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Re: [PSUBS-MAILIST] viewports

Dan,
           That is some really good insight,  something you wouldn't necessarily find in a book.   Thanks,  I only hope I can return the favor some day to somebody in the future.  
 
What about the viewports that do not have that 45 degree taper, just squared off.   Don't you have some viewports like that?   I've been thinking of maybe using that type because of the ease of machining them and just having a small tapered one in the hatch.
 
On the flat viewports does your o ring also go in that little chamfer space? And all the same would apply as far as the size of the lens and the placement of the urethane sealant.
 
Thanks
 
Brian
----- Original Message -----
From: Dan H.
Sent: Tuesday, December 20, 2005 05:57
Subject: Re: [PSUBS-MAILIST] viewports

Brian,
 
The Acrylic people can tell you what the thermal expansion of acrylic is.  I talked to them when I was doing mine but I forget the details right now.  It's pretty great, as most plastics are.  On a small diameter viewport, it's not a big deal because there aren't that many inches of acrylic to expand and contract, but a large diameter lens changes dimension quite a bit. 
 
My first design followed the recommendation in PVHO for a flat lens with a 45 degree chamfer on it's outer edge to contain an O-ring seal.  The O-ring hits in three places.  One is on the 45 degree chamfer on the lens, the other is on the viewport housing and lastly on the cover ring that holds the O-ring and lens in place. 
 
The PVHO standards also call for a flat urethane cushion to be placed between the lens and the housing.  I guess that's so any out of flatness in either the lens or housing can be cushioned. 
 
The problem was that the hot, 130 degree, diameter and the cold, 30 degree, diameter varied so much that a viewport housing made to fit the lens when at it's largest, will be big enough to leave a big clearance gap when the lens is cold.  The gap between the lens and housing when cold was more then the O-ring manufacture recommended.  I talked to the O-ring engineers and they said that a three point contact application as this application is in PVHO actually forces the O-ring to extrude at a lower pressure.  They said, this coupled with a gap that could grow quite large is not a good design.   If the O-ring did extrude in this design, it may also take the urethane disk with it and blow the whole thing into the sub. 
 
So what did I do?  I did what worked for Captain Kittredge for years.  I machined my lens smaller to create an 1/8 inch gap all around it circumference and I made the housing so it was about fifty thousants deeper then the lens was thick.  When I installed the lens, I imbedded it in urethane sealant.  This created a flat bed of urethane for the lens to rest against and a 1/8 inch ring of Urethane to seal the gap around the circumference of the lens.  For good measure, I put the O-ring in the 45 degree chamfer I had around the edge of the lens also.  The O-ring is sort of redundant, but if the urethane ever separates, the O-ring will still keep the seal and as long as the urethane fills the gap, the O-ring can't be extruded. 
 
I know I don't have the expansion room I was first trying to have.  But, I don't have the lend pressing against bare metal and I have a good seal.   You can't knock success and Captain Kittredge had success with this method for thirty years. 
 
Dan H.
----- Original Message -----
From: Brian Cox
Sent: Tuesday, December 20, 2005 1:02 AM
Subject: Re: [PSUBS-MAILIST] viewports

Maybe make the lens and then measure it hot and cold, then machine the viewport housing to the hot dimension and get the right size o ring for whatever the gap there is.
 
Bri
----- Original Message -----
From: Dan H.
Sent: Monday, December 19, 2005 20:03
Subject: Re: [PSUBS-MAILIST] viewports

Yep!  Plexiglas is a trade name for Acrylic. 
 
Some one wrote that you should get a copy of PVHO.  It's a good idea if you can get a look at it.  In there, you'll find the answers to most of your questions. 
 
The one thing that PVHO didn't answer for me was how to deal with thermal expansion of the acrylic lens.  It's a great set of standards to design with if your viewport is in a pressure chamber in a building, but a sub viewport can be exposed to 130 degrees F in or better in the hot sun and 30 degrees F in cold water.  Mine has already seen that extreme.  If I followed the PVHO exactly, my external O-ring seal would now be stuffed in the annulus between the lens and the viewport housing after my deep water test in cold water. 
 
Dan H.
----- Original Message -----
From: Brian Cox
Sent: Monday, December 19, 2005 9:32 AM
Subject: Re: [PSUBS-MAILIST] viewports

"Cast" Acrylic is what to use - correct ?
 
Brian
----- Original Message -----
From: Dan H.
Sent: Monday, December 19, 2005 04:51
Subject: Re: [PSUBS-MAILIST] viewports

Joe,
 
Look up Acrylic in a search, find a manufacture you like and either E-mail or call them with an inquiry about where the nearest stocking distributor is in your location.  That works for most anything your looking for.  Get a few distributors names and you can bounce pricing between more then one if you like.
 
Isn't the internet a great thing? ;-)
Dan H.
----- Original Message -----
From: JOE
Sent: Monday, December 19, 2005 12:54 AM
Subject: [PSUBS-MAILIST] viewports

Looking for a plastic supplier for viewports, in the USA
 
Thanks
Happy holidays
 
Joe