Do housing main o-ring seal designs make sense?

I recently got my first dSLR and am puzzled about the main o-ring seal design. A lot of housings use a design in which the o-ring is placed in a groove which is barely wider than the o-ring diameter. It then makes a face seal against a mating flat surface in such a way that a gap exists between the two metal surfaces. As you descend, the pressure compresses this further so that the gap shrinks and the compression on the o-ring increases. I guess the idea is that the seal "gets better" at depth. It works, but strikes me as plain bizzare. The problem that I see is that it seals poorly at the surface giving rise to the oft-lamented flood in the rinse tank. Moreover the gap that exists at low pressure can trap dirt and debris.

If you go to any textbook on o-ring seals, you will find that they NEVER recommend such a configuration. Usually the o-ring grove is made approximately 15% shallower and 50% wider than the o-ring diameter (depending on the material and the pressures required), so that when you close the face seal, the two metal surfaces TOUCH. The compression is set and doesn't change as you pressurize the seal. With this configuration, it is easy to achieve seals up to 5000psi, which is WAY higher than anything we need in scuba.

I don't know the history, but my personal speculation is that early on someone tried the scuba design, and it worked, and became the dogma in housing design. Although we think of scuba as exerting a "high" pressure on the seal, the max depth of most housings is around 200ft. This is only a pressure of less 100psi, which is peanuts compared to the pressures that o-rings can handle. Therefore it doesn't make any difference if the seal is properly designed or not; they basically all work. Even so, it strikes me as weird.

Is there a method to the madness, or am I missing something?

On many design (our's included) the latch when closed apply substantial pressure on the o-ring, dus compressing it even before entering the water, simple but efficient, i would be an issue in the rinse tank if said o-ring where not compressed, then a fart from a shrimp would cause havoc in your gear bag ,

Regards

I just checked the compression of the o-ring on my Sea&Sea 5D housing, by wiping off all residue on the back plate face, closing it up opening it and looking at the width of the "new" residue laid down by the o-ring. The o-ring is almost dry at the moment, so there's no extra spread due to excess grease.

The line of residue is 2mm wide, for a 4mm o-ring, and it's consistant all the way round the housing.

The compression on the o-ring is applied by a set of three strong springs per latch and judging by the force required to close the latches with an o-ring in place, compared to without, I'd say they were putting on a decent amount of compression.

The concern that Steve has aired I think is an interesting one to be aware of and it is good to check the amount of compression and its consistancy around the main seal when checking out housings. As I mentioned above though, always wipe the surface clean before testing topside as an o-ring that has seen 3 ATM or so will have swept out a wider line of grease.

BTW, I wish shrimp wouldn't do that! No wonder some religions see them as unclean.

Martyn

Next to last paragraph on http://www.ikelite.com/web_pages/slrone.html says it all:

"The housing "O" ring seal is a masterpiece in simplicity compared to designs that require stuffing the "O" ring into a groove."

I agree. this o-ring seal is very easy to clean as well....

I am missing something. What is so hard about putting an o-ring into a groove? It takes all of 10 seconds.

From the responces I have gotten, the justification is that the "housing" o-ring design allows low enough clamping pressure that the suitcase latches can be used to close the main seal. To fully compress the o-ring requires higher clamping pressure than the latches can supply. Therefore one would need to switch to some sort of screw type enclosure which would make opening and closing the housing more of a chore.

I guess in the days of film this made a lot of sense since you needed to get into the housing after every dive to change the film. In the digial era, it makes less sense. I only open and close the main door of the housing once a day, usually in a hotel room. I sometimes open the port seal to change lenses, but I usually shoot one lens all day. Given the choice, I would trade convenience for durability.

Also, I like to blow off my housing with compressed air after the fresh water rinse to reduce the amount of water that drys on the surface. Several people have cautioned my about doing this since I could potentially blow past the o-ring (since it is undercompressed on the surface). I am very careful and so far have not seen any sign of problems. Again, a fully clamped design would make this a non-issue.

Another small potential benefit is that in the fully compressed design, the back plate would not move with depth. In the standard design, this plate moves with depth as the o-ring compresses. The less movement, the less chance there is of actuators getting misaligned.

I don't anticipate such a design becoming available, and besides I am going to stick with my housing for the foreseable future, but one can dream...

In answer to the question:

Yes! - The system is tried and tested and it works. Pre-loading the main 'O' ring ensures that it seals at the surface and when the ambient pressure reaches the sprung seal 'pressure' (force), the housing is at sufficient depth to ensure that there isn't a problem here either. From my experience, main 'O' rings are pretty tolerant and will accept a surprising amount of abuse (oh yes, I do know that abuse can be too much!) and are pretty simple to maintain. I suspect that the question should be something more like " Is there a potentially better main sealing system?" to which the answer is probably yes but at a substantially higher cost.

Hi Steve,

it seems to me there are two issues that you are concerned with here. Firstly the low pressure preload condition that you feel may be inadequate to keep out dirt and compressed air blasted water. Secondly there is the total pressure sealing of the o-ring joint, for example you mentioed o-ring seals being able to go up to 5000 psi.

The first concern is largely covered by the track record of the current standard designs. The "slot and face" joint is mechanically similar to Ike's ledge design in that the compression is set by the geometry of the slot/ledge and the tension of the latches. As you say ramping up the compression could increase the localised security of the o-ring, and help keep out stuff you "try" to blast in with compressed air, but at the cost of usability. I find I do change cards/batteries more than once a day when I've got good subjects so I'd hate an inconvenient mechanism.

In general compressed air isn't a real risk if you have a nice narrow gap between the sealing faces like on most housings, because there is no ram effect to channel the high pressure air into the o-ring seal. Most of it just spills out without generating much more over pressure than you can create by offering your lips up to the crevice. If it did, compressed air would be much better than it is at clearing the crevice of water. That's my observation, YMMV.

As for the high pressure sealing case, as long as the internal crevice formed between the two housing halves is tight enough to stop the o-ring from extruding into the housed cavity then the seal is by design self reinforcing. That is to say with increasing external pressure the o-ring is force deeper and deeper into the this inside corner of the slot, making the seal better and better. That's the reason for o-ring lubrication of course, to allow the o-ring to shift into this tighter position. You know that from your o-ring textbooks, but some people may not have been told or have figured this out yet.

Martyn