Re: [PSUBS-MAILIST] viewport calculators outputs

["Dariusz Hoffmann" <darekhoffmann@gmail.com>]

Thank you Jon for your reply. UP 4.5 requires mssql server 05, and
.net framework 2.0. Both freeware.

Safety factor of 6 seems pretty high to me. I mean, many subs has a
safety factor of 3, or even 2.5. I was thinking about a safety factor
of 3.5. I think it is a good compromise between safety, machining
loss, dynamic pressure on turns/stops, and costs of overengineered
sub.
I understand that viewports are spots of higher risk, and thus they
should be tougher then all the rest. But what view would I have
through 5" thick acrylic window? I suppose it could be pretty
distorted. Is this kind of data also present in Stachiw's book ?

darek



2008/9/29  <jonw@psubs.org>:
> Hi Darek,
>
> Interesting software and free!  I'm going to check this out in more detail
> although too bad they had to throw MS SQL into the mix.  Anyway...
>
> The PSUBS viewport calculator is based upon graph points located in
> Stachiw's book.  Although I'm sure he used mathematics during his career,
> it's my understanding that most of the information in his book is based upon
> years of practical testing of the material.  So it's not surprising to me
> that his numbers may come out more conservative than using engineering
> formulas in isolation.  However, because the graphs in his book have
> printing remnants and artifacts that need to be addressed such as ink
> thickness and clarity, and the graphs contain a sloping line that does not
> always intersect a direct xy plot point; in some cases the resulting number
> has to be interpreted.  All these considerations lead to the PSUBS
> calculator being somewhat conservative in its output, which is better than
> the alternative.  When you also consider that the limited availability of
> acrylic material thickness from retail markets rarely match an intended
> depth capability perfectly, you more than likely are going to have to "round
> up" to the next larger thickness anyway.
>
> You have to consider as well, that the physical dimensions of acrylic are
> not consistent, perhaps even within the same sheet.  For example, the
> thickness can vary across a large diameter (not a huge amount, but
> measurably) due to the way the sheets are manufactured.  This means you may
> have a viewport that meets or exceeds the calculated failure depth, and you
> may have one or more others that don't.  Hence, implementing safety factors
> which differentiate between "operating" depth and "failure" depth.  A
> simplistic example just to get the point across to those who may not
> understand where we're heading here, are blowing up balloons.  Take a bag of
> balloons and blow them up until they pop.  Do they all require the exact
> same amount of air before they burst?  Not likely.  Some balloons will burst
> sooner than others even though they may appear to all be the same
> physically.  So how do you assure that you don't burst a significant portion
> of the balloons such that you ruin the party?  You only fill them up part
> way by implementing some "safety factor" that you decide is appropriate so
> they don't pop in your face as you are filling them.
>
> All the failure points you have listed below are catastrophic data points,
> or the depth where failure is likely assured.  You'll note on the PSUBS
> calculator that according to Stachiw, the maximum safe operating depth for
> the viewport size you have specified is 60 meters.  Which brings us to your
> question "But I wonder which one of these is closest to the truth..."
> Without meaning to sound flip, I think the answer is that nobody can answer
> your question definitively, and by the way, the failure depth is not as
> interesting as the maximum operating depth which is going to include a
> safety factor.  In other words, the difference between all the failure
> depths you have documented appear to be within a safety factor of one (1),
> which is not very reassuring with all the manufacturing issues related to
> acrylic viewports.  Given the specs for the viewport you defined, you
> shouldn't be operating within 60 meters of any of the calculated failure
> depths and therefore which catastrophic failure point is closest doesn't
> really matter.
>
> Regarding a drawing for edge support,
> 1) go to www.psubs.org
> 2) click on Research Links at the top of the page
> 3) click on Design & Fabrication
> 4) click on Design Tools
> 5) click on "Viewports" at the left menu
> 6) click on "1-ATM Flat Disc Acrylic"
> 7) click on "Bearing Ratio"
>
> Or just go to this link
> http://www.psubs.org/design/viewports/1ATMFD/bearingratio.html
>
> This is a simple drawing of edge support, but does not include the retainer
> ring which is also required.
>
> I can't provide all the details in an email, so if you don't have Stachiw's
> book, please purchase one.  It has everything you need to know to fabricate
> viewports with safety factors that to date have resulted in a perfect safety
> record.
>
> Jon
>
>
>  -----Original Message-----
> From: owner-personal_submersibles@psubs.org
> [mailto:owner-personal_submersibles@psubs.org]On Behalf Of Dariusz Hoffmann
> Sent: Monday, September 29, 2008 7:27 AM
> To: personal_submersibles@psubs.org
> Subject: [PSUBS-MAILIST] viewport calculators outputs
>
> Hello Psubers, after testing 3 different viewport calculators I am little
> confused, because it seams that they're giving different results in failure
> depth. Obviously I can use the lowest output in my considerations and even
> overengineer it. But I wonder which one of these is closest to the truth...
>
> Flat disc, acrylic viewport;
> Outer diameter: 750 mm
> Free (viewing) diameter: 600 mm
> Thickness: 90 mm
>
> Analysis resulsts (same material, sea water):
>
> Under Pressure v 4.5 - Failure at 393.11 meters
> Under Pressure v 3.01 (dos) - Failure at 383.1 meters
> Flat disc calculator (PSUBS site) - Failure at 338 meters
>
>
> Another question:
> - edges fixed
> - edges simply supported
> ... what does it mean? A simple drawing would be nice :)
>
>
> Thank you,
> regards
> Darek



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