Chris Ross
Contributor
O-rings need to be loaded to work properly, the pressure from outside pushes the o-ring into groove and provides sealing pressure. The deeper you go the higher the sealing force. If you do the calculations, assuming a vacuum of 200 mBar (same pressure difference as the housing being at a depth of 2m) the force holding a 120mm port in is about 23kg (50 lb). I've seen reports around that 200 mBar is a common vacuum used for these systems. That force helps ensure that a bump can't dislodge a port or break the sealI hope this isn't too much of a hijack. I've always wondered why you use vacuum in a camera housing instead of pressurizing it. Seems a bit counter-intuitive to create a suction in order to keep something out. Inquiring minds and all that.
Unloaded o-rings are easier to dislodge or move to break the seal, this is why cameras will flood in the rinse tank. Having the vacuum on the housing means the port and rear door are being held closed by significant force. You also don't want to pressurize the housing as you would go through a zero pressure differential about 1-3m below the surface - when this happens the o-ring will shift from one side of the groove to the other and if it hangs up it may leak then. Vacuum systems are well proven for leak detection and also for sealing.