650 



2 - 



was extonsivoly used as an empirical gauge for the comparison of the efficiency of charges, tests 

 of completeness of detonation and of primer efficiency, etc. and to this end its design was 

 considerably modified. 



The original 12" diametor gauges, besides being bulky had the serious objection that they 

 could not be used at ranges less thvi 75 feet from a 300 lb. charge. A s-tandardi sed design with 

 a diaphragm 0.2" thick and 6" effective diameter was accordingly produced which has been in 

 continuous use ever since. 



A photografh of the standard gauge is reproduced in Figure 1 and shows the method of 

 mounting. The body of the gauge or 'pot' consists of a massive cylinder of steel or cast iron 

 10" long and with wide flanges 10" diameter. The internal diameter of the cylinder is 6" and a 

 circular fillet of hard steel is inserted at each end flush *ith the surface of the flange to 

 provide a be^iring ring round the effective circumference. The edge of this fillet is rounded 

 slightly to prevent shearing of the diaphragm. 



The utmost care was taken to select uniform diciphragms by using only the purest 

 electrolytic copper. Even with these precautions, certain differences were sometimes observed 

 with different batches of copper and in all later trials the diaphragms were selected by a 

 Vickers diamond hardness test. It was found that the diaphragms with very few exceptions lay 

 within the Vickers diamond hardness range us to 65, and for any particular series of trials, 

 diaphragms with the same hardness numbers were chosen as far as possible. 



This gauge could be used at ranges close to 30 feet frcm a 300 lb. T.N.T. charge without 

 bursting the diaphragm, but dt this range the measurements were not usually consistent and longer 

 ranges were preferred. 



The static energy and pressure calibration curves for the gauges are reproduced in Figure 2. 

 Similar curves obtained independently after a lapse of 20 years were not appreciably different. 



It was soon established that the volume of the dishing was proportional approximately to 

 ,W /d and as measurements accumulated the relation was shown to be very close to W ' /d ' . 



It was observed that the depression did not vary with the orientation of the diaphragm, 

 that oriented directly away from the charge having as large a depression as that facing. 



In addition to the 6" gauge, smaller gauges of 3" and 1" diameter were also sometimes used. 

 None of these gave such consistent reading as the 6" and they were not used extensively. It was 

 noticed, however, that the depression tended to vary nearly with vr'' especially with large 

 charges, suggesting that the small gauge measures a parameter approximating to peak pressure, as 

 might be expected from its dimensions relative to that of the pulse itself. 



Until 1945 the gauge had been used entirely as an empirical 'damage' meter aind no attempt 

 was made to determine the parameter of the pressure pulse which was recorded. 



An attempt was then made by Dr. E.N. Fox and Mr. W. T. Rollo to calculate the depression 

 which might be expected in a diaphragm gauge from a pressure pulse of given form. The report 

 on this calculati'on was issued in the report "Undex pot gauge analysis". It must be pointed 

 out that the larger discrepancy between calculated and experimental measurements of the vdlune 

 depression, although rendering the gauge of little value as a means of determining absolute 

 parameters, does not in any way vitiate the comparison of empirical 'damage' for which the gauge 

 was originally devised and for which it has proved so valuable over a period of almost thirty 

 years. 



