tolerances, in regard to roundness and uniformity of wall thickness, the 

 designer can set the operational depth of the structure as close to its 

 theoretical collapse pressure as he desires, in which case, the difference 



between the operational depth and the collapse depth is a true safety 



factor. 



The second approach is a compromise between fabrication cost and the 

 desirable strength to weight ratio of the structure. In such a case, 

 quality control and dimensional tolerances are relaxed sufficiently to 

 bring the cost of fabrication down, and yet the tolerances are not so 

 relaxed as to make the structure lose the attractive strength to weight 

 ratio imparted to it by the good mechanical properties of the material. 



Both approaches to the design of external pressure vessels have their 

 place in engineering practice, and in this particular example, both have 

 been applied to the design and fabrication of the first practical glass 

 and ceramic oceanographic instrumentation capsules, which has resulted in 

 a cost ratio of approximately five to one. Since the cost difference is 

 considerable, it bears examining as to what the additional cost has pur- 

 chased for one capsule system in comparison to the other capsule system. 



DIVEAR Capsule System 



The hull of the DIVEAR capsule^ [Diving Instrumentation Vehicle 

 for Environmental and Acoustic Research] (Figure 12) has been designed for 

 a low stress level so that considerable deviation from nominal dimensions 

 could be tolerated without failure of the hull at its operational depth. 

 The hull of DIVEAR is a Pyrex cylinder (Figure 13) which has an OD of 16 

 inches, is 58 inches long with integral ribs, and has a wall thickness of 

 13/16 inch. The glass hull was fabricated by fusing several tubular glass 

 sections (Figure 14) that had been hand blown separately within an exter- 

 nal cylindrical mold. Since each of the tubular glass sections was 

 actually a flat- bottom jar whose bottom had been only partially removed, 

 the remaining bottom rim served, after extensive flame working on a lathe, 

 as a rib for stiffening the hull. After all of the glass sections were 

 fused together, the ends of the hull were ground and the whole structure 

 annealed. Since the primary concern during the fabrication of the glass 

 hull was the evaluation of standard hand blowing and flame working tech- 

 niques for their applicability to the construction of underwater vehicle 

 structures, no special emphasis was placed on the elimination of air 

 bubbles or external ridges in the glass welds. Thus the finished glass 

 structure (Figures 15-16) has pronounced ridges and some trapped air 

 bubbles in the areas of the fusion welds. No attention was paid, also, 

 to the optical properties of the finished glass structure. Nevertheless, 



280 



