btwn dives, do you leave your camera latched up in its housing?

[Shasta_man]

Aside from the specs, I would say, no, it's not a good idea to leave your camera in the rinse tank longer than necessary to rinse off. Lots of thoughts why, like it being the lowest pressure time and with banging around into other housings, some have had it flood in the tank. I personally don't open it unless I need to, thereby avoiding introducing another point of potential failure. I use larger memory sticks to be able to do so, and usually the battery, also extended if possible, is the reason for opening. Of course, there is a point at which larger memory adds risk because instead of losing just this dives pictures, the larger stick had a few dives on in before you flooded it.

 

[spt29970]

Do I read this right, your findings are that the groove width of your housing is actually narrower than the thickness of your o-ring?
And if you were to close your housing so the housing parts touch, which you say it does when you vacuum your housing, the groove height will be reduced to 0.100?

That sounds too small. Rubber is incompressible, so when you squeeze it from 0.139 down to 0.100, that rubber has to move somewhere. That's why you should have a wider groove. Granted, it will change it's shape to look more like the groove, so going from circle to square cross section will help, but it feels like something is off. A closed groove with 0.1 height and 0.136 width won't fit an o-ring with 0.139 cross section diameter. Unless I understood you wrong?

You understood correctly. The o-ring groove I measured is slightly narrower than the thickness of the o-ring, which violates the design guidelines which suggest that the width should be 1.5x the thickness to accommodate lateral expansion. I think the idea was to hold the o-ring in the groove during assembly. A standard groove would have problems keeping the o-ring from popping out where it bends around the corners.

I don't know why they choose this odd design, but in my experience o-rings can be very forgiving. You can design a groove that is very far from the design guidelines and it will often still sorta work. My personal feeling is that this is why many housings have problems in the rinse tank. With "good enough" designs, they will seal so long as they doesn't get jarred or banged. If it fails it can be chalked up to user error.

I recently upgraded to the EOS 5D3 and they changed the groove design. Instead of using a straight groove, the new housing uses a dovetail profile. This retains the o-ring without having to use a channel that is so narrow. I haven't measured it in detail, but it looks to be much closer to the guidelines than the design in used in the original 5D housing.

One last thing. I have the vacuum system on the new housing, and I can verify that under vacuum it closes metal-to-metal. I am not entirely sure whether that is true for the original 5D housing. In their defense I have never had the system flood due to the back plate seal failing. I did have a flood with the original 5D housing because it didn't have a lock for the front port. The bayonette mount came untwisted and the port fell off. The new housing has a lock that prevents this from happening on the housing end.

 

[Golo]

Okay, I checked the values for an o-ring cross section of 0.139, a groove width of 0.136 and a groove height of 0.100. (By the way, all those values are imperial (inch), right?)

If those values are correct and assuming no tolerances, then the fill rate is at 110%. And as I said, while rubber can change shape under compression, it can not change in volume. So either the housing will not go into contact (or at least bends), leaving a gap just big enough for the rubber material to squeeze in, or something gets damaged. Either is not exactly good. You would not want the o-ring to squeeze/bend over the edge of the groove.

Squeeze (compression) would be within recommendation at 28%. A fill rate of 100% would be achieved at a height of 0.110, and at 0.115 height you'd have 95% fill rate with a squeeze of 17%, which would still be within recommendation.

And you are right, you can intentionally make a groove narrower than the cross section of the o-ring for it to not drop out during assembly, however that is not good design. You need squeeze for a seal, and you need space for the rubber to move to, no way around it. A design with a groove narrower than the o-ring cross section means you can't close the groove, risking damage at the groove edge when under pressure.

But you are right, at the moderate pressures under water plus the ability to easily exchange the o-ring for underwater housings you can probably get away with a lot of "bad design".

If you mean what I think you mean with dovetail design, then yes, that is a better design. The standard is to make the inside diameter of the groove bigger than the inside diameter of the o-ring, so the o-ring stretches and keeps in place that way (which is also the case with my housings, and they don't pop out during assembly).