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Re: [PSUBS-MAILIST] Cast dome cost



Hi FrankD
As always thanks for the info. What concerns  me is the o ring on the k350 is only 1/4'' thick and protrudes 0.035'' can this be correct that's not much .Don't you think a flange gasket would be safer covering the entire surface of the ring?
All the best
GlenSA
----- Original Message -----
Sent: Wednesday, September 02, 2009 7:01 PM
Subject: Re: [PSUBS-MAILIST] Cast dome cost

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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