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Hi Frank, I've seen many hatches that go metal to metal, and a good many that don't, with out going way beyond there general operational depths. The last hyperbaric door my relative built and had the O-ring groove cut into. He just made the groove width a little narrower then the O-ring cross section diameter, taking into account for the powder coating that would go on the door and the surfaces of the O-ring groove. He's had no problem with the O-ring staying in. Also on my original K-250 hatch, the groove is just a simple square cut, and the O-ring cross section diameter is larger the the width of the groove. It says in really well. The O-ring on mine is pretty thick, so I don't know if the whole O-ring would fit into the groove at 111 psi external pressure on the whole hatch surface, as you would expect at 250 fsw. It's hardness would of course make a difference in how much it resist the pressure. The K-250 plans call for a 15 degree angle cut on the inside groove surface. The K-250 plans call for a 10 degree angle cut on the inside groove surface. I would go with the newer plans myself. The K-250 plans call for a .25" cross section 55 durameter hardness O-ring, and a groove width or .262" to .265, and a OD of 24". So to put the 21" ID O-ring in, your going to have to stretch it, and snap it in. So both tight and loose work. Which is better for a particular sub application? My thinking currently is that for subs that goes less then 250 fsw, the square groove will server you fine. But the other configuration with the angled inside wall, to give the O-ring enough room to still have a gap on the exterior side of it, if it's being used to seal against exterior pressure, is best. Oh by the way Frank, my model of the O-ring in a groove with the angle groove on the outside was done a long time ago before I knew better. Regards, Szybowski From: ShellyDalg@aol.com Date: Mon, 7 Sep 2009 16:59:48 -0400 Subject: Re: [PSUBS-MAILIST] No Machining Hatch Sealing Concept To: personal_submersibles@psubs.org Hi Glen. yes, the mouth of the groove is larger than the "O" ring. On mine, which is taken directly from the Parker book, the groove is just under 1/16 inch wider than the "O" ring rubber ( 1/4 inch round )
Of course, they measure it in thousandths and I don't have the info right here but that's pretty close.
The depth of the groove is smaller than the "O" ring so a little rubber sticks out.
The cross section of the groove measures enough square inches so that even when the hatch is pushed down metal-to-metal there is still empty space in the groove.
I could probably find the printed copy in my sub files but it's pretty long. Best bet is to just download it from their site. It's free and is there so people can design things using just the charts and different types of applications that need "O" rings.
Frank D.
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