654 



IV DISCUSSION 



8, Accuracy of tha Method 



A comparison of the standard deviation of any of the pressures 

 reported here with those obtained from piezoelectric measurements^^/ 

 shows that the dome velocity method is of about the same precision. 

 The percentage deviation is about the same for all of the pressures 

 studied except for the highest value of wV3/r in the tank series. The 

 difficulty in this case can be understood by inspection of Fig. 10. The 

 initial velocity is so great and the retarding influences of friction 

 and gravity appear so early that the camera speed is not sufficient to 

 resolve the phenomenon. The only recourse here is to use the greatest 

 slope observed between the first dome appearance and the preceding fram* 

 as the closest possible approach to the correct slope. The recorded 

 result for this point corresponds therefore to the single observation 

 giving the greatest velocity and does not represent an arithmetical mean 

 as do all the others. 



There does not appear to be any choice between the three cameras used 

 with regard to attained accuracy. However, the Fastaz or Eastman High 

 Speed negatives allow a more detailed study if such is considered 

 desirable. 



Uorc scatter is apparent in the surface tension series. Since every 

 other condition was identical with the other tank series, this might very 

 well be due to slight changes of the surface tension from shot to shot 

 even though the measured veulues of samples in the laboratory ware nearly 

 identical. It is known^^) that the surface tension of dilute solutions 

 of surface active materials gradually diminishes with time from the value 

 obtained at a freshly prepared or swept surface. No attempt was made in 

 the course of this experiment to keep the "atge" of the surface constant 

 between shots since it was impossible to maintain conditions in accord 

 with the practices of surface chemistry. 



9, Vfdue of the Method for the Determination of the Peak Pressure 



The precision of the dome velocity method has been established as 

 being as good as that of other accepted methods for the determination of 

 peak pressure. The limitations of the method are 



(1) that only peak pressure can be estimated, 



(2) that the range of wV3/r values is limited to greatai 

 than about 0.2 because the inaccuracy due to the unsatis- 

 factory knowledge of the tension T in Eq. (2) becomes 

 increasingly important in this region of lower values, and 



(3) that only one measurement is obtained from each shot whereas 

 with gauge methods, for example, the number of observations 

 is limited only by the number of gauges and/or recording 

 channels available. 



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