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

[laikabear]

I had been wondering about this too, so thanks for asking and thanks to everyone who replied. I am thinking of buying my own tanks and have been trying out a few different types of rentals.

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

I tried this calculator out.

They recommended to use 1 cubic foot/ min if you don't know your RMV. I don't know mine so that's what I used. I also left the 200 psi "buffer" which hopefully will take care of the "can't breathe tank to zero psi" problem as well as a slight gauge inaccuracy.

I did a few calculations (I used 77 cf for the volume of the AL 80):

Rock bottom for a dive to 30 feet: AL 80 - 700 psi, HP 100 - 600 psi, LP 80 - 600 psi, LP 95 - 500 psi
60 feet: AL 80 - 900, HP 100 - 900, LP 80 - 800, LP 95 - 700.
85 feet: AL 80 - 1600, HP 100 - 1400, LP 80 - 1300, LP 95 - 1100
100 feet: AL 80 - 1800, HP 100 - 1600, LP 80 - 1400, LP 95 - 1200

Any comments? Do those numbers seem reasonable to those with more experience?

 

[elgoog]

I've done the calculations manually for HP100 as an exercise and the numbers are practically the same as what that calculator gives you.
The only thing I do differently is instead of using working SAC, I use twice my average logged SAC. Mainly because I haven't separately figured out my working SAC and using double the value I know is a reasonable approximation of what it would be in an urgent/OOG/panic situation.

 

[stuartv]

this works better on smaller, lower-pressure tanks.

I think you mean that this works better on higher pressure tanks.

150psi on a 3500psi 80 is 3.4 cu-ft.
150psi on a 2400psi 80 is 5 cu-ft.

Thus, you can breathe 76.6 cu-ft out of the HP tank where you'd only get 75 cu-ft out of an LP tank.

 

[stuartv]

did a few calculations (I used 77 cf for the volume of the AL 80):

Rock bottom for a dive to 30 feet: AL 80 - 700 psi, HP 100 - 600 psi, LP 80 - 600 psi, LP 95 - 500 psi
60 feet: AL 80 - 600, HP 100 - 900, LP 80 - 800, LP 95 - 700.
85 feet: AL 80 - 1600, HP 100 - 1400, LP 80 - 1300, LP 95 - 1100
100 feet: AL 80 - 1800, HP 100 - 1600, LP 80 - 1400, LP 95 - 1200

Any comments? Do those numbers seem reasonable to those with more experience?

I've only done <10 dives to over 100 feet with an AL80, but I have never had anyone suggest to me that I should start my ascent when the tank pressure got down to 1800. That seems WAY high, to me.

A direct ascent from 132' (40m) at 10m/min only takes 4 minutes (with no safety stop). That's an average depth of 20m, which is 3ATA.

3ATA for 4 minutes with 2 divers, both huffing away at 1cu-ft/min is 24 cu-ft. That would be 930psi from an AL80. Allowing 170psi as "unusable", that's still only a requirement of 1100psi. And that's for an ascent from 132' - not 100'.

If you really want to allow for a 3 minute safety stop at 15', I would figure on everyone being a little calmer by then, so RMV of 0.75, instead of 1. So, that's basically 1.5ATA for 3 minutes for 2 divers at 0.75, which is an additional 6.75 cu-ft of air, which is another 260 psi in an AL80.

So, even if you want a 3 minute safety stop, you only need 1400psi when you start your ascent (from 132').

1800psi for a turn pressure from 100' sounds way too high, to me. And, personally, I would simply plan on a direct ascent with no safety stop for that worst-case scenario. Hopefully, if it actually came to pass, at least one of us would stay calm enough that we would actually get to 15' and have enough gas left to do a safety stop - even if it wasn't a full 3 minutes.

All that said, I expect if I don't someone else will point out that assuming a worst-case scenario RMV of 2 cu-ft/min, combined, for both divers, might be on the low side. If both divers are normally at 0.75, and they both double their breathing, giving a combined RMV of 3, instead of 2. Thus needing 50% more air. If that's what you want to plan for, then you'd still only need about 1550psi to start your ascent (from 132') and make it to the surface (with no safety stop).

 

[tursiops]

I think you mean that this works better on higher pressure tanks.

150psi on a 3500psi 80 is 3.4 cu-ft.
150psi on a 2400psi 80 is 5 cu-ft.

Thus, you can breathe 76.6 cu-ft out of the HP tank where you'd only get 75 cu-ft out of an LP tank.

I don't know what I was trying to say.

 

[RJP]

I think you mean that this works better on higher pressure tanks.

150psi on a 3500psi 80 is 3.4 cu-ft.
150psi on a 2400psi 80 is 5 cu-ft.

Thus, you can breathe 76.6 cu-ft out of the HP tank where you'd only get 75 cu-ft out of an LP tank.

"Math... or it didn't happen."

 

[RJP]

So, even if you want a 3 minute safety stop, you only need 1400psi when you start your ascent (from 132').

1800psi for a turn pressure from 100' sounds way too high, to me.

Don't confuse "turn pressure" with the pressure you might have when beginning your ascent.

Turn pressure is when you "turn" your dive and start heading back to the exit point from the furthest point of your dive. A direct ascent from that furthest point may not be practical/desirable. Consider a case where you're on a 400ft wreck, with the dive boat tied in on the stern of the wreck. You're swimming along and reach the bow. You see that you have 1400psi remaining. Is that enough to get you back to the upline, do your ascent, make your stop, and get out of the water... or do you intend to do a free ascent 400ft away from the dive boat?

Similarly you might be in Bonaire. Do you want to do a nice leisurely swim back to where you parked your truck, or would you rather ascend at the furthest point of your dive and walk back?

 

[CuzzA]

@stuartv I think you're being a little aggressive with the time for a controlled safe ascent including the mishap, IMO. 4 minutes for you and a buddy to surface from 130' from the moment of catastrophe at the time you should ascend is not a lot. Of course you COULD ascend faster... Hell, one breath CESA to the surface, but nobody wants to do that.

There's a lot of assumptions going on with the calculator. One being that it will take one minute to sort out the problem at depth, what if it takes three minutes and you didn't account for any stops in your rock bottom calculation. Well, you're either going to have to do a faster ascent or run out of air.

Keep in mind, that something bad just happened at 130 feet, it's safe to assume it's going to take time to sort out the problem at depth, plus you're now doing a very uncomfortable ascent with two divers tethered together in what could be less than ideal conditions (current, poor viz, etc.). I'd rather be on the conservative side than the bare minimum. Of course, in reality if I'm at 130', I would have brought 130 cubic feet of gas plus an appropriately sized bailout/pony and planned my dive based on that, rather than trying to suck the last cubic foot out of an AL80.

 

[lerpy]

When it comes to an open water dive my policy is I am on the line and getting set to head up one the needle cracks 1000# remaining.

 

[NAM001]

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.

Totally agree and add to that a little gage error and it is even worse. They dont teach these things in OW Only AL80 tanks and accurate gages.