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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
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... |