Beuchart tank
Reef Fish (Large Nassau Grouper)
Bill <ma...@cs.fsu.edu> discusses the (by now) familiar spec:
>>>>cu ft pres liters diameter height weight
>>>>190 300 bar 18 8.03" 31.29" 87 lbs
>A Scubapro Steel 95, which weighs 39.8 pounts (empty, I presume), is
>0.6 pounds positive when empty. It dimensions are 8.02 inches diameter
>(only .01 difference) and 23.8 inches long. Rounding both off to
>8.00 inches diameter, the difference in external volume, assuming that
>the rounded ends are nearly the same shape, is 16*3.1416*(31.29-23.8),
>or 376.5 cubic inches. Hence the difference in lift is about
>14 pounds (in sea water) and the 190s must be about 33 pounds negative
>when empty and nearly 48 pounds negative when full| Say it ain't so.
Okay, "it ain't so", because:
1. Sanity check. No manufacturer will be stupid enough to make a tank
that is 33 lbs. negative (empty), even if some diver would use it. :-)
2. The empty/full buoyancy of a tank cannot be derived as a mathematical
function of the external dimensions of a tank. They are separately
calibrated/rated.
3. Reality check, using the (faulty) method of analysis/derivation which
was described in a later post, on a couple of actual tanks with their
given dimensions (but with KNOWN buoyancy at empty and full).
The original derivation was explained (simplified) as follows:
> :...: I've checked and the calculations are
> approximately correct. For a rough estimate with fewer opportunities
> for errors, assume the 190 is a perfect cylinder. Its outside volume
> is then 1584.6 cubic inches, or 25.97 liters. It then displaces 25.97
> kg or 57.2 pounds in fresh water and is about 30 pounds negative.
> Even assuming the 87 pound weight was full they are still (by this
> estimate, which is low) 15 pounds negative when empty. In any case,
> two Scubapro 95s hold the same amount of air and are lighter both
> in and out of the water, if the specifications given for the 190s
> are correct.
I verified the above calculations (on a spreadsheet; just to make sure
I was using Bill's method as he did and described) for the 190, and
then did the same for a steel 72 and a steel 95 to compare the
"theoretical" buoyancy so derived against the "actual" buoyancy of
those tanks. This is what the figures look like:
(w) (d) (w)-(d) ACTUAL
tank diameter height weight Volume displace buoyancy buoyancy
volume (inches) (inches) (lbs.) (cu.in.) wt. (lbs) empty empty
190 S 8.03 31.29 87.00 1,584.63 57.22 29.78 ???
95 S 8.02 23.82 37.20 1,203.32 43.45 -6.25 1.50
* 72 S 6.84 20.47 28.60 752.18 27.16 1.44 -4.60
* I used one of these tanks once, on a liveaboard; and was the only time
(22 dives actually) I was OVERweighted when I dove without any weights.
The specs for the 95 (95.1) and 72 (71.4) steel tanks (and their rated
buoyancy when empty) are taken from a table in NAUI's Advanced Diving
manual (1989).
Note the particular 72 steel (there are others with different specs) is
more NEGATIVE than the 95 steel when empty (by 6.1 lbs), whereas Bill's
method of derivation would have said it's more POSITIVE (by 8.69 lbs)|
If you're still unconvinced that Bill's method of derivation was faulty,
this should convince you: Note in that method, (d) is derived simply
as a function of the external dimension of the cylinder. This number
is subtracted from the actual weight (w) of the tank to derive the buoyancy
at empty. The critical missing element in that derivation is of course
the THICKNESS of the tank material which determines (w) -- thus, by
varying the thickness of the steel, the manufacturer can leave the tank
at essentially the same external dimensions and make the 72 S weigh more
than 87 lbs., or less than it's actual 28.6 lbs., and set the buoyancy
at empty to whatever it (the manufacturer) pleases. The same holds for
the 190 S. The manufacture could easily have made it NEUTRAL (or even
positive) at empty while keeping the other external variables/dimensions
essentially intact. The difference in buoyancy of a tank between full
and empty is of course determined by only the mass of air in the tank
between full and empty.
Back to the riddle posed by <mmi...@hubcap.clemson.edu (Michael N Milde)>:
>>>Would YOU get in the water with anything that weighed 87 pounds??? :-)
Why not? While in the water, it COULD be no different (in buoyancy) from
the one that weighed 29 lbs. But carrying that 87 pounds BEFORE you get
in the water is something else. ;-)
-- Bob.
Bill Mayne
>It's time for me to jump net and put in my 6 peso's (US $.02) worth.
>Bill <ma...@cs.fsu.edu> discusses the (by now) familiar spec:
>
>>>>>cu ft pres liters diameter height weight
>>>>>190 300 bar 18 8.03" 31.29" 87 lbs
>
> >A Scubapro Steel 95, which weighs 39.8 pounts (empty, I presume), is
> >0.6 pounds positive when empty. It dimensions are 8.02 inches diameter
> >(only .01 difference) and 23.8 inches long. Rounding both off to
> >8.00 inches diameter, the difference in external volume, assuming that
> >the rounded ends are nearly the same shape, is 16*3.1416*(31.29-23.8),
> >or 376.5 cubic inches. Hence the difference in lift is about
> >14 pounds (in sea water) and the 190s must be about 33 pounds negative
> >when empty and nearly 48 pounds negative when full| Say it ain't so.
>
>Okay, "it ain't so", because:
>
>1. Sanity check. No manufacturer will be stupid enough to make a tank
> that is 33 lbs. negative (empty), even if some diver would use it. :-)
That has not been demonstrated.
>2. The empty/full buoyancy of a tank cannot be derived as a mathematical
> function of the external dimensions of a tank. They are separately
> calibrated/rated.
It can, to a very good approximation. Buoyancy is a function of weight
and external volume. External volume is strictly a function of external
dimensions. Inaccuracies creep in only because the dimensions don't
completely describe the volume. In particular a perfect cylinder
enclosing the tank has more volume than the actual tank, because of
the rounded ends of the tank. Hence the straight cylinder calculation
is an upper bound on external volume. The difference between this and
the true value, for a tank of a given diameter, is nearly the same
for two tanks of different length but equal diameter. In the case of
the steel 95s and 190s the difference is about 4 pounds.
>3. Reality check, using the (faulty) method of analysis/derivation which
> was described in a later post, on a couple of actual tanks with their
> given dimensions (but with KNOWN buoyancy at empty and full).
>
>The original derivation was explained (simplified) as follows:
>[Perfect cylinder calculation.]
>
>I verified the above calculations (on a spreadsheet; just to make sure
>I was using Bill's method as he did and described) for the 190, and
>then did the same for a steel 72 and a steel 95 to compare the
>"theoretical" buoyancy so derived against the "actual" buoyancy of
>those tanks. This is what the figures look like:
>
> (w) (d) (w)-(d) ACTUAL
> tank diameter height weight Volume displace buoyancy buoyancy
> volume (inches) (inches) (lbs.) (cu.in.) wt. (lbs) empty empty
>
> 190 S 8.03 31.29 87.00 1,584.63 57.22 29.78 ???
> 95 S 8.02 23.82 37.20 1,203.32 43.45 -6.25 1.50
(Note correction already posted reversing signs above.)
The actual empty buoyancy of a 95, according to specs I have, is +0.5.
After you correct for the difference of about 4 pounds between the cylinder
estimate and the true value this gets to within about 2 pounds of the actual
value, and within one pound using the actual buoyancy you give, which is not
bad.
>If you're still unconvinced that Bill's method of derivation was faulty,
>this should convince you: Note in that method, (d) is derived simply
>as a function of the external dimension of the cylinder. This number
>is subtracted from the actual weight (w) of the tank to derive the buoyancy
>at empty. The critical missing element in that derivation is of course
>the THICKNESS of the tank material which determines (w)...
I was taking weigh values as given. The thickness of the metal has nothing
to do with the outside volume.
> -- thus, by
>varying the thickness of the steel, the manufacturer can leave the tank
>at essentially the same external dimensions and make the 72 S weigh more
>than 87 lbs., or less than it's actual 28.6 lbs., and set the buoyancy
>at empty to whatever it (the manufacturer) pleases.
They can do this only by increasing the working pressure, since thicker
walls mean less internal volume, for the same external dimensions. But
it would be stupid to put much more metal than is needed into a tank,
except, of course, for a safety factor.
> The same holds for
>the 190 S. The manufacture could easily have made it NEUTRAL (or even
>positive) at empty while keeping the other external variables/dimensions
>essentially intact.
They could not have done this. They need a certain internal volume,
proportional to the inverse of the working pressure. To make the tank
neutral with the same external dimensions they would have to make the walls
thinner. There is a limit to how thin they can make the walls and still have
them withstand the working pressure.
> The difference in buoyancy of a tank between full
>and empty is of course determined by only the mass of air in the tank
>between full and empty.
So true. It is amazing how many people don't seem to understand that.
At least we agree on something. :-)
>>>>Would YOU get in the water with anything that weighed 87 pounds??? :-)
>
>Why not? While in the water, it COULD be no different (in buoyancy) from
>the one that weighed 29 lbs. But carrying that 87 pounds BEFORE you get
>in the water is something else. ;-)
But evidently it is different in buoyancy. It is quite a bit more negative.
To make a 190 which was nearly neutral, with the same materials, would
require a bigger external volume, and that might not be acceptable.
As for the weight of tanks before getting in the water, that is the
toughest part of diving for me (besides paying for it), using double
104s which weigh over 100 pounds when full.
Bill Mayne
Craig Campbell
>>>>>Would YOU get in the water with anything that weighed 87 pounds??? :-)
Yes. I use twin 95's and still require some weights in to cold water dives
I do. Negative tanks allow me to loose the weight belt (which I don't like),
and have more insulation (which I do like). What do twin 95's weight?
Less than 104's, but not by much.
Pardon?!
craig
--
Craig Campbell | These opinions are entirely the result of a
Systems Engineer | small butterfly crashing into the far side
NCR Canada, | of the planet. Chaos Rules.
an AT&T Company | I speak for no one, except, possibly, myself.
Bill Mayne
>Who know who wrote:
>
>>>>>>Would YOU get in the water with anything that weighed 87 pounds??? :-)
>
The problem with the Beuchat 190s is not so much weight as buoyancy. They
weigh just a little *more* than two 95s, but they are about 30 pounds
negative when empty. The 95s are nearly neutral when empty. Big difference.
Beuchat had a representative showing the new tanks at the NSS-CDS
workshop last weekend. The biggest one he had was a 140. The actual
sizes were specified in liters, but he translated. He thought that list
price on the 190s is $595, and declined to speculate on what they will
actually sell for.
>I do. Negative tanks allow me to loose the weight belt (which I don't like),
>and have more insulation (which I do like). What do twin 95's weight?
>Less than 104's, but not by much.
A set of 95s weighs about 10 pounds less than a set of 104s. This difference
is more significant that it might appear.
Bill Mayne
Craig Campbell
>In article <1993Jun16.1...@cattnts.canada.ncr.com> cr...@cattnts.canada.ncr.com (Craig Campbell) writes:
>>Who know who wrote:
>>>>>>>Would YOU get in the water with anything that weighed 87 pounds??? :-)
>>Yes. I use twin 95's and still require some weights in to cold water dives.
>The problem with the Beuchat 190s is not so much weight as buoyancy. They
>weigh just a little *more* than two 95s, but they are about 30 pounds
>negative when empty. The 95s are nearly neutral when empty. Big difference.
Yup, that's an important detail. I still think I'd like to try one. Up here,
I hang drop wieghts on me like a christmas tree for the really cold water
dives. (Although I must in all fairness admit that when posssible I ditch
them until decomp.)
Heard any first or second hand reports of actual dive experiences with these
babies? A single tank instread of doubles sound appealing to me. Or imagine
a twin set of these? Lots of air, but just maybe it's float tube time...
>Bill Mayne