106 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



large transient pressures. The system is illustrated in Fig. (1-5)3 and 

 consists of four circular tourmaline disks stacked on both sides of a 

 steel plate. The tourmaline disks are coated with a conductive 

 material and connected to the terminals. The entire assembly is 

 coated with an insulating protective layer. Gauges of this type have 

 been used for the measurement of pressure amplitudes ranging from 

 0.03 to 6,700 atm (0.5 to 100,000 lb/in. 2 ). 



Because of the small size of the pressure transducer (about \ in. 

 diameter) , the distortion of the pressure field is usually negligible. The 



distortion can be further reduced 

 by orienting the disk-shaped 

 transducer edgewise to the propa- 

 gation direction of the pressure 

 wave. 



The output impedance is high. 

 An equivalent circuit for a piezo- 

 electric transducer is shown in 

 Fig. (1-2)64. The dynamic re- 

 sponse of the transducer falls off at low frequencies and becomes zero 

 for static pressures for reasons explained in 1-25. The response to 

 high-frequency pressure fluctuation can extend over a considerable 

 range. In the system described by Arons and Cole, the response begins 

 to fall off between 10 and 20 kc and reaches zero in the vicinity of 

 100 kc. 



Change of the crystal temperature causes spurious output voltages 

 (pyroelectric effect). The error can be considerable; in the system 

 described by Arons and Cole, a change in temperature of 1 °C produces 

 a charge equivalent to that caused by a pressure change of 13 atm. 

 However, a temperature change of this order of magnitude occurs 

 rarely during the short time of a pressure-transient measurement 

 and can also be kept low by thermal insulation of the transducer. 

 Undesirable signals are also likely to arise in the cables connecting 

 the pressure-sensing element with the accessory equipment. The 

 effect has been studied by Cole. 1 



Fig. (1-5)3. Piezoelectrical pressure 

 transducer [from A. B. Arons and R. H. 

 Cole, Rev. Sci. Instr.. 21, 31 (1950); by 

 permission]. 



1-54. Electrokinetic Pressure Transducer 



If a liquid such as water is brought in contact with a solid body 

 such as a glass particle, an electric field of molecular thickness will 

 arise at the interface between the two substances. Under the in- 

 fluence of this field, the glass as well as some of the water molecules 



1 R. H. Cole, Rev. Sci. Instr., 21, 32, footnote 3 (1950). 



