MEASUREMENT OF PRESSURES 163 



As a result of the considerations outlined and others, tourmaline 

 pressure gauges have been the main reliance in determination of tran- 

 sient pressures. There are certain difficulties which must be considered 

 in any use of tourmaline or other piezoelectric gauges. The first of 

 these is the fact that, regarded as a circuit element, a piezoelectric gauge 

 has the fundamental characteristics of an ideal generator in series with 

 a small capacitance, of a few hundred micromicrofarads at most, which 

 represents the electrostatic capacitance of the crystal.^ The gauge is 

 therefore inherently a high impedance device, a characteristic which 

 introduces several complications in its use. Most devices suitable for 

 recording fast transients are basically voltage sensitive and have a finite 

 direct current resistance. The connection of such a resistance across 

 the gauge thus provides a leakage path for the charge developed, and 

 the voltage response to a suddenly applied and maintained pressure, for 

 example, is an exponential decay, the rate decreasing as either leakage 

 resistance or circuit capacitance is increased. The product of these two 

 factors determines the rate of decay and must therefore be sufficiently 

 large, a condition usually best achieved by making the leakage resist- 

 ance large, as increased capacitance shunting the gauge reduces the 

 voltage developed. 



The necessity of essentially open circuit conditions for direct cur- 

 rent or low frequencies not only presents requirements to be satisfied by 

 the recording equipment to which the gauge is connected, but also 

 presents problems in making this connection. If the electrical line is 

 long enough, the time required for signals to be transmitted down its 

 length is comparable with times of interest in transient pressure meas- 

 urements and attention must be paid to the way in which such trans- 

 mission takes place. It is a familiar fact that improper termination of 

 electrical lines results in reflected electromagnetic waves and a distortion 

 of the voltage at the end of the cable. This distortion is eliminated if 

 the line is terminated by an impedance equal to the so-called surge im- 

 pedance of the cable, which is quite closely equivalent to a resistance of 

 the order 50-100 ohms. This value of resistance is very far from values 

 of megohms necessary to prevent appreciable leakage of charge and 

 special measures are therefore necessary to minimize distortion of this 

 kind. 



Another difficulty associated with high impedance gauges is the 

 vulnerabiUty of such circuits to unwanted signals. Pickup from dis- 

 turbances in the neighborhood can be minimized by proper shielding 

 and grounding of circuits, but another source of disturbance encountered 

 in use of electrical cables, known as "cable signal," must be carefully 

 considered. This phenomenon manifests itself as electrical charge de- 



^ This result is readily obtained from the basic piezoelectric equations (see Refer- 

 ence (3) for example), but is sufficiently evident not to justify a detailed proof here. 



