Rule of Thirds & Shallow Rec diving

[Foxfish]

So we're getting close to agreement on the numbers for the scenario I posted. I suspect the differences are related to rounding errors. If you began your ascent with a reasonable amount of air to end with the surface with 50 b you'd have about 80 b which was the amount calculated for an emergency ascent.

The next question is whether the 80 b from 30 m for a 12 L tank is realistic for an ascent from 30 m.

Some argue that you need an extra 20 b to prevent your tank flooding and as a buffer. Is 20 b really enough? What if your breathing rate exceeds 30 L/min? What if you have exceeded your NDL and need to do extra deco stops? What if there is a downward current that prevents you ascending at 9 m/min? What if during your safety stop you inadvertently fall back to 15 m and your computer signals that you need to redo the 3 minute stop? What if your buddy loses a fin on the way up and can't achieve the required ascent rate? What if you have a free flow from your tank after your buddy has run out of air etc. etc. Maybe you should carry a spare tank of air? If you had no other option that may be a consideration? In the case of OW/AOW divers you can do a CESA?

On the other hand, what if I for go the safety stop or start the ascent soon after the OOA. What if my buddy and I don't panic and our air consumption remains at 15 L/m each. Now instead of using 80 b I need 19 b.

The point is that estimating gas reserves is not a black and white issue. Claiming you will run out of air and die if you start your ascent with 80 b is simply nonsense. You should be monitoring your air during the ascent. If you are getting low, be prepared to for go the safety stop, swim faster or do a CESA. This is a rare life threatening emergency.

 

[victorzamora]

So we're getting close to agreement on the numbers for the scenario I posted. I suspect the differences are related to rounding errors. If you began your ascent with a reasonable amount of air to end with the surface with 50 b you'd have about 80 b which was the amount calculated for an emergency ascent.

No, we're not getting "closer" on our numbers. They used the WRONG numbers, and nearly died from it.

The next question is whether the 80 b from 30 m for a 12 L tank is realistic for an ascent from 30 m.

No, we're talking about 95b minimum, with ZERO reserve for bad SPG measurements. So, consider a minimum of 15b for SPG inaccuracy. 110b for a 12L tank.

Some argue that you need an extra 20 b to prevent your tank flooding and as a buffer. Is 20 b really enough?

Yes, it should be.

What if your breathing rate exceeds 30 L/min?

That's why you use a breathing rate high enough for worst case. If you don't think 30L/min is sufficient, then use a higher number.

What if you have exceeded your NDL and need to do extra deco stops?

Now you're thinking. That's additional gas you should add to your MinGas if you think that's a concern.

What if there is a downward current that prevents you ascending at 9 m/min?

Same as above.

What if during your safety stop you inadvertently fall back to 15 m and your computer signals that you need to redo the 3 minute stop? What if your buddy loses a fin on the way up and can't achieve the required ascent rate?

Again, good for you for thinking about it.

What if you have a free flow from your tank after your buddy has run out of air etc. etc.

So, this is where you have to start thinking realistically. You can't plan for EVERYTHING going wrong. How much going wrong are you okay with? In caves, the common thing is: if any 2 failures occur, you should be able to make it out safely. If a 3rd occurs, getting out is a perk. In rec diving, is 2 enough? Is it too many? I like diving SM, especially if deep. That mitigates that possibility, and gives me a large pony.

Maybe you should carry a spare tank of air? If you had no other option that may be a consideration?

Yes, a pony bottle is a HUGE benefit on a lot of dives....and a lot of people advocate diving with one. Sizing the pony also requires some MG/RB calculations, otherwise you'll run out of pony gas.

In the case of OW/AOW divers you can do a CESA?

Yes, but it could end up with 1 or 2 bent divers, even if doing CESAs from recreational, NDL, limits. Plenty of people take "undeserved hits" because of ascending too quickly. Two guys I know did, they were in examples above.

On the other hand, what if I for go the safety stop or start the ascent soon after the OOA. What if my buddy and I don't panic and our air consumption remains at 15 L/m each. Now instead of using 80 b I need 19 b.

This is where your logic fails. If you plan for best-case, perfect world scenario, then you'll run out of gas when real-world scenarios occur. Wouldn't you rather surface with extra gas rather than run out of air?

The point is that estimating gas reserves is not a black and white issue. Claiming you will run out of air and die if you start your ascent with 80 b is simply nonsense.

You're right....but claiming that starting an ascent with 80b from 30m will cause an OOA with 2 divers, each with a 30L/min breathing rate isn't nonsense.....it's completely accurate math. The variables can be tweaked for your liking, adding or removing conservatism. How little conservatism are YOU okay with? I like to hope for the best but plan for the worst. It seems like you're hoping for the best, expecting it, and ignoring anything that could go wrong.

You should be monitoring your air during the ascent. If you are getting low, be prepared to for go the safety stop, swim faster or do a CESA. This is a rare life threatening emergency.

I agree, but CESA and forgoing the safety stop CAN RESULT IN BENDS OR DEATH. Even within recreational limits. Even within NDL. There are LOTS of examples of it happening. My point is, you SHOULD be monitoring your air, but you shouldn't HAVE to monitor your air on ascent because you should KNOW you'll be fine no matter what happens. THAT is planning ahead.

 

[Foxfish]

On the other hand, in every single year, the triggering event most associated with fatalities (other than cardiac events) is OOA.

That was also the conclusion of a joint PADI and DAN study a couple of years ago. As a result of that study, PADI has greatly improved its gas management information and practice in the OW class in the new standards being implemented next year.

What does that prove? It may be that a lot of divers ignore the 50 b rule. Or it may be that the events that occur during an OOA emergencies that occur are nothing like what is predicted.

 

[victorzamora]

It may be that a lot of divers ignore the 50 b rule.

I've already covered this. I then asked for feedback, and got nothing. Giving feedback would help. You're constantly dodging the questions that show how wrong you are.

Or it may be that the events that occur during an OOA emergencies that occur are nothing like what is predicted.

They AREN'T like anything we've practiced. A real emergency is MUCH more stressful. Period. That's why we practice. So we can use what we know to help for when things really happen, because you can never train for a real-world accident, you can only prepare. Giving yourself enough gas is the best way to ensure your survival. I can't remember who has it, but the original quote is something like:

When things go tits-up, you will never "rise to the occasion." You will fall as low as the skills that you have completely memorized.

 

[Foxfish]

Victor you seem to have overlooked my question in post #298.

 

[victorzamora]

First of all, can you REALLY get on my case about missing one question? I just answered 40 of yours, and you've dodged TONS of mine. Not very fair.

As for your question in post #298....No, they didn't. They were at 20ft and were slightly negative and about to race for the surface as one of them sucked the last molecule out of the tank. My buddy and I were swimming towards them with long hoses out and completed our safety stop safely. They would've blown off their SS, if they had enough breath left in their lungs to make it there.

 

[Drin]

We're 306 posts into this topic and Foxfish is as obtuse as he was when the conversation started. I've never met anyone so honestly incapable of comprehending basic logic designed to save his life and that of his dive buddy, so I only see two options:

1. Foxfish is a troll.

Or

2. He's looking to be proven incompetent when the insurance company is investigating the death of his dive buddy from Type II DCS.

"But my OW class instructor SAID 50b was enough!"

-Adrian

 

[Foxfish]

The story so far then Vctor is as follows

All of these numbers were gotten post-incident, as I wasn't watching their air or profile....but it's what I remember being told. I think they were at 90ft when they started the ascent. I think they were at 1100psi when they started their ascent. Neither of them had any reason to be breathing heavy as it was a good dive. Their depth/time profile was fairly square, descending to the sand and then working their way up from 100ft-90ft along the wreck. It was the second dive of a two-tank dip, all running PDCs with no added conservancy and relatively aggressive algorithms. We were all running a really hot nitrox mix.

The point is: They were diving exactly as you prescribed. 80b is 1176psi, which you said you'd use to ascend from 100ft (30m). They ascended slightly later from slightly shallower. It would've been fine, but right at ascent one of the yoke o-rings blew out and emptied the tank with surprising speed. They ascended together like a textbook (we had been practicing for our DM course, so they had just done like 30 air-sharing ascents in a pool) and ran out of gas right before they reached their safety stop. Both were pretty comfortable in the water. Literally everything you were saying you'd do almost got them killed.event really sticks out in my mind. It just so happens that it was fantastically similar to what your described profile would be.

They were at 20ft and were slightly negative and about to race for the surface as one of them sucked the last molecule out of the tank.

The ascent profile was:

A. Begin ascent at 27 m (90 ft), tank O ring seal blew
B. Ascend and get to 6 m where they ran out of gas (20 ft).

Breathing rate was normal. The divers were comfortable in the water.

Let's assume for a moment that they were using steel 12 L tanks, air consumption rates for both divers were 20 L/min and the ascent rate was a constant 9 m/min. If so, the calculated pressure drop in tank is 22 b. But you said they started with 80 b and ran out of air. So they used nearly four times the volume of gas you'd expect on a reasonably well rehearsed emergency ascent.

Something seems amiss here. But let's assume that your story is accurate for a minute. What conclusions do you draw from this incident?

 

[victorzamora]

I said they HAD been calm. No idea what happened to their breathing rates after all hell broke loose

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[GCullen94]

I just realized foxfish is the same person that asked and started this thread http://www.scubaboard.com/forums/advanced-scuba-discussions/470422-how-low-spare-air.html We are dealing with someone that likes to push the envelope in terms of running low on gas. Foxfish there is NO POINT in running low. Get bigger cylinders or doubles if you want more gas. Don't put you and your buddy's life at risk because you want to run liberal gas plans.

To those that are following this thread and have learned something from it, like Tattooedjeeper, thank you. That made all of this arguing worth it. I hope some of the almost 10,000 people who have viewed this thread by this point have also gotten something out of it. I am sorry for loosing my cool a few times, I do believe this thread has helped others though. I hope people can see that running Min Gas calculations isn't hard, doesn't take much time and makes the dive much safer in the end.

GC