PST LP 104 -- buoyancy stats

[large_diver]

Can someone give me some ACCURATE buoyancy figures for a PST LP104?

I thought they were about -8 when full. I was just at my LDS and they showed me a glossy PST brochure that said more than -12 when full.

Weird.

The same brochure showed that the LP120 is LESS negatively buoyant (~ -9, I think) when full......????

I was all set to buy a 104 since I thought they had similar buoyancy to my LP98.......now I'm confused again

Thanks.

 

[Uncle Pug]

Originally posted by large_diver
I thought they were about -8 when full. I was just at my LDS and they showed me a glossy PST brochure that said more than -12 when full.

not counting the valve the 104 is -0.7 empty and the 120 is -0.4 empty....

Add the weight of whatever gas and volume of gas you are going to use to those figures.

Even glossy brochures can have typos...
Unless of course they were filling with argon to 4K.

 

[WreckWriter]

My usual source for such info http://informatics.tju.edu/pcdm/tanks.htm shows them as 5.3 negative full, 2.5 negative empty. That would be with a valve I think as it's tough to get them full without



Tom

 

[roakey]

Originally posted by WreckWriter
My usual source for such info http://informatics.tju.edu/pcdm/tanks.htm shows them as 5.3 negative full, 2.5 negative empty.

Can’t be, at least on Earth. A 104 swings about 8 pounds from full to empty.

There's a LOT of bad information out there about cylinder buoyancy, some of it even in glossy brochures. I’ve never come up with a way to explain this, except that maybe the average marketing guy or gal is in marketing because they flunked physics.

Air weighs about 1 pound for every 13 cubic feet. That’s how I arrived at 8 pounds (104 / 13 = 8). Buoyancy is dependent on both mass and displacement, but since the cylinder’s displacement [for all intents and purposes] doesn’t change when filled, only its mass, or to simplify its weight effects the cylinder’s buoyancy.

So it’s literally impossible for a 104 to swing only about 3 pounds from full to empty (-5.3 to –2.5). The swing is one of the very first things I look at when I’m looking at cylinder buoyancy numbers to see if the numbers are the least bit believable. And I’ve seen glossy brochures that have unbelievable numbers in them!

One of the more accurate pages I’ve come across is:

http://www.diverlink.com/gear/tankspecifications.htm

It may not be perfect, but it’s close and has information on a wide number of cylinders.

It lists a 104 as swinging from –2.5 empty to –11.1 full. This is 8.6 pounds, which makes it pass the ballpark test.

Roak

 

[WreckWriter]

Originally posted by roakey

Can’t be, at least on Earth. A 104 swings about 8 pounds from full to empty.

Interesting. The site I posted is maintained by a DAN physician, it's generally pretty accurate. My 104s are on order so I wasn't able to see what is apparently a glaring error. My bust.

Then again, if you scroll down on the site you posted you'll find conflicting info, including the numbers I posted <shrug>

Tom

 

[large_diver]

Exactly....thus my reason for the question.

scubadiving.com has the following information.....for the lp 104 it says -8.4 full (apparently including valve)

http://www.scubadiving.com/gear/27tanktips/stats.shtml

So Pug says about -8.7 (using air -- I assume the valves have a negligible impact)......Roakey says -11.1 (but the diverlink site he references which is also my normal reference says both -11.1 and -12.6).

Hmmmm

 

[Rick Murchison]

Originally posted by WreckWriter


Interesting. The site I posted is maintained by a DAN physician, it's generally pretty accurate. My 104s are on order so I wasn't able to see what is apparently a glaring error. My bust.

Then again, if you scroll down on the site you posted you'll find conflicting info, including the numbers I posted <shrug>

Tom

Well, it isn't accurate in this case. Physics is physics and Roakey's right on. I don't know where the folks who post these figures get 'em, or how the bogus numbers gain credibility, but if you don't see a swing in the weight of about .08 pounds per CF then the numbers are wrong. Period. (Nitrox weighs a little more, trimix a little less, but .08's pretty close(that's the same as roakey's 13CF/pound))
Rick

 

[Uncle Pug]

Originally posted by large_diver
So Pug says about -8.7 (using air -- I assume the valves have a negligible impact)......Roakey says -11.1 (but the diverlink site he references which is also my normal reference says both -11.1 and -12.6).

Don't assume... go weigh a valve... then add it to the total...

But the figure you need to start with Chris is that manufacturer's own figure for negative buoyancy empty...
From there on out everything is variable:
What valve?
What gas?
What amount of gas?

PST says that their LP 104 is -0.7# empty

 

[roakey]

They've added a lot of stuff to that site, I just went to the table I usually use and gave those numbers. Going back I now see that we have three sets of numbers as large_diver and WreckWriter have pointed out.

Hence the confusion over cylinder buoyancy. The -11.1 versus -12.6 *might* be novalve/valve, I don't know. I DO know that the -5.3 full/-2.5 empty swing CANNOT be correct. That swing is on the order of a 40 cubic foot cylinder.

The ONLY thing you can get a good handle on is the swing weight of a cylinder from full to empty, so I'll reiterate that the DAN physician's data HAS to be wrong. I really doubt its his fault, he probably got it from a [supposedly] reputable source, such is the sad state of cylinder information.

Information like this is where the "steels don't change buoyancy as much as ALs" myths start.

-12 to -4 is probably in the ballpark, but that's a "feeling" I have from diving with my 104s.

Roak

 

[Rick Murchison]

Originally posted by large_diver
Exactly....thus my reason for the question.

scubadiving.com has the following information.....for the lp 104 it says -8.4 full (apparently including valve)

http://www.scubadiving.com/gear/27tanktips/stats.shtml

So Pug says about -8.7 (using air -- I assume the valves have a negligible impact)......Roakey says -11.1 (but the diverlink site he references which is also my normal reference says both -11.1 and -12.6).

Hmmmm

Since I want to know what the real effect of the tank's buoyancy is, what I do is ignore every published number on tank buoyancy there is. I take the tank out to the pool, and with a homemade balance suspended from the diving board, I hang the tank with 500psi in it under water on one end of the balance bar and see how much lead it takes in a bag suspended under water from the other end to balance it. So I know that *my* PST 95's with valve and *my* regulator attached are -11 pounds each, and full they're -17.5 pounds each. That is, compared to an aluminum 80, which takes a pound of lead to sink at 500 psi with my reg on it, I can remove 12 pounds of lead when I use this tank (or 23 if I have 'em doubled up) - and that's the number that really counts.
(I don't apply a salt water correction to individual pieces of equipment, but to the final total system, so I do all my measurements of individual pieces in the pool)
Rick