How much air to leave in reserve with different sized tanks?

[JohnnyC]

An AL80 is at 77.4 cuft, depending on manufacturer.

LP and MP tanks give the volume at the + pressure so that it is 80 cuft at 2640#, which will leave you at 72.7 cuft at 2400#.

HP will give your stated volume at pressure, depending on manufacturer.

http://www.indianvalleyscuba.com/services page/Tank Inspection/information/CYLINDER SPECIFICATIONS.pdf
This link and a calculator will get you as close as you need to be.


Bob

Thanks for that link, I'll bookmark it for sure. The numbers were all hypothetical to prove a point to the OP. I figured that since so much of the information is dependent on manufacturer that speaking in generalities was enough to get the point across to OP.

 

[Diving Dubai]

Just to make an additional point. And well done to the OP for raising the question.

Yes do work out the Rock bottom and reserves for your gas requirements based on tank sizes and depths. But also work out thos efor what your buddy might be carrying.

Case in point. I dive a 15l steel filled to 232 bar. My wife dives 12l steels at 232bar.

On my slate I have my rock bottoms for my tank and hers. The reason being that for any gas pressure I have more volume. Her SAC is better than mine but the Rock bottoms are calculated on my SAC for both because its the greater. But because she has the lesser volume as a reserve, her tanks size is the governing factor.

If we are diving with different buddies then this info is at hand ( and includes different tank sizes) so its easy for the bail out plan

 

[Kharon]

No the standard practise for OW level divers is to leave the same amount in reserve for all tanks - just means less time underwater with a smaller tank.

Reason being - your safety stop is the same length so same gas requirement and the reserve is for the exact same emergency contingency so why would you leave that short?

Wrong, wrong, and wrong!!!

(Tank) Rated Vol / Working Pressure = cf/psi

For an 80cf 3000psi aluminum tank 500 psi ≈ 13 cubic feet (12.9 if you want to be picky).
77.4/3000 = .0258 then 500psi X .0258 = 12.9 CF
For an 80cf LP tank with a working pressure of 2640 it's .0303cf/psi.
13cf/.0303cf/psi = 429psi.

In other words 429 psi in an 80cf LP tank is the exact same amount of air as 500 psi in an 80cf aluminum. Has absolutely no bearing on safety stop. 13cf is 13cf unless Boyles Law has been repealed.

Other formulae you might be interested in:

cf/psi x psi used = cf used
cf used / dive length = Raw SRMV
raw SRMV / deep or ave atm = deep or ave SRMV (atm = (depth/33) + 1)\
(Target Depth / 33) + 1 = Depth Factor
SAC x Depth Factor x Dive Factor = RMV
RMV x Dive Length = Air Required
Useable Air / RMV = Allowable Dive Length

Definitions:

SAC = surface air consumption (measured at surface at rest).
RMV = respiratory minute volume
SRMV = surface (calculated) respiratory minute volume
Dive Factor is an estimate dependant on temp,current, work rate, etc. and varies with each dive.

 

[stuartv]

In other words 429 psi in an 80cf LP tank is the exact same amount of air as 500 psi in an 80cf aluminum. Has absolutely no bearing on safety stop. 13cf is 13cf unless Boyles Law has been repealed.

Yes, but 429psi in an LP tank is less USABLE air than 500 psi in an AL80 (as explained in my previous post), and that's what is really more important for a scuba diver.

 

[tursiops]

Yes, but 429psi in an LP tank is less USABLE air than 500 psi in an AL80 (as explained in my previous post), and that's what is really more important for a scuba diver.

Kharon is only talking about how MUCH air in in the tank, not whether or not you can GET IT OUT of the tank.....which I agree is kinda important.

 

[Bob DBF]

As for how much you can get out of a tank is dependant upon whether you have a balanced second stage. The first stage will be wide open if tank pressure is below the set IP. The spring pressure in the second determines how far you can breathe it down which will be much lower in a balanced second, with one of the new over breathing seconds it might be even lower. Of course it would be dependant on the depth. Next time I'm emptying a tank I'll check.

I do agree one should have a reasonable amount of air in their tank to deal with an emergency on the surface, where you might need to float another diver, or have to submerge to avoid a problem on the surface. I have been in both situations and having air made all the difference between an annoyance and a clusterf**k. Of course this won't necessarily be seen to be important until it happens to you once, I hope you have the air.


Bob

 

[Frosty]

personally at your stage of diving I would run the same numbers as your instructor suggests regardless of tank size/type.
Im not for a second discounting the very valid reasoning of those that have posted above.
My reasoning is that your attention is kinda like a Pizza pie.
Right now a heck of a lot of your pizza is being used up with the simple mechanics of diving.
Task loading yourself with trying to remember what PSI to ascend at with whatever tank is just adding to the amount of "used up" pizza.
Revisit the question and answers when you have a few more dives under your belt.

 

[Kharon]

Kharon is only talking about how MUCH air in in the tank, not whether or not you can GET IT OUT of the tank.....which I agree is kinda important.

So you are saying that you can breathe down 13cf in an al80 farther than you can from a LP 80 steel? On what do you base that? You should get the exact same amount of air out of either using the same equipment at the same depth - 1st & 2nd stage. Again 13cf is 13cf.

 

[stuartv]

So you are saying that you can breathe down 13cf in an al80 farther than you can from a LP 80 steel? On what do you base that? You should get the exact same amount of air out of either using the same equipment at the same depth - 1st & 2nd stage. Again 13cf is 13cf.

I explained this in post #8 of this thread.

100psi in a 2400 psi tank is more air than 100psi in a 3000 psi tank is more air than 100 psi in a 3500psi tank. Assuming all these tanks hold the same amount of air (e.g. 80 cu-ft) when full.

If you're gear and depth permits you to breathe a tank down to X psi before you cannot pull anymore from it, then a LP80 will have more left inside than an HP80*. Ergo, you got less out.

*I left AL80 out of this statement because I haven't done the math to determine whether the fact that the AL80 only really holds 77.4 offsets the fact that the LP80 has a bigger water volume. It could be that in that specific example, they are the same, or maybe even that the LP80 will give you more air. It just depends on the specific numbers.

 

[tursiops]

So you are saying that you can breathe down 13cf in an al80 farther than you can from a LP 80 steel? On what do you base that? You should get the exact same amount of air out of either using the same equipment at the same depth - 1st & 2nd stage. Again 13cf is 13cf.

Think of it this way: what if you had 13cf in a 500cf, 3000psi tank? The pressure in that tank would be 13/500 x 3000 = 78 psi; this is not enough to make your second stage function (it expects an Intermediate Pressure of about 135psi) and probably not your first stage either. So those 13cf are just sitting there, and you can't get them out of the tank. For an 80cf 3000psi tank, you reach the 150psi level in the tank with 1/20 of the tank full, or 4cf (using all nice round numbers). So you better start breathing that tank before you get to 150psi, or it will stop providing air to you, even though there is air in it. and it would be unsafe to assume you are OK until your SPG gets down to 150 (or 135psi), because the gauges are not very accurate. In an LP80 2400 psi tank, you reach that 150psi cutoff with 5cf still in the tank.

So the issue is (as you say) having enough cf in the tank that you can ascend, safety-stop, etc, but the issue is ALSO can you get all those cf out of the tank; and this works better on smaller, lower-pressure tanks.