16^ MEASUREMENT OF PRESSURES 



with edges at 45° and 135° to the optic axis and a normal stress apphed 

 only to one pair of edges results in a shearing strain, the other pair and 

 the faces being protected by a rigid housing. 



The advantage of sensitivity to hydrostatic pressure is, of course, 

 the fact that no mechanical protection need be provided to permit only 

 certain directions of strain being effective. Any such housing in- 

 creases the effective size of the gauge, which is undesirable for gauges 

 which must respond to pressures rapidly changing in space and time, 

 and is also liable to shock resonances developed by the applied pressure 

 wave which falsify the crystal response. From this viewpoint, there is 

 no advantage but rather a definite difficulty in using quartz rather than 

 tourmaline, and there is no advantage to quartz from the point of view 

 of sensitivity, the two differing by only a few per cent. Although both 

 Rochelle salt and ADP have much higher sensitivity, they also suffer 

 from the disadvantage of having no hydrostatic response and must 

 therefore be protected from unwanted stresses. The crystals are 

 physically much less rugged, an important consideration when explosion 

 pressures are involved, and Rochelle salt is very hygroscopic, likewise 

 a serious problem when used underwater or in humid surroundings. 

 Nevertheless, occasions may arise when the higher sensitivity attainable 

 outweighs these drawbacks and both Rochelle salt and ADP gauges 

 have been used to a limited extent for such purposes. (The statements 

 made here refer of course to measurements of explosion pressures rather 

 than underwater sound studies w^here the much lower pressure levels 

 usually rule out tourmaline.) 



As applied to explosion tests, Rochelle salt also suffers from its large 

 temperature effect. This is due largely to ferroelectric properties which 

 make its electrostatic capacitance vary by a factor of 10, becoming very 

 large in the neighborhood of the Curie points at — 10° C. and 25° C. 

 The open circuit voltage changes by only about 20 per cent over the 

 same range, and if the crystal is connected directly to a preamplifier or 

 other high impedance circuit, the latter condition is quite closely real- 

 ized. This method of use presents obvious difficulties when the ampli- 

 fier must function properly during the passage of intense shock pres- 

 sures. The other alternative, of connecting the crystal to a cable of 

 considerable length, loads the crystal to such an extent that the output 

 signal is determined by the coulomb sensitivity and its large tempera- 

 ture variations. These difficulties are present to a much smaller degree 

 in the less sensitive ADP crystals which nevertheless have about ten 

 times as large a response for a given size as tourmaline. ADP gauges 

 have been used to a limited extent for explosion measurements and, for 

 relatively large scale measurements at least, have showed considerable 

 promise. A further difficulty with Rochelle salt gauges is transient 

 distortion encountered with slowly changing pressures. 



