206 MEASUREMENT OF PRESSURES 



has the disadvantages that a sine wave, being a smooth curve, is not 

 ideally suited for precise interval measurements, and is rather difficult 

 to calibrate with high accuracy. A further objection is that the record 

 of a single frequency gives virtually no indication of transient response 

 characteristics. A better procedure is to separate the two types of 

 calibration and perform each in a more convenient and useful way. 



The Q-step principle of calibrating piezoelectric gauge circuits has 

 already been discussed. As was pointed out in section 5.7, this method 

 of applying a known transient voltage, and in effect a known quantity 

 of charge, provides a complete transient response calibration of the 

 entire electrical system (cable, amplifier, cathode ray tube), except, as 

 ordinarily used, for effects of distributed parameters in a long cable. 

 The most useful type of transient voltage for purposes of analysis is 

 usually a step, or Heaviside unit function, although sometimes other 

 functions, such as a negative exponential, may be desirable. Various 

 means of generating such transients have been developed, which have 

 in common an accurately known source of d-c voltage and a conveniently 

 controllable switch (either mechanical or electrical). A simple form of 

 transient generator, consisting of a high-capacity dry-cell and precision 

 voltage divider switched by a relay and associated control circuit, may 

 be adequate for some purposes. For more critical applications, more 

 elaborate circuits have been developed which include an electronically 

 regulated supply switched by a vacuum-tube "gate" circuit, a means of 

 delaying the operation for an adjustable interval after a synchronizing 

 pulse, and an internal potentiometer circuit for checking the calibration. 



For timing calibration, pulses of sharply peaked wave form can be 

 provided from standard crystal or tuning fork frequencies and their 

 submultiples obtained by multivibrator or scaling circuits. These 

 pulses can be applied to the cathode ray tube as a signal or intensity 

 modulation, or used to control a glow discharge tube mounted optically 

 adjacent to the tube, which thus provides timing marks on a moving film. 



Summary. To sum up the methods of transient recording, most of 

 the reciuirements can be met by extension or adaptation of standard 

 electronic and photographic techni(]ues. The important consideration 

 is that such equipment must be capable of giving quantitatively reliable 

 records dependably imder field conditions. Most existing commercial 

 equipment falls short of these demands and, although some instruments 

 can be modified to perform acceptably, it is usually more satisfactory 

 to build the equipment specifically for its intended purposes. 



5.10. Experimental Arrangements for Pressure Measurements 



Although the arrangement and handling of gear for locating explo- 

 sive charges and measuring equipment involves few points of funda- 

 mental scientific interest, it is a necessary part of the experimental work 



