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

[chi_tortoise]

Maybe a dumb question but I'd like to know the answer.

In my open water diver class we used aluminum 80 tanks filled to 3000 psi and always left 500 psi in reserve and 300 for safety stop, meaning begin ascent at 800psi. Calculated time to turn dive based on 3000-800. Recently I dove with a steel tank 80 c.f. but only 2400 psi when full.

If diving low pressure tank do you leave less psi in reserve? What about high pressure steel tanks filled to 3400?

 

[boulderjohn]

It's a great question, and you will no doubt get a number of different answers depending upon the responder's preference for the theory of how much to hold in reserve. Here is the generic response: figure out how much you need in gas volume, then learn how to convert that to PSI for the given tank. There are mathematical process you can use to give you precise numbers, or you can do simple ball park estimates. For example, 800 PSI is a bit over a quarter of an AL 80, so figure around 20 cubic feet. A quarter of an 80 cubic foot tank with a 2400 PSI fill would be 600 PSI. An HP 100 would get to 20 cubic feet in 1/5 of its PSI, or about 700 PSI. (For those of you who want to get anal, yes, I know that none of those numbers are exact.)

BTW, 800 PSI in an AL 80 only works for a certain depth range. If you are deeper than that range, it is not enough.

 

[Neilwood]

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?

 

[NAM001]

PSI is no measure of what you should have. You dont breath PSI, you breath cu ft. figure the cu ft out and find the appropriate psi that will give you that. 1000 psi on a single and its 500 on doubles. Also don't forget about spg error. boulder john said it pretty well. Which by the way you should check the accuracy of your spg or AI on your computer. You can often see a 10% error in an AI.

And to the OP it is not a dumb question by any measure.

 

[JohnnyC]

Remember that volume is a function of pressure, and available gas volume is contingent on knowing both the volume at service pressure, and being able to calculate volume for any given pressure. So using PSI isn't a true indicator, you need to know the actual volume available. You need to know the true capacity too, as an AL80 isn't actually 80 cuft.

But, to give you an idea, let's just pretend that an AL80 is actually 80 cuft at 3000 psi. An LP80 is 80 cuft. at 2400 psi, an HP80 would be 80cuft at 3442 psi. As you can see, they all hold the same volume of gas, but because it's dependent on pressure, you can't simply go by a pressure reading on your SPG to determine if you've got "enough." These are purely hypothetical, but you get the idea.

 

[CuzzA]

This should help. DiveNerd - Rock Bottom Calculator, Imperial Units

 

[Bob DBF]

Remember that volume is a function of pressure, and available gas volume is contingent on knowing both the volume at service pressure, and being able to calculate volume for any given pressure. So using PSI isn't a true indicator, you need to know the actual volume available. You need to know the true capacity too, as an AL80 isn't actually 80 cuft.

But, to give you an idea, let's just pretend that an AL80 is actually 80 cuft at 3000 psi. An LP80 is 80 cuft. at 2400 psi, an HP80 would be 80cuft at 3442 psi. As you can see, they all hold the same volume of gas, but because it's dependent on pressure, you can't simply go by a pressure reading on your SPG to determine if you've got "enough." These are purely hypothetical, but you get the idea.

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

 

[stuartv]

...Here is the generic response: figure out how much you need in gas volume, then learn how to convert that to PSI for the given tank. There are mathematical process you can use to give you precise numbers

If you're going to pursue exact calculations, I think it would be good to note the following:

You can never breathe a tank all the way down to zero. And, for all other parameters being equal, the psi that you CAN breathe your tank down to does not depend on the tank size. If your current conditions allow you to breathe your tank down to 100 psi before it goes dry, then it will go dry at 100 whether it's an AL40 or double HP120s (in the same conditions, current gear being part of your current conditions).

As this is the "Basic" forum, I'll keep it simple and say that, for calculating numbers that are a bit closer to accurate, you would do well to assume that you can't breathe a tank down past 100 psi (this number being hugely subject to debates that I have seen). So, an AL80 at 500 is actually 400 psi of reserve + 100 psi unusable. 400 psi in an AL80 is roughly 10 cu-ft.

So, to achieve the same reserve in, for example, an HP120, you would convert 10 cu-ft to psi (using the tank factor), giving roughly 300 psi. Add that to your 100 psi that you can't use and the result is that a reserve of 500 psi in an AL80 is pretty close to the same usable gas as 400 psi in an HP120 (when you assume 100psi unusable).

Double 120s would be 150psi for 10 cu-ft, so a reserve of 250psi to be the same as a single 120 at 400 psi or a single AL80 at 500 psi.

As I said, the 100 psi of unusable gas is a total estimate. The actual amount could be less or it could be (to make up a number) 200 psi. It depends on your depth and your regulators. It will never be less than the ambient pressure. E.g. if you're at 33 feet depth, the unusable gas in your cylinder will never be less than 29 psi (2 ATM). If you're at 132 feet depth, the unusable gas will never be less than 74 psi (5 ATM). Really, it will always be more, but how much more depends on your regulators. With an unbalanced 1st stage, it could be quite a bit more.

 

[Storker]

BTW, 800 PSI in an AL 80 only works for a certain depth range. If you are deeper than that range, it is not enough.

Totally agree.

I've realized that my gas consumption is decent, but not stellar (partly, but not totally, due to the fact that I dive dry, and for several reasons diving dry often comes with a somewhat increased gas consumption compared to WS diving). Since my computer automatically gives me my average consumption I have a decent idea of both the average and normal range, so I've made myself a small table giving min pressures at some relevant depths. I've assumed a worst case scenario, having to share gas with my buddy, and both having an elevated consumption due to stress (when the bovine manure intercepts with the rotating ventilation device, your BP and breathing rate goes up).

For a 10L tank (about 10% less than the water volume of an Al80), my personal min pressures are:
30m/100': 110 bar/1600 psi
25m/80': 100 bar/1400 psi
20m/65': 85 bar/1200 psi
15m/50': 70 bar/1000 psi
10m/30': 60 bar/900 psi
5m/15' (safety stop): 50 bar/750 psi

Google "minimum gas" or "rock bottom" for more info. If you're living in imperial unit country, you might want to take a look at NWGratefulDiver.com

 

[2airishuman]

Maybe a dumb question but I'd like to know the answer.

I don't think it's a dumb question. I am new to diving but have thought about this quite a bit, too.

In my open water diver class we used aluminum 80 tanks filled to 3000 psi and always left 500 psi in reserve and 300 for safety stop, meaning begin ascent at 800psi. Calculated time to turn dive based on 3000-800. Recently I dove with a steel tank 80 c.f. but only 2400 psi when full.

This reflects common practice but I don't think it's a sufficient reserve except for shallow dives where running out of air during ascent is acceptable. You have to decide what risks you're willing to take and how much of an obligation you have to your buddy should your buddy encounter an OOA emergency at the worst possible point in the dive.

For a deep dive, I think about the cylinder pressure at which ascent must begin (also called the "rock bottom pressure") as the:

Minimum pressure at which the first stage regulator will function at the surface + air needed to safely end the dive

I use 100 PSI as the minimum pressure at which the first stage will work, although others use lower or higher values.

I calculate the air needed to safely end the dive as the sum of these things, multiplied by two, because I plan for my buddy having an OOA emergency at the worst possible time, that is, right before planned ascent:

- air needed to identify that buddy is OOA, swim to buddy, start sharing air, and prepare for ascent
- air needed for ascent
- air needed for safety stop
- air needed on surface to reach safety

I disregard the distorting effect of depth on tank pressure gauge readings because it's 60 PSI max at recreational depths. Disregarding it makes the calculations slightly more conservative, not much.

I figure these in cubic feet and convert them to PSI based on the design capacity and pressure of the cylinder I'm going to use.

This should help. DiveNerd - Rock Bottom Calculator, Imperial Units

That site does a good job of automating some of the calculations. It does not include air needed on the surface, however.

Here's an article you may find helpful:

NWGratefulDiver.com

If diving low pressure tank do you leave less psi in reserve? What about high pressure steel tanks filled to 3400?

Yes, if the tanks have the same capacity when filled to their rated pressure.

With typical tank sizes, depth plays a larger role. For a 70 foot dive you need about a 50% higher reserve than a 35 foot dive.