Re: Viewports gobbed on with silicon

[protek@shreve.net]

Vance,
    I admit that I should have a relief valve on my hull for safety.  I do
have a barometer that I can set each time I begin a dive.  I also have a
manual method for relieving pressure differential.  I also have a hatch
design whereby I can loosen the hatch and allow the hatch to act as a
relief without the possibility of opening.  But, consider this; relief
valves have a minimum amount of pressure for activation.  If a psubber used
a valve that, for example, has a 0.5 PSI opening pressure, then an 8 inch
diameter window that has a 6 inch unsupported area would have applied some
14 pounds of force pushing the window outward.  If you consider that there
is some sort of perimeter seal, like silicone, on the outside of the 8 inch
window, the total force in theory could go as high as 25 pounds.
    Another factor for this relief valve is the size.  Keep in mind that if
a sub makes a rapid ascent from depth and there is a pressure buildup
inside the hull the valve may be insufficient in relieving it fast enough.
My point is simply.  Don't just use silicone seal to hold windows in.  Use
a retainer ring of some sort with an approved seal or O-Ring etc.  This
technology has been around for many years.
    It also has not been mentioned to my knowledge, but acrylic plastic
flat plane viewports have, (I believe from memory) a required force of 10
pounds against the seat at all times for initial seating.  "Acrylic Plastic
Viewports", Jerry D. Stachiw, 1982.  This may also be in the PVHO standards.

Gary Boucher

>A simple check valve on a hull stop used is the norm for ABS and other 
>certifying agencies. You must have some way to relieve internal pressure. 
>Waiting for the hatch to burp is an invitation to unseating a viewport.
>Vance
>
>