W8 MEASUREMENT OF PRESSURES 



steel ring of suitable size, for example, rings ten or fifteen feet in diam- 

 eter of one inch steel rod. The entire rig is then lowered from a boom 

 or overhead cableway to a suitable depth for firing. 



The successful firing of charges weighing hundreds of pounds under 

 controlled conditions is at best a rather complex operation involving 

 considerable manpower and equipment, including vessels to handle 

 gear, in addition to the basic gauges and auxiliary instruments. Any 

 such explosions must be fired in reasonably quiet water, usually with an 

 available depth of eighty feet or more. Even with purely mechanical 

 gauge instrumentation, rather large and cumbersome rigs are required 

 to provide known positions of gauges and charge. Work of this kind 

 done by the U. S. Navy Bureau of Ordnance at Solomon's Island, 

 Maryland has involved the use of pipe frameworks supported from the 

 surface by large logs or telegraph poles, the whole gear being set from 

 large barges. In some British work, the gear has been set from a mov- 

 ing vessel and supported by floats, as has also been done in the work at 

 Woods Hole. The latter work, employing the schooner "Rehance," has 

 given the most comprehensive data for large charges on effects of explo- 

 sive weight, shape and composition, variation of explosion pressures 

 with distance, and so forth, the instrumentation including both piezo- 

 electric and mechanical gauges. For most of the work, the gauges and 

 charge have been disposed along a steel cable forty feet below the sur- 

 face, the various elements being supported by floats, and the whole rig 

 kept taut by towing. ^^ Electrical firing line and cables from the piezo- 

 electric gauges are led along the surface on floats to the Reliance, a 

 seventy-six-foot Gloucester schooner fitted out with electrical recording 

 equipment, laboratory and shop space, and sources of electrical power. 



A typical rig of the type described is shown schematically in Fig. 

 5.26, the insets giving details of the various gauge mountings. On some 

 shots, as many as 8 piezoelectric gauges, 25 ball crusher gauges, 8 dia- 

 phragm, 8 Modugno and 8 mechanical momentum gauges have been 

 employed, and at the date of writing 450 large charges of various kinds 

 have been fired. For further details of rigging and handling gear, de- 

 tailed analysis of experimental techniques and measurements, and so 

 forth, the research reports should be consulted (22, 23) . 



In all explosive w^ork the hazardous nature of the work must con- 

 stantly be kept in mind and adequate safeguards against misfires, im- 

 proper handling, and so on, set up and observed. The characteristic of 

 explosions that they happen only once adds to the difficulty of good 

 measurements, because the instruments must be functioning properly 

 at the time of firing if any results at all are to be obtained from that 



2^ The methods used in measurements of large charges (50 to 1,000 pounds of 

 explosive) at the Underwater Explosives Research Laboratory (Woods Hole) are 

 fully described in a report by J. S. Coles (22), 



