How to Know When To Start Heading Up

[Doc Harry]

I really like the two references about Rock Bottom gas and Grateful Diver's gas management article.

Grateful Diver's article touches on the subject of turnaround pressure but doesn't go into detail. Here is the detail

There are 3 scenarios which determine your turnaround gas:
(1) 1/3 available
(2) 1/2 available
(3) All available

In a 1/3 available scenario, you must return to your descent point in order to ascend. For instance, if you descend a mooring line in strong current, then you're going to need to ascend back up the mooring line to get to the boat.

From Bob's article:
Available gas = Fill Pressure - Rock Bottom Gas

Example: AL80 filled to 3,000 PSI, Rock Bottom Gas=800psi
Available gas = 3,000 - 800 = 2,200 PSI

You can use 1/3 of this available gas (1/3 * 2,200 = 730 psi) to swim upstream from the mooring line. Obviously you'll need another 1/3 (730 psi) to get back to the mooring line. You'll need to keep 1/3 (730 psi) in reserve to get your buddy back to the mooring line in case of emergency.

To confirm your math is correct, the sum of thirds and rock bottom = 730 + 730 + 730 + 800 = 3,000 psi

In a 1/2 available scenario, you want return to your descent point in order to ascend, but it's not mandatory. For instance, if you descend from an anchored boat with no current, then you want ascend to the surface at the anchor line, but if something goes wrong you always ascend anywhere and swim back on the surface.

Using the same pressures as above:
Available gas = Fill Pressure - Rock Bottom Gas
Available gas = 3,000 - 800 = 2,200 PSI

You can use 1/2 of this available gas (1/2 * 2,200 = 1,100 psi) to swim away from the anchor. Obviously you'll need another 1/2 (1,100 psi) to get back to the anchor. You don't need a reserve for your buddy because you can always just go to the surface and swim back to the boat on the surface. But you do need your rock bottom gas for your buddy so that you both can reach the surface in an emergency.

To confirm your math is correct, the sum of halves and rock bottom = 1,100 + 1,100 + 800 = 3,000 psi

In an all available scenario, you can ascend anywhere. For instance, a drift dive where you can stay down until you reach rock bottom gas.

Using the same pressures as above:
Available gas = Fill Pressure - Rock Bottom Gas
Available gas = 3,000 - 800 = 2,200 PSI

You can use all of this available gas (2,200 psi) on the dive. When you reach rock bottom gas, the dive is over, ascend with your buddy and the boat picks you up where ever you are.

To confirm your math is correct, the sum = 2,200 + 800 = 3,000 psi

Obviously these are general guidelines when all divers have the same tanks and same fill pressures. These guidelines must be modified as the conditions dictate. For instance, if the conditions are particularly bad you may want to increase your rock bottom gas, thus reducing your available gas. This means you turn around sooner, get back to the boat with more gas, and have more gas in reserve in case of emergencies.

And this explanation doesn't even touch on the subject of dissimilar tanks.

Common Errors:

Using absolute 1/3s or 1/2s without first subtracting out rock bottom gas. Very common mistake in novice divers trying to manage their gas. For instance, using the first scenario above, it is a mistake to use an absolute 1/3 (one third of 3,000 psi = 1,000 psi) instead of using 1/3 of available gas (one third of 2,200 psi = 730 psi).

Or, in the second scenario, it is a mistake to use an absolute 1/2 (one half of 3,000 psi = 1,500 psi) instead of using 1/2 of available gas (one half of 2,200 psi = 1,100 psi).

Using 1/2 when you should be using 1/3.

Not accounting for current. For instance, starting the dive by swimming down current instead of up current.

Not using a large enough rock bottom gas.

Not adjusting the numbers when you've got a short fill.

Not making appropriate corrections for dissimilar tanks or dissimilar fill pressures or grossly dissimilar SAC rates.

GRADUATION QUIZ:

Explain why so many people get into trouble diving to 100 feet on the Spiegel Grove with a single AL80 filled to 3,000 psi.

(HINT: First find your rock bottom gas and then, because of the current, you'll be using thirds. Calculate your thirds and your turn pressure. What would the numbers be if you erroneously used absolute thirds instead of thirds of available gas? What would happen if your buddy ran out of gas right when you reached turn pressure? Run the numbers for this last scenario assuming both absolute thirds and thirds of available gas. Where and why do you run out of air?)

 

[The Chairman]

Great synopsis, Doc Harry.

This sure beats the old days when we were waiting for the breathing to get harder. Nothing gave you a pucker like reaching for the reserve rod only to find that it's already DOWN. :homealone:

 

[SkimFisher]

An excerpt from Rock Bottom and Gas Management for Recreational Divers which I believe will be very useful to the OP. It's pretty basic - which is what I like about it. "Basic" is a good place to start.

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.

Start there and then read the entire article for some more good info.

 

[fnfalman]

Maybe you should check the stats and learn that the majority of deaths happen within 60' of the surface before you get to confident in you're risk management.

So, because of the statistics we should treat every recreational dive as a technical dive with double tanks, pony bottles, redundant mask, redundant timer, redundant compass, redundant lights?

 

[OldNSalty]

So, because of the statistics we should treat every recreational dive as a technical dive with double tanks, pony bottles, redundant mask, redundant timer, redundant compass, redundant lights?

but I thought we recreational divers carried all that stuff because we like to pretend we are technical divers. Nothing impresses the ladies more then having doubles and a pony at 45 feet in warm Florida waters

 

[fnfalman]

but I thought we recreational divers carried all that stuff because we like to pretend we are technical divers. Nothing impresses the ladies more then having doubles and a pony at 45 feet in warm Florida waters

I thought that chicks dig dive watches?

 

[OldNSalty]

I thought that chicks dig dive watches?

No. Its the master diver card you dork

 

[ScubaInChicago]

So, because of the statistics we should treat every recreational dive as a technical dive with double tanks, pony bottles, redundant mask, redundant timer, redundant compass, redundant lights?

for what???? it's a recreational dive....I think you're trying to sterotype me as someone i'm not.....

 

[fnfalman]

for what???? it's a recreational dive....I think you're trying to sterotype me as someone i'm not.....

Then what's the point of saying that the statistics said most deaths occured in 60-ft or less dives?

You either take the time to plan your dive; hopefully to suit your dive spots and buddy, or you don't. Statistics of deaths at what depths have nothing to do with it.

You don't plan a 60-ft open water dive in a cove like you would a 150-ft wreck penetration dive, but you still have to plan that 60-ft open water dive according to its inherent risks and your dive skills.

This last weekend, I went out on a two-days dive charter with most of the divers getting OW certified. On the second day after they've passed their cert dives, I took two divers out and I made them plan their dives in accordance to their recently learned knowledges, experiences and skills. They wanted to "follow me" and I told them that they need to make their own dive plans even though the water is only 35-ft to the sand. I watched them do their planning and gave some pointers, we all agreed to the plan (bottom time, max depth, compass headings, hand signals, emergency procedures, etc.) then we went on to have a fun, non-incidence dive.

 

[ScubaInChicago]

Then what's the point of saying that the statistics said most deaths occured in 60-ft or less dives?

Because it's the truth....It's just not good for the scuba business to concentrate on that fact......

You either take the time to plan your dive; hopefully to suit your dive spots and buddy, or you don't. Statistics of deaths at what depths have nothing to do with it.

My earlier post have stated the above....and I added in that the rule of thirds as someone was using it would leave that poster in a hairy predicament if something were to go wrong.

You don't plan a 60-ft open water dive in a cove like you would a 150-ft wreck penetration dive, but you still have to plan that 60-ft open water dive according to its inherent risks and your dive skills.

Outside of adding decompression......you need the exact same information to formulate a plan for both above dives.....RB and the applicable gas rule (all use, 1/2, 1/3 rds).

This last weekend, I went out on a two-days dive charter with most of the divers getting OW certified. On the second day after they've passed their cert dives, I took two divers out and I made them plan their dives in accordance to their recently learned knowledges, experiences and skills. They wanted to "follow me" and I told them that they need to make their own dive plans even though the water is only 35-ft to the sand. I watched them do their planning and gave some pointers, we all agreed to the plan (bottom time, max depth, compass headings, hand signals, emergency procedures, etc.) then we went on to have a fun, non-incidence dive.

Did you teach them gas management and how to calculate their turn pressures? Did you explain the all use, 1/2, 1/3 rds rules?......If you say you did, great....I think it's a shame that OW students are not given the tools to safely plan a dive with reserves for their buddies.