Diminishing Returns and Gas Calcs for Cave Filled LP Tanks?

[UCFKnightDiver]

I can show you photographs of my aluminum 80s that have “S80” and “3AL-3000” stamped in them. Doesn’t change the fact that they hold 77.4 ft.³. Not 80.

The claims on most packages are exaggerations. Why should scuba tanks be any different?

I understand what you're saying tmassey! I just wanted to point out that the cylinders are indeed marked as that. I do see that the original hydro has a + stamped next to it, which seems to align with what you were saying regarding the 2640 pressure.

 

[tbone1004]

View attachment 644917 View attachment 644918

@tbone1004 I'm pretty sure these are pics of the service pressure and tank volume on my Faber 108s

That is in fact the service pressure, but it is NOT the working pressure, which is 2640 due to the + rating. The volumes are calculated based on that + rating and is industry standard for umpteen million years has been to always quote 3aa scuba tank volumes at their fill pressure with the 10% overfill. This goes all the way back to the LP72's.
Note below that the manufacturer only uses the working pressure in their tables, and not the service pressure for the 3aa cylinders.
https://kaplanindustries.com/wp-con...-Faber-Steel-Cylinder-Specifications-2019.pdf

 

[tmassey]

That is in fact the service pressure, but it is NOT the working pressure, which is 2640 due to the + rating.

I’m pretty sure I used working pressure in my comments above, which as T-bone brought out is incorrect: 2400 is the service pressure. I’m not going to go back and change them, but wanted to note the inaccuracy.

The stupid plus Hydro thing is one of the most annoying things about LP tanks. The amount of old wives tales, ignorance, and flat out lies told in self interest are overwhelming. That’s an advantage that aluminum and high-pressure steels have: one pressure, one rating, one volume, etc.

At least modern LP tanks almost always have the REE number stamped into them now. Doesn’t usually mean you’re going to actually get the plus, but at least it’s not virtually impossible to get the plus rating.

 

[tbone1004]

I’m pretty sure I used working pressure in my comments above, which as T-bone brought out is incorrect: 2400 is the service pressure. I’m not going to go back and change them, but wanted to note the inaccuracy.

The stupid plus Hydro thing is one of the most annoying things about LP tanks. The amount of old wives tales, ignorance, and flat out lies told in self interest are overwhelming. That’s an advantage that aluminum and high-pressure steels have: one pressure, one rating, one volume, etc.

At least modern LP tanks almost always have the REE number stamped into them now. Doesn’t usually mean you’re going to actually get the plus, but at least it’s not virtually impossible to get the plus rating.

how did you back calculate the water volumes? Or did you measure? I know @Luis H actually has measurements on all of his bottles...

 

[tmassey]

how did you back calculate the water volumes? Or did you measure? I know @Luis H actually has measurements on all of his bottles...

Where? In this thread, I simply used what the original poster said (16L), which turns out is probably wrong. I just did the basic math to expand 16L at 248 bars to cubic feet, and 108ft3 at 2640 to volume at 3600. They matched pretty closely. (I wasn’t going to muddy the point by bringing Z factors in, unlike certain overachievers. ) His only real mistake was dividing by 2400 instead of 2640. Well, that and trying to get such a highly exact number and compare it with a completely different tank’s highly exact number.

In the scuba tank specifications, all of those are intentionally pulled from information published directly by the manufacturer. However accurate that may be, which probably isn’t gold plated in the first place. A perfect example is the OMS tanks and the Faber tanks differ in *some* of their specifications, when we all know the OMS tanks were made by Faber!

And the moral of the story is: it pointless to try to be this precise. It’s impossible for anyone to speak authoritatively and precisely about the volume of any given scuba tank, unless they have personally verified the water volume of that specific tank. You mentioned that Luis has specific water volumes of his tanks. I’d love to know how those compared to the nominal values from the manufacturer.

The next time I get a Hydro I should see if the Hydro place can give me the exact water capacity. But frankly, they’re doing so many tanks, I don’t know they want to take the time. But I can ask.

 

[tbone1004]

@tmassey I meant on your chart with the PST's which I've never seen water volumes for. Worthington and Faber published theirs but I don't know if I've ever seen PST's.

 

[UCFKnightDiver]

@tmassey are those buoyancy characteristics for tanks in salt water? ~+2 positive when empty with valve for the Faber LP108s seems a little off, at least for freshwater.

 

[tmassey]

@tmassey are those buoyancy characteristics for tanks in salt water? ~+2 positive when empty with valve for the Faber LP108s seems a little off, at least for freshwater.

I’ll say what I said in the tank specification thread: all of the details for all of the measurements are from the manufacturer, and the link where I found those specifications is included in the spreadsheet. You do have to be careful, because different manufacturers list their characteristics differently: with or without valve, completely empty or 500 psi, etc. That was why it was so important for me to only list information that could be tied directly back to a manufacturer-provided data set. That way, each person could examine the information and see how it applied to other tanks and their own needs.

Now as for whether any of that information is actually accurate: I have no clue, nor do I claim anything. You’re not the first person to disagree with what’s on those charts, nor will you be the last. I can’t tell you if it’s right, I can only tell you where it came from.

It was asking the kind of questions that you are asking that caused me to waste a day of my life in collecting and collating that chart. When I saw the differences and variations in manufacturer supplied information, I realized that it was impossible to try to be precise. And that is before you include the fact that air, Nitrox, oxygen and Trimix all work differently inside scuba tanks, and work differently at different pressures! (All is relative! Nothing is real!)

Seeing all that allowed me to relax about the tiny details of mathematical calculations surrounding scuba tanks, and focus much more about general characteristics that fit me more specifically: length, width, general capacity, etc. And then use the tanks that fit me the best, and let go of the tiny details.

So, for example, I use HP 120s because I prefer the trim characteristics and smaller cross-section over 130/104’s. I lose a little bit of gas. But, given real world gas characteristics, not as much as you would think. And if the difference between doing a dive and not doing it is the difference in gas between 120s and 130s, maybe I should just bring a stage bottle instead, and have a lot more gas!

Anyway, I know that doesn’t answer your question specifically. But it’s what I got!

 

[tbone1004]

@tmassey are those buoyancy characteristics for tanks in salt water? ~+2 positive when empty with valve for the Faber LP108s seems a little off, at least for freshwater.

+3 in salt, tank only. Per the distributors website.
It displaces about 1100ci per the website so call it 4.75gallons so it would be about +2 in fresh.
I just weighed a Thermo manifold valve, admittedly without bonnet or plug n it because it was in the ultrasonic, but it was 1lb 8.6oz, so call it 1.6lbs with a plug and a negligible difference in buoyancy for salt vs. fresh.
Faber LP108's=+1.5lbs in salt when empty, +0.5 in fresh with a standard valve.
Since we established that it holds 138cf when cave filled, and let's be real, no other pressure matters than 3600 when comparing bottles ;-), 138*.0807=11.13
Faber LP108's= -9.8lbs when full in salt, and -10.8lbs when full in fresh with a third weighing 3.7lbs

 

[DiveClimbRide]

Never have I been more thankful that I use metric since reading this thread.