Sec. 1-5] MECHANICAL INPUT TRANSDUCERS 105 



a compensating nonlinear network the author has succeeded in ob- 

 taining an output current that is a linear function of the pressure. 

 The dynamic transfer function is lower by about 20 per cent than the 

 static transfer function because of the variation of temperature 

 which accompanies the adiabatic compression. 



The system has been used in a pressure range from to 200 kg/cm 2 . 

 Water and gases dissolved in the benzene may cause a hysteresis ol 

 the pressure-capacitance characteristic. The method is primarily 

 applicable to nonpolar substances. 



The capacitive method can be used to measure pressure in gases; 

 the dielectric constant of air (19°C) changes from 1.0006 at 1 atm to 

 1.0548 at 100 atm. 



Variations with pressure of the dielectric constants of solids (Rochelle salt) 

 have been reported by D. Bancroft, Phys. Rev., 53, 587 (1938); of barium 

 titanate by B. M. Sul and L. F. Vereshchagin, Compt. rend. acad. sci. U.R.S.S., 

 48, 634 (1945); of ionic crystals (MgO, LiF, NaCl, KC1, KBr) by S. Mayburg, 

 Phys. Rev. ,79, 375 (1950). Pressure cells for the determination of the dielectric 

 constants at elevated pressure that can be used for pressure measurements are 

 described by M. G. Vallauri and P. W. Fosbergh, Jr., Rev. Sci. Instr., 28, 198 

 (1957), and D. W. McCall, Rev. Sci. Instr., 28, 345 (1957). 



1-53. Piezoelectric Pressure Transducer 



Certain piezoelectric crystals, if exposed to pressure from all sides 

 ("hydrostatic pressure"), will develop a polarization in a preferred 

 crystal direction; the polarization charge gives rise to an output 

 voltage. Piezoelectric pressure transducers are primarily useful for 

 the measurement of transient pressures of the order of several thou- 

 sand atmospheres and of a time range from a fraction of a second to 

 more than ten microseconds. 



Not all piezoelectric crystals are hydrostatically sensitive; pri- 

 marily tartaric acid, tourmaline, and sucrose have been used for 

 pressure transducers. The sensitivity ranges from 5.6 x 10 -12 cou- 

 lomb per (lb/in. 2 ) for sucrose to 21 x 10~ 12 coulomb per (lb/in. 2 ) for 

 tartaric acid. The frequently used tourmaline has a sensitivity of 

 11 X 10 -12 coulomb per (lb/in. 2 ). The output voltage is a linear 

 function of the pressure. 



A very simple piezoelectric pressure transducer consisting of a 

 spark plug on which a crystal is mounted is described by Lawson and 

 Miller. 1 Arons and Cole 2 have described the construction and investi- 

 gation of a piezoelectric pressure transducer for the measurement of 



1 A. W. Lawson and P. H. Miller, Rev. Sci. Instr., 13, 427 (1942). 



2 A. B. Arons and R. H. Cole, Rev. Sci. Instr., 21,31 (1950). 



