original position. Therefore, the first three seal designs, a, b, and c, in 

 Figure A-1 , would find it difficult to efficiently seal after the habitat surfaces. 

 Neither the retainer ring, gasket, nor 0-ring seal no. 1, respectively, would be 

 able to follow the creep displacement of the viewport; the necessary compres- 

 sion of the components for sealing would be lost. These designs function 

 perfectly for submersibles because their dive times are short, thus minimizing 

 the effects of creep. 



0-ring seal no. 2 and 0-ring seal no. 3 are both able to follow the 

 displacement of the viewport and to maintain the seal. A compressible 

 gasket might be employed at the retainer ring-viewport interface, not as a 

 seal, but as a spring to maintain pressure on the viewport for low-pressure 

 sealing. Operationally, an 0-ring that is stretched around the viewport 

 (0-ring seal no. 3) is much easier to work with than the reverse (0-ring seal 

 no. 2). The design in Figure A-2 appears to exhibit operational advantages 

 over the other four designs. 



The success of such a seal system (Figure A-2) has already been 

 proven under long term tests in the ocean. Jenkins and Reinhart'' ^ tested 

 such a design, a conventional 0-ring seal in an angular flange, for 189 days 

 at a depth of 5,900 feet in the Pacific Ocean. The 0-ring gland materials 

 were carbon steel, aluminum, and clad steel. The 0-ring material was nitrile 

 (Buna N) with a hardness of 90 Shore A durometer. The lubrication was 

 petroleum based, although a silicone grease is recommended to preclude any 

 deleterious effect on the acrylic. Results of the tests indicated: (1 ) no 

 deterioration of the 0-ring material, (2) no significant change in 0-ring hard- 

 ness or resilience, (3) no significant amount of water absorption by the 

 0-ring, and (4) no leakage past the seal system. 



Economy 



The total cost of a viewport assembly could be reduced if an 0-ring 

 seal was used rather than a lapped-joint seal. Lapped-joint seals are extremely 

 expensive due to: (1 ) manual lapping of viewport, (2) tight tolerances on 

 dimensions, (3) additional machining for smooth surface finishes, and (4) 

 absence of interchangeability of viewports. Even for low-pressure sealing, 

 seal systems a, b, and c in Figure A-1 require tight tolerances to insure that 

 the viewport seats at the correct level in the flange. 



0-ring seals no. 2 and no. 3 on the other hand, can seal anywhere on 

 the faying surfaces so the tolerances need not be so tight. Between the two 

 designs, no. 2 and no. 3, the latter appears to be the more economical. In 

 initial fabrication, it would be more economical to cut the groove in the 

 easily machined acrylic. Also, if the flange is to be clad for corrosion pur- 

 poses, the process is more economical if it is not necessary to contend with 



35 



