Max Depth vs. Cylinder Capacity

[Marek K]

I've seen several references here recently about an apparently new rule of thumb, that you shouldn't plan a dive deeper in feet that the capacity of your cylinder in cubic feet.

So with an 80 CF cylinder, you shouldn't dive deeper than 80 ft.

Where did that originate? Using that guideline, it would be pretty difficult to get AOW certified without a 100 CF cylinder...

 

[D_B]

Thats from Lamont's very good gas planning paper ... the complete paper http://www.scriptkiddie.org/diving/rockbottom.html
It's just a general rule of thumb that has excptions, It's also premised that it's for a new diver and is referring to the new divers usually high gas consumption rate and how it would not allow meaningful bottom time/gas reserves at increased depths.
I also like the, easy for me to remember, OW, HP100 tank, rock bottom rule of thumb .. depth x 10 + 300 = psi to start up
.... Snip ...

Rules of Thumb

Warning

These rules of thumb are not the whole story. They are simplified as much as possible to make them more widely accessable. As with all massively oversimlified rules they may not apply well in every circumstance and are necessarily aimed at the worst case of beginning divers and harsher conditions. They are hopefully simple enough for all divers to be able to employ with minimal math and should be able to be recalled even through the haze of narcosis.

Tank Size

The rule of thumb is that you should not dive to a greater depth in fsw than your tank capacity in cu ft. This works reasonably well for most beginning and intermediate recreational divers and caps the depth that an Al80 should be dove to to 77 fsw (Al80 == 77.4 cu ft). A steel 100 tank should not be dove past 100 fsw and a steel 130 should not be dove past 130 fsw (which is the limits of recreational diving as well).

In some circumstances, experienced divers in good viz and warm waters may do dives to 100 fsw routinely on Al80s so this rule may clearly be stretched.
It is probably not a good idea to be doing dives to 130 fsw on Al80s under any circumstances, though, and inexperienced divers (100 dives or less) doing coldwater dives to 100 fsw on Al80s are what this rule is squarely aimed at preventing. Also, the diver with 100 dives who think they're okay with an Al80 at 100 fsw in warm water and good viz is probably at the edge of being overconfident.

Simplified Rock Bottom Values

For HP120/HP130/LP104/LP95s (big tanks) use a rockbottom value equal to 10 times the depth in fsw that you are at. So, for an X8-130 high pressure Worthington tank at 110 fsw, you must leave the bottom with at least 1100 psi.

For Al80s/HP100s/LP80s/LP72s (small tanks) use a rockbottom value of 10 times the depth in fsw plus 300 psi. So, for an Al80 at 60 fsw, you must leave the bottom with at least 900 psi left in the tank.
For both of these rules, never use a rockbottom value of less than 500 psi. Be aware that this simplified extrapolation breaks down as you go deeper. For an E8-130 at 130 fsw the appropriate rock bottom value is closer to 1600 psi. If you are diving deeper than 100-110 fsw you should be able to follow the full rockbottom and SAC rate calculations below.

 

[Marek K]

Thanks for the info.

From Lamont: "In some circumstances, experienced divers in good viz and warm waters may do dives to 100 fsw routinely on Al80s so this rule may clearly be stretched."

Hmmm... Well, the rule is quite evidently stretched all the time. Again using the AOW example, I doubt many students do their required deep dive with a 100 CF tank.

 

[D_B]

I didn't spend any time at 100ft for mine, we took turns and only just just long enough to do some exercises and then back up to 70ft ... It's not like diving to 100 feet and do an entire dive there

 

[elmer fudd]

I'm not sure it makes a lot of sense though. There are just too many variables involved. Some divers weigh less than 100 lbs. and some are over 300. Some of us dive in 80 degree water and some in 40 degrees and of course some divers are in very bad shape, poor swimmers and prone to anxiety while others are confident and athletic.

I know it's just a rule of thumb, but considering just how different peoples SAC rates can actually be, I'm not sure it's a good one.

 

[k ellis]

Ive been to 90 feet on an 80 and maxed out my NDL on regular air and surfaced with plenty of gas to spare (Though I was on nitrox) Your consumption is what actually dictates if you can reach the NDL or how short in length your dive should be.

A really good rule of thumb is you should live by the 3rds rule and dive to within a third of your gas before you ascend. You should have consumed within 2/3rds gas before you surface and leave your last 1/3 for any unexpected problems you have if you cant make it up in time. so when you dive with 3000 pounds you should try to plan to surface before 1000 pounds if possible.
this would allow if you had some reason you needed to slow your ascent or even stop for a short period of time youd have enough air to get you to the surface.

between 500 and 1000 should be contingency with the dive idealy ending close to 500 lbs.

 

[D_B]

It does not apply to everything, and maybe not even to most .. but

inexperienced divers (100 dives or less) doing coldwater dives to 100 fsw on Al80s are what this rule is squarely aimed at preventing.

 

[D_B]

his rule of thumb ... 90ft x 10 + 300 = 1200 psi ... works for me in OW
and .. I can do that underwater in my head at different than planned for depth , shallower, or a bit deeper

 

[on_two_wheels]

Hell, even the depth x 10 + 300 guideline has lots of variables. If I do a dive on the yukon and turn the dive with 1300 psi in my tank, that's over 50% remaining on a no deco dive. Figure ascending from 100 feet including a safety stop you're looking at 7 minutes. So I'm back on the boat with over a 1000 psi? No thanks.

Bottom line is learn to plan your gas for your dive including a proper safety margin. You'll get more out of your dives than using all these rules of thumb and such.

 

[MauiScubaSteve]

Since the OP framed his questions around Advanced Open Water, the rule of thirds is kind of over his head perhaps?

Rock Bottom, where one plans for catastrophic failure of the buddies air supply at the last second of max depth NDL, giving one enough reserve to get safely to the surface, could be figured with or without safety stop and 500 surfacing psi, as long as you are diving without overhead obstruction, no?