Adam, you are partially right, but you have confused the hardness of the o-ring material (durometer) with the o-ring resiliance, which is the tendency to become permanently deformed. The durometer rating is related to the clamping pressure required to make the seal, whereas the resilience depends primarily on the material used and the percent compression. If you have enough clamping pressure (ie bolts) you don't need to use a soft material. For example Buna-N o-rings are inexpensive and have good resilience, which is why they are so commonly used. A face seal using a 70 durometer Buna-N o-ring with a 15% compression is good for over 3000psi without even trying. If you use bolts, you get plenty of clamping force. You don't need a torque wrench, you can simply tighten the screws until you hit the hard stop of the two surfaces touching. The problem is that the suitcase clamps can't do this, so the housing vendors rely on an incomplete face seal and let the water pressure complete the seal. To me this is a poor trade-off and explains the tendency for housings to flood in the rinse tank. Frankly, the pressures experienced in recreational diving are trivial for a good o-ring design, so unless the o-ring is physically damaged, issues like compression set should be almost irrelevant. If you are interested, there is more than you ever wanted to know about designing seals at http://www.logwell.c...er_Handbook.pdf
I too need to give this thread a rest.