m MEASUREMENT OF PRESSURES 



veloped as a result of stress on the cable and, in transient pressure 

 measurements, results in an increasing potential difference between the 

 cable conductors as more of the cable is struck by the pressure wave. 

 The exact cause of the effect is not fully understood but in coaxial cables 

 appears to be developed between the dielectric and outer sliielding braid 

 by some sort of frictional process. Its effect is evidently particularly 

 serious in measurements of small pressures following an initially much 

 greater pressure rise, as are encountered, for example, in an ordinary 

 shock wave. 



Another type of effect closely associated with piezoelectricity is the 

 pyroelectric effect in crystals, in which electric charge is developed as a 

 result of pressure changes. This effect may be a genuine one or a "false 

 pyro charge," which is really piezoelectric charge induced by strains set 

 up in the crystal by differential expansion from temperature changes. 

 In tourmaline a temperature rise of 1° C. has been found to develop a 

 charge corresponding to a decrease in pressure of about 200 Ib./in.^ 

 This effect is eliminated if both measurement and calibration are car- 

 ried out under adiabatic conditions, but temperature changes in either 

 situation resulting from conduction into the crystal of heat externally 

 developed can cause appreciable errors. Fortunately, the effect is not 

 appreciable for practicable conditions in underwater pressure measure- 

 ments and calibration. It can, however, cause serious errors in air- 

 blast measurements where proportionately much greater temperature 

 changes are encountered and the durations are much longer. 



Piezoelectricity is of course not the only pressure-sensitive electrical 

 phenomenon w^hich can be utilized as a measuring technique. Some of 

 the more promising of other possible methods are use of pressure coeffi- 

 cient of resistance of metals (e.g., manganin) or electrolytic solutions; 

 magnetostriction, in which the flux changes of magnetic alloys resulting 

 from stress develop induced voltages in suitably arranged coils; the 

 condenser microphone principle, in which mechanical displacements 

 under stress cause capacity changes in an electrical circuit. None of 

 these principles has as yet been developed into gauge designs as widely 

 adaptable as the tourmaline or other piezoelectric gauges but some of 

 them present attractive possibilities. 



5.5. Response Characteristics of Gauges 



An effect which must be considered in any gauge design is the rela- 

 tion of its size and mechanical properties to its response to rapidly 

 changing pressures. Obviously, the ideal gauge for such measurements 

 would be of vanishingly small size, so as to offer no obstruction to pres- 

 sure waves or flow in the medium. This is hardly a practical idealiza- 

 tion and the effect of finite size and specified mechanical properties of a 



