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Hi Dean. Good point. Even though we work very hard to get out hatch flanges
flat and smooth, there's always going to be some slight difference between the
two flanges. A "O" ring will still seal it up once the pressure is applied.
For Glen....the hatch dogs are mainly to hold the hatch down and sealed
when you are near the surface. Once you submerge, the pressure on the hatch
compresses the "O" ring. The pressure near the surface isn't always enough to
prevent leaks, so the hatch dogs pull the hatch down until you reach ten to
fifteen feet, then the hatch seals itself. It's tempting to tighten the hatch
dogs because they will become loose at depth once the pressure builds up
but don't. When you come back up, the dogs will be too tight and either you use
a hammer to get them loose or you go back down to loosen them.
Let's say you have a 24 inch hatch....At 100 feet deep that's about 3
atmospheres. Multiply the square inches of your hatch by the difference between
inside and outside pressure and it's a BIG number ! That's how much pressure is
being applied to the hatch.
That little "O" ring is being flattened inside it's groove, and being
forced into the tiny gap between the two steel flanges. It won't leak. I've seen
hatch flanges with several layers of paint on them and they are a long way from
perfectly flat with the paint layers and scratches etc. It still seals
good.
Most guys are using 1/4 or 3/8 "O" rings with a #70 hardness.
Remember to get the groove machined with a slight taper on the inside wall
and size the "O" ring to fit snug. That way it stays put when you open the
hatch.
Frank D.
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