182 MEASUREMENT OF PRESSURES 



a water surface and placing the gauge to be calibrated in the water. 

 The pressure in the water will be double that in the air, except for 

 divergence or attenuation of the wave and nonacoustic reflection 

 (negligible for pressures less than 10,000 lb. /in. 2). The duration of the 

 wave so formed is determined by the lengths of direct and reflection 

 paths, but is easily made adequate. A necessary precaution is careful 

 determination of the initial pressure and temperature of the air in the 

 tube, to both of which factors the velocity is sensitive. With suitable 

 experimental modification, the "shock tube" methods used in air-blast 



Q=KAP _^ 



€ 



CABLE 



> 



t 



V 



1 



-O 



Fig. 5.15 "Q-step" calibration circuit. 



calibrations may thus very well provide an excellent dynamic means of 

 calibration, the chief disadvantage being the low pressure levels con- 

 veniently obtainable. 



D. Calibration standards. The simplest absolute pressure standard 

 is the free floating piston, or dead weight tester. For routine measure- 

 ments to which it is applicable, a good quality Bourdon gauge, periodi- 

 cally calibrated against a dead weight tester, has been found very satis- 

 factory for pressures up to 10,000 Ib./in.^, a sufficiently large value for 

 most work. Another method for low pressures in air, based on the 

 pressure-shock velocity relation, has already been mentioned (part (C) 

 above). If much calibration of gauges is done, it is extremely con- 

 venient to use calibrated tourmaline crystals as a secondary standard, 

 ideally in conjunction with a null comparison method. Carefully cali- 

 brated crystals are dimensionally stable and extensive checks have 

 shown no significant aging effects. 



Electrical calibration of the piezoelectric gauge circuit and asso- 

 ciated recording equipment is also necessary and a very useful tech- 

 nique for this purpose and for calibration of pressure measurements is 

 the so-called "Q-step" calibration circuit, shown schematically in Fig. 

 5.15. When the calibration response is to be recorded, the condenser 

 Cs is connected in parallel with the gauge circuit, and the voltage Vp 

 developed by pressure P is 



Vp = 



KAP 



C-\-Co + Cs 



