[PSUBS-MAILIST] Overpressure on dome

[Alec Smyth via Personal_Submersibles]

Yep, this is why the cabin pressure gauge and the OP valve are right in
front of the pilot. In a K250 the dome makes it particularly apparent, but
I would suggest the same approach be taken in any sub with viewports.
Temperature changes are routine and on some dives the hull can feel like an
ice maker. It's normal for temperature change to reduce cabin pressure, but
I've never encountered the opposite situation. In my mind it's the small,
slow, unnoticed air leaks that you want to look out for. Not a SCUBA tank
exploding but a valve stem that leaks a tiny amount of air. Those would
also mess up life support if it were automatic and based off cabin
pressure. So just keep your eye on that cabin pressure gauge, it's there
for a good reason and is sensitive. For the record, the OP valve cracking
pressure is adjustable with a knob, it's not hard set at 1/2 psi.

Best,

Alec

On Wed, Aug 3, 2016 at 10:10 PM, Alan James via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

>   Hi all,
> need some group input here.
> I was looking at a picture of Snoopy's dome that Doug posted on Facebook.
> (below)
> The 6 dome retainers that are made of plastic were making me nervous.
> There have been a couple of cases of domes blowing off. I think  George
> Kitrige
> was one of them. So I wanted a bit of analysis on this scenario in general.
> What sort of overpressure can you expect at a maximum on a dive?
> Doug's overpressure valve operates at .5psi but if you were 3psi
> overpressure
> the valve wouldn't operate till you were 5ft from the surface & you would
> have a short
> time to get the pressure down. Also wave movement would factor in &
> fluctuate the pressure quickly at that depth.
> I think the K250 dome is 24" diameter. I calculated out that there would
> be 452lb
> pressure on the dome retainers for every 1psi overpressure. I based this
> on the area
> of a 24" disc, or should I be basing it on the area of the dome? (Sean)
> At 3psi that would be 226lb lifting force on each of those 6 plastic
> retainers!
> What is a good safety factor here? (Sorry for picking on Snoopy Alec)
> Cheers Alan
>
>
>
>
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