180 



MEASUREMENT OF PRESSURES 



sufficient to eliminate or greatly reduce pyroelectric and time constant 

 errors. The method does not conveniently provide an absolute calibra- 

 tion of known pressure amplitude, and has so far only been used as a 

 null method for comparing an unknown crystal against a standard one 

 the constant for which has been otherwise determined. The method 

 has shown considerable promise and should be a very convenient one, 

 but has not as yet been fully tested. 



An absolute method employing travelling acoustic Avaves has been 

 developed by the Underwater Sound Reference Laboratory, operated by 



(a). Comparison of high pressure static with acoustic reciprocity calibrations of tourmaline gauges 

 (sensitivities expressed in /i^icoul./lb./in.^). 



(b). Interlaboratory comparison of gauge crystal calibrations (sensitivities expressed in 

 /i /xcoul./lb./in.2) . 



Table 5.1. Calibration of tourmaline piezoelectric gauges. 



Division 6, NDRC during the war. This method provides valuable 

 information on the response characteristics of gauges, as discussed in 

 section 5.5, but is not satisfactory for obtaining primary calibrations of 

 explosion pressure gauges because of its limited accurac}^ and the very 

 low pressure level. The limited accuracy is partly due to experimental 

 difficulties in calibrating a steady state sound field (interference effects 

 from reflections at boundaries of the finite medium), and partly results 

 from the low sensitivity of gauges intended and suitable to measure 

 much higher pressures than are obtained with acoustic sources. A pres- 

 sure of 100 lb./in.2 is very low on the scale of underwater explosions, but 

 sound projectors rarely develop pressures of more than a small fraction 

 of an atmosphere under calibration conditions. This great difference 

 is reflected in the very low signal developed by a tourmaline gauge 

 tested by such pressures, and has the more fundamental disadvantage 

 that any nonlinearity of gauge response with increasing pressure will 

 not be detected. It is interesting that, in spite of these sources of error, 

 the agreement between the steady state absolute calibrations and static 



