8.55 VIII.8.6: Structure Monitoring and Leak Detection: The complete loss of a hull plate is immediately apparent, but a means of detecting plate cracks is needed. A cracked plate leaks,so a general leak detection system serves as a hull structure monitoring system. Scanning the outer surface of the hull for traces of leaking gas is difficult, and would only indicate leaks in the outer hull. Since the hull is compartmented, continuous measurement of atmospheric pressure differences between the compartments and the hull is a much simpler and more thorough means of indicating leaks and plate failures. The main inner hull volume is 3030 times that of the largest compartment volume, so it serves as an effectively constant pressure reference even if it is leaking. If all compartments are kept at a lower pressure than the main hull volume, an increase in a compartment's pressure indicates a leak in the inner hull to that compartment, and a decrease in pressure indicates a leak in the outer hull. There is an even number of compartments in the circumferential direction around the hull, and all compartment bulkheads meet only at four-way corners, so that compartments form a sort of closed checkerboard pattern, distorted in size in the endcaps. If alternate compartments are kept at two different pressure levels (analogous to the red and black squares on a checkerboard), all bulkheads have a pressure difference across them. A reduction in the difference between the measured pressures of two adjacent compartments then indicates a leak irr their shared bulkhead. (Both compartment pressure levels must still be lower than the main hull pressure for hull leak detection.) The leak detection system indicates only the leaking hull or bulkhead in any compartment; it does not indicate the exact cracked plate. But such a crack would be long enough to be detected easily with available bridge inspection equipment (8.36). The shielding material has to be removed to locate a crack in the outer hull or bottom of a bulkhead, but this has to be done for the repair anyway. Commercially available semiconductor pressure sensors claim accuracies better than ~1% of range, but field use of uncalibrated units cannot be expected to get uniform performance of better than
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