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Hi Glen. If the "flat seal" you're talking about is the hatch gasket, it
needs to be an "O" ring.
A flat gasket won't seal the hatch. It may be OK on a shallow dive to 10 or
20 feet if both surfaces were machined to very close tolerances, polished,
and the gasket was perfect, and the hatch dogs were closed using a hammer to
tighten them down with. But much deeper and it will start leaking. Even though
the steel looks thick and strong, it will flex when REAL pressure is applied by
going deeper.
With an "O" ring, the rubber is being forced into the crack between the two
mating surfaces and as the pressure increases the crack gets smaller because the
"O" ring is being squashed. Eventually the two steel rings "touch" and the crack
between them is measured in thousandths. The rubber "O" ring is being forced
into that crack and it won't leak.
A flat gasket however has imperfections in the rubber, and there's always
going to be SOME portion of the mating surfaces that aren't perfectly flat. As
the pressure builds, the steel will flex, and it will leak.
One suggestion here....Make the "O" ring groove with a taper so the "O"
ring has to stretch a tiny bit to get it in the groove. Not much of a
taper...maybe only 3 to 5 degrees from plumb.
It's called a "dove tail" groove. This holds the "O" ring in place when the
hatch is repeatedly opened and closed.
An "O" ring joint when properly fabricated will last a long time,
and will seal even if the two machined surfaces aren't perfect. The "O" ring
itself is protected some what inside it's groove and carrying a spare is easy
and inexpensive.
I got my "O" ring information from the Parker catalogue, but it's much
cheaper to get the "O" rings from McMaster-Carr. I had a machinist plane off the
flanges and cut the groove for $300.
Some guys use a flat rubber gasket under their flat acrylic view ports but
they still put a thin layer of Sikaflex in there to make up for the small
irregularities in the steel and rubber gasket. Others use an "O" ring while some
use just the Sikaflex without the rubber gasket.
Get the book by Stachiw from the psub site. It's money well spent.
Frank D.
Frank D.
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