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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.
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