Re: Proper Weighting in COLD or WARM water (was Re: Cruise Ship Dive)


Dave C wrote:
> Reef Fish wrote:
> > Dave C wrote:
> > > Reef Fish wrote:
> > > > Dave C wrote:
> > > > > Reef Fish wrote:
> > > > >
> > > > > > BUOYANCY control is EVERYTHING in diving -- except it's a skill
> > > > > > seldom acquired by the Cold-Water trained divers.
> > > Snip
> > > > > I beg to differ, Bob.
> >
> > We had one of the few meaty discussions I've had in rec.scuba.* for
> > some time.
>
> It was good for me, too. Cigarette, Bob?
>
> 8^)

No thanks. Gave it up cold turkey in August 1992, NOT for health
reasons, but getting damned tired of the anti-smoking zealots. :-)
My air consumption rate got a little WORSE after I quit. <G>




>
> > Since we had more agreements in the ESSENTIALS of weighting and
> > buoyancy, I'll cut this short and provide mainly some clarifications
> > for issues you didn't catch or misunderstood.
>
> SNIP
> > > > > I have a quick rule of thumb for ROUGHLY estimating weighting required
> > > > > by undergarments is to put them in a plastic bag and ball them up with
> > > > > a little compression and estimate the volume they will displace in the
> > > > > water.
> > > >
> > > > That's a very bad "rule of thumb". That's not even proper physics!
> > > > Proper calibration is no different for cold and warm water divers.
> > > > Find out how much ACTUAL weight you'll need to remain neutral (with
> > > > All air out of the BC) when you're doing your safety stop at 15 fsw
> > > > with 500 psi.
> >
> > > For a "ROUGH" rule of thumb, where's the problem with physics?
> >
> > The physics is Archimedes principle. So, how do you estimate the
> > COLUME of the displacement of your wadded plastic bag that is
> > partially submerged? (In cu.ft??) You can't! And if you're off
> > by as little as 20% of the volume, you're off by more than 12 lbs
> > according to Mr. Archimedes!

Note my key objection to your rough estimate was the "wadded plastic
bag partially submerged".
>
> For this rough estimate of weighting, one can estimate the volume of
> water that _will_ be displaced by the drysuit undergarments by a number
> of methods.

You didn't say that before! :-)
>
> Measuring the actual displacement IN WATER isn't necessary for such a
> rough estimate, if that's what you thought I was doing.

Yes! Archimedes required you to measure the DISPLACEMENT in water!

>
> Again, let me emphasize that this is a ROUGH estimate. How rough? Might
> be plus or minus 10%, but it saves time and effort.

That wasn't the problem. It was HOW you can get that close, even
though
10% is hardly negligible. :)

>
> So, Rube Goldberg here balls up the undergarments in a plastic bag,
> presses a little air out of the bag (to simulate the supposed "4 psi"
> experienced in actual drysuit use), and measures the volume, calculates
> the weight of displaced water.

Wait! You haven't determined how much water it displaced it YET.
The volume of the wadded plastic bag is immaterial. It doesn't
matter if it's 1 cu. ft. and 1 cu ft of water would weigh about
62.5 lbs. :-)

If you have ever taken a plastic bottle underwater, you'll realize
how SHALLOW you need to go before it is crushed by the pressure!
You can't measure the volume at the surface and pretend it's the
displaced volume as soon as you jump into the water. For a ROUGH
estimate, you have to put Boyle and Archimedes as buddies to work
together!

>
> Your choice: make it a rough sphere, measure the circumference and work
> from there (what was it for volume of a sphere, 4/3 times Pi times the
> cube of the radius? I forget).

You got the volume formula right, but see my preceding paragraph!

> Or, take a 5-gallon sheetrock mud pail with nice cylindrical shape and
> shove the bag of undergarments in the pail with slight compression and
> estimate the number of gallons occupied. You know the weight conversion
> from there.

You are getting WARM, but still not close.

Finally, the volume of the drysuit MAY or MAY NOT have much relevance
to the amount of positive buoyancy cause by the AIR trapped inside
the suit or its insulation garments.

For the Viking drysuit I used, its VOLUME was quite large (the way
you measure it), but I could dive with it in the pool, with no
insulating garment, with 10 lbs of lead, because I could dump nearly
all of the air between the suit and my body.

For insulating garments, you'll have to get Boyle to help.

> Why compress the undergarments to simulate 4 psi compression?

4 psi? :-) In your bath?

> Well, that's the reported typical pressure of the experts' recommended
> "proper" squeeze of the drysuit.

Yeah, beware of the physics-deficient 'experts'. LOL!!!
>
> That's because 4.5 psi is probably when your nuts pop! Goddam purists!

Really now?

At 100 feet, the water pressure is 59.25 pounds per square inch (psi),
I must have pretty tough nuts because I didn't even FEEL any cracking
pressure when I was diving down to almost 300 feet. :-)


> I use a little less than proper compression, naturally. 8^)
>
> Dave C

If I WERE Popeye (the rs one), I probably would say you're a ***. :-)

But I do suggest you study your PHYSICS a little bit more carefully,
before misapplying it on your nuts. <WG>

-- Bob.

.