Modifying rule of thirds

[RN]

Okay, this has turned into a very interesting discussion. There has been some vague reference to my next question, but no one has quite come out and stated anything specific on it (and no one may do it).

What (or is there) is the general consensus on modified thirds when swimming into the flow in a high flow system such as Devil's, Manatee, or JB? Does anyone penetrate past thirds knowing that the flow will help expedite the exit?

I've never done this, but the thought has been on my mind for some time. If I'm penetrating a high flow system and turning 1200psi in and coming out with only 600psi then is there an issue with penetrating to 1400psi, for example, knowing that you'll only use 700psi to exit? You would still be coming out with more than one third of your total gas left.

[kensuf]

Another good thing to do is really know how much you need to exit swimming. Not just numbers but actual proven amount. One of my first scooter dive was to swim back from 3000' at Ginnie. Took me about 40 min and used a bit less than 1/3 of my gas supply. That was pushing the scooter ahead and carrying a stage. I would have dropped both in a real situation.

Dropping the dead scooter makes sense. Dropping any viable gas supply doesn't. If you had a real situation, breathe the stage empty then jettison the empty tank.

[kensuf]

It was suggested that the rule of thirds be modified when diving solo. I'd like to hear suggestions for modifying this rule.

The rule of thirds is meant to be the most liberal of gas management rules you should use when diving. Diving in poor visibility, deep, low-flow, syphons, or solo should make you pause and consider more conservative gas management rules; maybe that is diving to 1/4ths, or some other fractional, or maybe breathing a stage bottle and saving your back gas for emergencies. Unfortunately, there is not a single hard and fast rule which would be correct for all situations.

Also, please consider this. Prior to the rule of thirds the general rule of air management was "half-plus 200". Many people survived cave dives using the half-plus rule, but also consider that back in the early 70s a typical cave dive was quite a bit shorter than they are now. A dive to the hinkel restriction might be considered a boring non-event today, but when blueprint was published in the 1970s it would have been an amazingly extensive dive.

Just some food for thought...

[chimie007]

Dropping the dead scooter makes sense. Dropping any viable gas supply doesn't. If you had a real situation, breathe the stage empty then jettison the empty tank.

Of course... I was implying breathing the stage empty first.. Never said I would drop the stage and scooter at the same time. Thought that was obvious that no one would drop a non empty stage.

[aainslie]

Well, I'll go here... I often use "half plus 200" on stages. I've done some spreadsheet work on this, and there's an added level of safety as you add stages, since this increases redundancy. i was surprised however at how SMALL the advantage is. So I only use 1/2+200 in high flow caves like Ginnie and Cow.

And I PROMISE that's conservative!

__________________________________________________ ___'

A slight change of pace.

Do this calculation - remember your PSI on entry and at turn, and look at your PSI on exit. Double the difference between turn and exit. Now take that as a % of the PSI consumed going in plus that number.

As an example, say you go in with 3600 psi and turn at 2400 psi. On exit you find that you're at 1800 psi.

You used 1200 psi going in, and 600 psi exiting. 2x600=1200. So to JUST survive the dive losing 1/2 your gas at turn you would have had to enter the water with 1200+1200=2400 psi of gas. You would have used 1200/2400 or 50% of your gas, and a bare minimum reserve would have been 50%.

That % is the min gas you need to exit safely, IN THAT CAVE AT THAT FLOW SWIMMING AT THAT SPEED etc etc. Add a safety margin depending on circumstances, and that'll give you a safe reserve.

Any safety you add will get doubled since it cuts into your ingress gas. So if we reserve 55%, that only gives us 45% for ingress, leaving a 10% gap in added safety.

So with tanks at 3600, we can now safely use 45% of 3600 or 1600 psi, making our turn p 2000 psi. And that's a safe turn.

Cow or Manatee will probably give you numbers similar to these.

But here's the interesting point. Assuming those calcs are right (and they're not far off from my memories of Cow dives) it's SAFER to dive to 2000 psi in Cow than it is to dive to 2400 psi in a no-flow cave.

Let's redo it in no-flow. You enter with 3600, turn at 2400 and exit at 1200. Your min gas needed is 1200 + 2x 1200 or - surprise! - 3600. Turn is at 33%. Now add 5% for safety and you should turn after using 28%, i.e. 2600 psi.

It is AS SAFE to turn at 2600 psi in Peacock (assuming you don't know where the alternative exits are) as it is to turn at 2000 psi in Cow (assuming my values for turn and exit p's are correct). it is way more dangerous to turn at 2400 in Peacock than it is at 2000 in Cow.

So if you want to be lazy and just turn at thirds, sometimes you'll be taking your life in your hands (no flow) and sometimes you'll be ridiculously conservative (high flow).

One caveat - you'd better be good at arithmetic before starting this stuff. "Math kills"!!

One other - sometimes caves change. There's a little no flow passage in Cow that completely changes these calcs. Be conservative in those situations.

So - I don't dive thirds any more. And I'm sure that offends some people. But at least I know my margin of safety. Do you?

[kensuf]

Of course... I was implying breathing the stage empty first.. Never said I would drop the stage and scooter at the same time. Thought that was obvious that no one would drop a non empty stage.

Don't take it personally because there was nothing said with any intent to flame. I've just learned over the years to never assume what you may think is obvious is clearly seen by everyone else.

[tflaris]

Well, I'll go here... I often use "half plus 200" on stages. I've done some spreadsheet work on this, and there's an added level of safety as you add stages, since this increases redundancy. i was surprised however at how SMALL the advantage is. So I only use 1/2+200 in high flow caves like Ginnie and Cow.

And I PROMISE that's conservative!

__________________________________________________ ___'

A slight change of pace.

Do this calculation - remember your PSI on entry and at turn, and look at your PSI on exit. Double the difference between turn and exit. Now take that as a % of the PSI consumed going in plus that number.

As an example, say you go in with 3600 psi and turn at 2400 psi. On exit you find that you're at 1800 psi.

You used 1200 psi going in, and 600 psi exiting. 2x600=1200. So to JUST survive the dive losing 1/2 your gas at turn you would have had to enter the water with 1200+1200=2400 psi of gas. You would have used 1200/2400 or 50% of your gas, and a bare minimum reserve would have been 50%.

That % is the min gas you need to exit safely, IN THAT CAVE AT THAT FLOW SWIMMING AT THAT SPEED etc etc. Add a safety margin depending on circumstances, and that'll give you a safe reserve.

Any safety you add will get doubled since it cuts into your ingress gas. So if we reserve 55%, that only gives us 45% for ingress, leaving a 10% gap in added safety.

So with tanks at 3600, we can now safely use 45% of 3600 or 1600 psi, making our turn p 2000 psi. And that's a safe turn.

Cow or Manatee will probably give you numbers similar to these.

But here's the interesting point. Assuming those calcs are right (and they're not far off from my memories of Cow dives) it's SAFER to dive to 2000 psi in Cow than it is to dive to 2400 psi in a no-flow cave.

Let's redo it in no-flow. You enter with 3600, turn at 2400 and exit at 1200. Your min gas needed is 1200 + 2x 1200 or - surprise! - 3600. Turn is at 33%. Now add 5% for safety and you should turn after using 28%, i.e. 2600 psi.

It is AS SAFE to turn at 2600 psi in Peacock (assuming you don't know where the alternative exits are) as it is to turn at 2000 psi in Cow (assuming my values for turn and exit p's are correct). it is way more dangerous to turn at 2400 in Peacock than it is at 2000 in Cow.

So if you want to be lazy and just turn at thirds, sometimes you'll be taking your life in your hands (no flow) and sometimes you'll be ridiculously conservative (high flow).

One caveat - you'd better be good at arithmetic before starting this stuff. "Math kills"!!

One other - sometimes caves change. There's a little no flow passage in Cow that completely changes these calcs. Be conservative in those situations.

So - I don't dive thirds any more. And I'm sure that offends some people. But at least I know my margin of safety. Do you?

I can see the clarity in your Math and have informally used this in High Flow Sysyems but instead of using the spreadsheet I have always based the runtimes from actually numbers or PSi when diving those systems.

I have used on a regular basis the 1/2+200 rule at Ginnie on a stage bottle but have maintained the Rule of 1/3's on my backgas for the what if's.

Very interesting refence between the math on a High Flow versus a Low Flow System.

Very simulating thread....

[Daedalus]

IMO if you want to bust thirds that is your business (and your buddy's) but you certainly should not be advocating it on the internet, even if your math makes sense to you. Just keep it to yourself and accept that you are breaking one of the cardinal rules of accident analysis when you do so.

I was taught and still believe 1/3 of your total gas volume is the maximum amount of gas you should use for penetration on a dive (in the most favorable conditions). I cant remember the last time I turned a dive right on my turn pressure, I usually turn 100 psi or more above it. If you want to drain your stages to 1/2 + 200 to make them easier to carry later in the dive or have them be "ditchable" in an emergency compensate for it off your backgas. 300 psi in an AL80 is approximately 100 psi in a set of double 104s. If I am breathing an AL80 stage down to 1700 instead of 2000 my new turn pressure on my backgas is 1/3rds plus 100.

[Line Squirrel]

Bob - I agree, I'm not going to say I have a lot of cave dives, but I've got a few and I have NEVER...NEVER dove past 1/3 of my back gas, or "side gas" as it where.

What you and your buddies do is your business but you dive with me and we turn on thirds or less period.

[Gary]

The first guy I heard of pushing thirds with with creative math was a guy at Little River who convieniently explained his method to a cave class before getting lost off the line and dieing nowhere near the exit. The last was John Robonson who pushed thirds without telling me as his buddy and nearly died as a result.

Both these guys pushed thirds, both thought they had justification to do so, both died in a cave.

You can follow Andrew's math spreadsheet on this if you want to but history shows this line of thinking is deeply flawed.