To find out actual air content of a tank, divide the volume by the working pressure then multiply by actual pressure (72/2400x3000=90cu ft.) At 3000psi your 72 becomes a 90, unless it was originally plus rated, in which case its operating pressure is 2640, making it an 82 at 3000psi.
The Final Word on + Ratings?
- Thread starter Locus
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
To find out actual air content of a tank, divide the volume by the working pressure then multiply by actual pressure (72/2400x3000=90cu ft.) At 3000psi your 72 becomes a 90, unless it was originally plus rated, in which case its operating pressure is 2640, making it an 82 at 3000psi.
halocline
Contributor
No this is not true. The plus rating does not change the capacity of a tank at a given pressure, it just allows 10% higher pressure to be filled 'legally'.
The specs I have for LP 72 are:
72 cu ft Standard
Service pressure: 2250 psi
Working pressure: 2475 psi (2250 psi +10%)
Actual air capacity: 71.2 ft3 (at a working pressure of 2475 psi)
Outer diameter: 6.9 in
Length without valve: 25.1 in
Empty weight: 26 lbs (w/o valve)
Buoyancy Empty: 0 lbs (w/valve)
Buoyancy Full: -5.4 lbs (w/valve)
Capacity at:
2500 71.9
2600 74.7
2700 77.6
2800 80.5
2900 83.4
3000 86.3
Yes, it allows a 10% higher pressure, but doesn't this change the figure for the working pressure, which you then plug into the above formula to determine actual cu ft content at other pressures?
From a couple of manufacturer's websites, I have gathered that a + rated tank needs to have its 10% overfill in order to be filled to its true capacity. Doesn't this make "overfill" a bit of a misnomer? If an lp95 actually holds 86.3 cu ft at 2400psi, then at 2640, having just reached its true capacity, I would simply call it a "fill", and anything less a short fill.
Before I did some research, I thought that they held 95cu ft at 2400psi, and that the +10% was an actual overfill. I liked my tanks more when I thought they held 104.5cu ft at 2640psi...
Ah, I was wondering after if it was one of the old 2250psi tanks. I should have asked. My mistake.
Last edited:
Eric Sedletzky
Contributor
This is what I was looking for, you're the man!
Do you think it's a big problem if I go into a shop somewhere and a kid that doesn't know an old 72 from an AL80 gives me a 3000 fill?
Not all the time, but once in a while?
Would you keep your mouth shut or would you say something?
I've never heard of a 72 that has a current hydro and is in good shape blowing up from a 3000 fill, but I have heard of AL 80's blowing up.
halocline
Contributor
I would and I have...but in general I'm happy with 2500 PSI fills. 90% of my local diving is less than 30 ft deep, I don't have to worry about running out of air too soon. To me the sweet spot would be 2700 PSI, because then you have a very slight overfill which surely will not cause any problems, and you have a nice compact neutrally buoyant tank with the capacity of an AL80, and it weighs 5 lbs less on land.
halocline
Contributor
Yes this is true, although I wouldn't say "true capacity", I'd say "advertised capacity." When a 3AA scuba tank is sold, the manufacturer claims volume based on the 10% + rating. Hence, the 'service' pressure of a LP95 is 2400, but the 'working' pressure is 2640. That LP95 only holds 95cft at 2640. The word 'overfill' is a bit of a misnomer.
My understanding is that the + rating came about during WW2 when supplies were short and there was a need for somewhat greater capacity in pressurized gas tanks. So the industry began the practice of allowing 10% increase in capacity with tanks in the 3AA standard, as long as those tanks had elastic expansion less than a given volume during hydro test. It's a U.S. thing; probably its about time for it to disappear and for the U.S. to do as other countries have done and simply approve the higher working pressure for all 3AA tanks. It certainly has been demonstrated over decades that 3AA tanks can easily handle the 10% increase in pressure.
Similar threads
