MEASUREMENT OF PRESSURES 171 



is attached to the commonly used copper-tubing cable as shown, and 

 the assembly is covered with a waterproofed coating. 



The coating operation is of great importance, as the completed gauge 

 must have high electrical resistance when in salt water and must re- 

 main watertight after being subjected to explosive shocks. The nec- 

 essary qualities for a coating material are: toughness and strength 

 without brittleness; freedom from air bubbles w^hich collapse under 

 transient pressures or tensions; good adhesion to the gauge assembly; 

 and freedom from spurious electrical signals. It is also highly desirable 

 that a thin coating be adequate, as the transient response character- 

 istics are affected by the overall size, and ease in application and re- 

 moval of the coating materially simphfies both the original construction 

 and the repair of a damaged unit. Many types of coating, such as 

 rubber molding, rubber tape and rubber cement layers, various lacquers 

 and plastics, have been tried with varying degrees of success, but the 

 best results have been obtained at Woods Hole using Zophar wax C276, 

 a compounded mineral wax. Two or more thin layers of this wax are 

 applied by dipping the gauge assembly into the molten wax and finally 

 a thin film of tough lacquer (Tygon TP21) is appHed. Although gauges 

 so coated are not immune to failure, they have by comparison with other 

 methods tried proved extremely rugged and easy to use and repair. 



The hourglass gauge constructed as outlined, either with a single 

 crystal or with 2 or 4 crystals, has been evolved and used in great num- 

 bers since 1941 and has proved quite successful. Most of the piezo- 

 electric measurements made in the United States have involved the use 

 of this or very similar designs. Gauges of this type have been used to 

 record pressures from values of the order of 100 Ib./in.^ up to 30,000 

 lb./in.2 or higher and have been employed both for shock and bubble 

 pulse measurement. In the interests of a more symmetrical construc- 

 tion and to obtain greater sensitivity, 2 crystals mounted face to face 

 have usually been used. The smallest practical crystal size w^ith the 

 hourglass construction is about 34 inch diameter by }/s inch thick, due 

 to the necessity for undercutting the edge. Crystals much larger than 

 3^ inch in diameter gave increasingly frequent gauge-to-gauge dis- 

 crepancies and changes in sensitivity, the cause of which w^as not defi- 

 nitely determined but probably resulted from lifting of the electrodes 

 from the crystal near its center. Difficulties with systematic discrep- 

 ancies between the smallest gauges on small charge work (1/10 to 5 

 pound charge weights) also indicated that the basic design, although it 

 led to gauges successfully used in accumulating a large amount of ex- 

 perimental data, still left much room for improvement. 



B. The Type B gauge. The basic principles of this gauge design 

 were originally developed for construction of large air-blast gauges, and 

 the primary consideration was elimination of the hourglass feature, so 



