been carried out to show the usefulness and reliability of the design for 

 both shock wave and bubble pulse measurements over a wide range of 

 pressures. 



C. Other piezoelectric gauge designs. In addition to the large British 

 gauges already mentioned, several interesting gauge designs have been 

 developed by Osborne (80) at the Naval Research Laboratory. The 

 most promising of these consisted of a small tourmaline element en- 

 closed by an oil-filled Neoprene bag, or sheets, supported by rings sev- 

 eral inches in diameter, leads from the electrodes being led out through 

 small plastic tubing. This construction was intended to eliminate 

 plastic coatings, which in earlier work of Osborne appeared to introduce 

 nonlinearity of response at low pressure levels (< 100 Ib./in.^), and 

 very small gauges of this type were used in measurements of small 

 charges, for example, detonators. Although the design has consider- 

 able advantages in such cases, the relatively fragile construction makes 

 it unlikely that such gauges would be practical for much larger charges. 



In measurement of small explosion pressure changes, tourmaline 

 gauges become difficult to use because of their relatively low sensitivity, 

 and as a consequence some work has been done on development of 

 piezoelectric gauges using the more sensitive Rochelle salt or ADP 

 crystals. In tests made at Woods Hole, 45° Z-cut crystals of these 

 materials with cemented foil electrodes have been mounted in a lucite 

 housing, as shown in Fig. 5.12, to prevent applications of pressure ex- 

 cept on one pair of edges. The coulomb sensitivities of such gauges 

 were of the order 400-500 and 115 mm coulomb per Ib./in.^ for Rochelle 

 salt and ADP respectively. The Rochelle salt gauges were used only 

 for qualitative measurements or timing purposes because of their large 

 temperature coefficients of capacity and variation in response to a tran- 

 sient pressure with the duration of the pressure. Tests with ADP 

 gauges wxre more successful and showed excellent quantitative agree- 

 ment with tourmaline gauges in recording shock wave pressure-time 

 curves; and calibration tests also indicated the absence of any signifi- 

 cant low-frequency transient distortion as was encountered with 

 Rochelle salt crystals. The primary difficulty with the ADP gauges 

 which has limited their usefulness is the fragile nature of the crystals, 

 which became gradually pulverized when used in large charge measure- 



