Surfacing at how much air?

[mike3939]

Tim Ingersoll once bubbled...
Remember there is nothing wrong with staying ten feet above everyone if you are a newbie. Your air will last longer and your ascent will be slower.

Best advice I've seen. At Coz you can see from 10-15' above the group just great.

BTW remember the current will be stronger that far above the bottom so you will not have to swim as hard to keep up, which will help you conserve your air. If you need to slow down just decend.

 

[cantlogon]

when I'm at the end of the dive, hanging out on a shallow reef, I like to keep there as long as possible so I don't have to come up & go back to work!:upset:

 

[Rick Murchison]

Diversauras once bubbled...
Did you practice blow and gos too?

WARNING! What follows may be of historical interest but is NOT to be taken as safe diving procedure. Diving this way will kill you!
Sure.
But a blow & go was only needed for some catastrophic equipment failure - and we had precious little equipment to fail. No SPG, no power inflator, no octopus, just a single hose (we called 'em "single hose regulators"). And in our part of the world, diving steel 72's, no wetsuit or weight belt either. It was quite obvious (if one paid attention) when the IP started to drop, and a normal ascent was then commenced. (One could not delay, however... when it was time to ascend it was mandatory to ascend).
Dive objectives were different too - we were hunters mainly, not sightseers. Bounce dives were the routine. No one even thought of owning more than one tank, and that had to last the weekend. So a typical dive was... mark the spot, snorkel directly overhead, hyperventilate, kick like hell for the bottom, take your first breath passing 30' or so, shoot a fish or two or (rarely) three, back to the boat, go to the next site, do it again. We never breathed from a regulator on the surface. Air was far too precious for that. Often we were looking for big stuff and using detachable spearheads attached to surface floats so a single fish was it for the dive. After the shot we'd surface, get in the boat, go get the float and haul in the fish. Any fighting with the fish was done from the boat.
Remarkably, in my circle of divers, no one ever had any catastrophic gas supply failures, got bent or seriously hurt or dead - and we were routinely doing profiles and managing gas in ways that today I wouldn't even consider, much less do.
WARNING! What you've just read may be of historical interest but is NOT to be taken as safe diving procedure. Diving this way will kill you!
Rick

 

[Rick Inman]

Rick Murchison once bubbled...

WARNING! What follows may be of historical interest but is NOT to be taken as safe diving procedure. Diving this way will kill you!
Sure.
But a blow & go was only needed for some catastrophic equipment failure - and we had precious little equipment to fail. No SPG, no power inflator, no octopus, just a single hose (we called 'em "single hose regulators"). And in our part of the world, diving steel 72's, no wetsuit or weight belt either. It was quite obvious (if one paid attention) when the IP started to drop, and a normal ascent was then commenced. (One could not delay, however... when it was time to ascend it was mandatory to ascend).
Dive objectives were different too - we were hunters mainly, not sightseers. Bounce dives were the routine. No one even thought of owning more than one tank, and that had to last the weekend. So a typical dive was... mark the spot, snorkel directly overhead, hyperventilate, kick like hell for the bottom, take your first breath passing 30' or so, shoot a fish or two or (rarely) three, back to the boat, go to the next site, do it again. We never breathed from a regulator on the surface. Air was far too precious for that. Often we were looking for big stuff and using detachable spearheads attached to surface floats so a single fish was it for the dive. After the shot we'd surface, get in the boat, go get the float and haul in the fish. Any fighting with the fish was done from the boat.
Remarkably, in my circle of divers, no one ever had any catastrophic gas supply failures, got bent or seriously hurt or dead - and we were routinely doing profiles and managing gas in ways that today I wouldn't even consider, much less do.
WARNING! What you've just read may be of historical interest but is NOT to be taken as safe diving procedure. Diving this way will kill you!
Rick

Yeah, back in the 70's, bouncing around was a normal part of the dive. There were no safety stops, and we never worried about deco. The rule was that you couldn't get into deco with a 72.
Yes, we called them single hose. In fact, at my first pool class, they were short a single hose reg and gave me a double hose. So my first breath underwater was on a good old double hose. I still remember that you have to tilt your head to the right to clear it (exhaust hose is on the left).
I also remember the little diddly they taught us in the NASDS class for an OOA:
Stop
Drop
Spit
Go
Blow
Flare
and you're THERE

We didn't think about having enough air for "sharing" 'cause with buddy breathing it was just one guy at a time anyway.

My buddy bought Scubapro's first Decompression Meter and we all thought it was a huge waste of money because we couldn't get the thing to go into the red with our 72s. But, ironically, he ended up getting a trip to the tank on Catalina Island anyway while he was still in the green. It was the summer of '75, and he and I bought a book called Diving California West and spent 60 or so days in a row making two-to-four dives a day down the CA coast, camping on the beach and living on the cheep, following the diving guide (we were both going into our senior year at high school). Near the end of August, I went home, but John kept diving, and four days later they choppered him to the chamber on Catalina Island. And he was still in the green.
Ah, the good old days...

 

[roakey]

[this is only slightly munged form a post I just put in the instructor's forum, so it might not be an exact match with references, but it's close...]

All I can say is wow, it appears that almost no one in here understands what real gas planning is. I see lots of generalities and hand waving, but no one made any attempt to demonstrate how to actually MANAGE their gas supply and how to have a PLAN in place as to when to turn the dive before jumping in.

First off, "500psi back on the boat" or "back on the ladder" is just plain wrong - this isn't planning, it's a guessing game. It's giving someone directions to your house like: "Turn one mile BEFORE the big red barn." If they guess the correct turn without any real information, they're OK. Otherwise they get back on the boat with 300psi or 700psi - they "lose" the guessing game. Or they really lose and try and get back on the boat with -300psi.

In addition, this guessing game will add more stress to an already stressful OOA situation. It probably takes the new OW student about one or two "real" dives to realize that they're playing a guessing game. After all, they don't know at the bottom when to turn the dive in order to get back on the boat with 500psi and they probably missed the mark by a mile on those first couple of dives! Now put them in an OOA situation and suddenly one or both divers will realize "HS! Do we have enough air to make it to the surface?" The hoovering begins, a bad situation gets worse and it's either just plain scary or we read about a diving death the next morning.

Replacing that unknown with a simple demonstration of gas planning and some REAL gas management rules will allow the OOA diver and donor to relax and know that they not only have enough gas to make it to the surface, but they can do so with a normal ascent rate and safety stops. They relax, consume less air, manage their buoyancy better and come back on board with a story, rather than shaking in fear wondering if they're going to get bent from a Poseidon-missile-like ascent.

What I'm going to describe is basically GUE's "Rock Bottom" calculation, with some simplifications in order to make the numbers easy so what's going on can be understood; once understood someone can go off and calculate exactly what they use for ascent rates, safety stops, etc. Think of the initial cut here "Rock Bottom for OW divers."

Here's the goal of correct gas planning:

Get two stressed divers to the surface safely from depth with normal ascent rates and safety stops.

So here are the assumptions, to simplify things:

80cf@3000psi cylinder
Stressed diver has a SAC of 1cf/min @ 1ATA
The divers spend 1 minute at depth to sort things out.
Ascent rate of 33ft/min (1ATA/min) - chosen just to simplify things.
1 safety stop for 2 minutes at 16.5 feet (1.5ATA - again, to simplify)
The dive ends with a direct ascent to the boat (no swimming back underwater)

So let's calculate how much air is required for the goal, stated above, at depths of 33, 66, 99 and 132 feet (2, 3, 4 and 5ATA):

For 33feet (2 ATA):

How much gas is consumed at 33feet for a minute to sort things out?
2ATA * 1 min * 1cf/min = 2cf.
Note: I'm going to drop the 1 cf/min calculation from here on out, since multiplying by 1 does nothing...

How much gas is consumed in the ascent from 33 feet to the surface?
33 feet deep/33 feet per minute = 1 minute.
What's the average ATA? 2ATA (@ 33 feet) + 1ATA (@ surface) / 2 = 1.5ATA
How many CF will one diver consume? 1.5 ATA * 1 minute = 1.5 cf of gas

How much is consumed at the safety stop?
1.5ATA * 2 min = 3cf

So ONE diver would consume 2cf + 1.5cf + 3cf = 6.5 cf of gas in this situation and two would consume 13cf.

What PSI is that? 14cf/80cf*3000psi = 487psi.

So to allow two divers to surface from 33 feet at a normal rate with a safety stop requires 487psi of gas, so the dive is called at 487 psi. Personally, I'd round this up to 500 psi.

For 66feet (3ATA):

Sort things out: 3ATA * 1min = 3cf
Ascent: ((3+1)/2 average depth in ATA) * 2min = 4cf
Safety stop: 2 min * 1.5ATA = 3cf (this is always the same, so we can just assume 3cf from here on out)

One diver's ascent consumption: 3+4+3 = 10CF, two would be 20cf.

Turn pressure: 20/80*3000=750psi

For 99feet (4 ATA):

Sort things out: 4ATA * 1min = 4cf
Ascent: ((4+1) / 2) * 3min = 7.5cf
Safety stop: 3cf

(4+7.5+3)*2 divers = 29cf, 29/80*3000=1088 psi turn pressure or 1100 psi.

For 132 feet (5 ATA):

Sort things out: 5cf
Ascent: ((5+1)/2) * 4min = 12cf
Safety stop: 3cf

(5+12+3) * 2 divers = 40cf, 40/80*3000=1500 psi turn pressure.

In conclusion:

Now that you have some really hard numbers, pad if you like with a hundred or two PSI... The important part is you have some REAL gas planning numbers to offer your students, something they can use DURING the dive rather than something to see how well they guessed once they're back on the boat. Personally, I'd demonstrate the calculations for the 66foot dive. Expect 90% of your student's eyes to glaze over, but for most in the back of their minds they'll know they will have enough air to safely ascend with an OOA diver, which won't be lost on them if it ever happens...

After they unglaze, offer the following numbers for their gas planning (and it's real planning now, what pressure to turn the dive DURING the dive!)

Less than 30feet, call the dive at 500psi MINIMUM
31-60feet, 750psi MINIMUM
61-100feet, 1100psi MINIMUM
101-130feet, 1500 MINIMUM

And stress that this allows a safe, unhurried ascent with the required safety stop. Also emphasize that the limit is not based on a square profile; if you do a dive that starts off going to 130, your turn is 1500psi, if at say, 2000psi you ascend to 100feet, now your turn is 1100psi. If you further ascend to 60feet, do you can now wait until 750psi. This can also be used to limit your dive. if you're at 60feet and you're at 1000psi, and you see a neat thing 20feet below you, you CAN'T descend because that'll put you at less than your limit (1100psi at depths between 60-100 feet).

End of basics. Now for extra credit:

Personally, I wouldn't do only one safety stop at 16.5 feet on a 120 foot dive, so what are the REAL numbers for, say, a 120 foot dive?

If I was doing a 120 foot dive, my profile would be from 120 feet, ascend at 30ft/min to 60feet, 1 min deep stop at 60, 30 ft/min to 30, 1 min "deep stop" at 30, then my normal safety stops of 1 min at 20, 2 min at 10.

So, how much air to sort things out for a minute at 120 feet?
(120/33feet per ATA + 1) = 4.6cf
Ascent: 120/30 = 4 min
CF used during ascent: ((4.6+1)/2) * 4 min = 11cf
CF used during 1 minute 60 foot deep stop:
60/33+1 = 2.8cf
CF used during 1 minute 30 foot deep stop:
30/33+1 = 1.9cf
CF used during 1 minute 20 foot safety stop:
20/33+1 = 1.6cf
CF used during 2 minute 10 foot safety stop:
(10/33+1) * 2min = 2.6cf
Total CF required for ascent: 4.6+11+2.8+1.9+1.6+2.6 = 24.5cf
For two divers: 24.5*2 = ~50cf, 50/80*3000=1875 psi turn pressure for the dive.

So you only have 1125psi to consume at 120 feet, at which point you turn the dive. That's about 13 minutes of bottom time at a SAC of 0.5.

Hope this helps some folks with real gas management...

Roak