Modifying rule of thirds

[aainslie]

I still don't get why it's reckless. Care to explain?

I'm suggesting keeping the SAFETY MARGIN constant, not the RULE.

Thirds is like setting a speed rule. We all know that increased speed increases the probability of an accident. but we also know it's a function of the road conditions. So we a) alter speed based on e.g. weather (slowing down in fog, etc) and b) alter speed limits based on road conditions (e.g. 65 on a highway, 35 in a town).

My suggestion is to implement something like the speed variation based on road conditions, which (like cave flow) are reasonably invariant. Thirds is like setting a speed limit of 50... whether you're on the highway or in the middle of High Springs. And flow is a very objectively measurable aspect of a cave.

You should add more margin if the cave is narrow... silty... unknown... deep... etc. etc..

Attached is a sheet with the calcs I put in the earlier discussion. Note I put a piece in there to add a psi "safety net" for conditions other than flow. And notice how easy the arithmetic is. Measure psi on entry, on turn and on exit. Input next dive's starting psi. Based on just those 4 numbers, plus the addition of a safety margin, you can keep a safety margin that stays constant as cave flow changes. I am ONLY controlling for flow. You need to control for other sources of gnarliness.... which btw you'd better be doing with thirds also.

It's another reason I hate thirds. it's such a simplistic rule that people think it's good enough. It isn't. It's frequently too liberal.

[billybones519]

I still don't get why it's reckless. Care to explain?

I'm suggesting keeping the SAFETY MARGIN constant, not the RULE.

Thirds is like setting a speed rule. We all know that increased speed increases the probability of an accident. but we also know it's a function of the road conditions. So we a) alter speed based on e.g. weather (slowing down in fog, etc) and b) alter speed limits based on road conditions (e.g. 65 on a highway, 35 in a town).

My suggestion is to implement something like the speed variation based on road conditions, which (like cave flow) are reasonably invariant. Thirds is like setting a speed limit of 50... whether you're on the highway or in the middle of High Springs. And flow is a very objectively measurable aspect of a cave.

You should add more margin if the cave is narrow... silty... unknown... deep... etc. etc..

Attached is a sheet with the calcs I put in the earlier discussion. Note I put a piece in there to add a psi "safety net" for conditions other than flow. And notice how easy the arithmetic is. Measure psi on entry, on turn and on exit. Input next dive's starting psi. Based on just those 4 numbers, plus the addition of a safety margin, you can keep a safety margin that stays constant as cave flow changes. I am ONLY controlling for flow. You need to control for other sources of gnarliness.... which btw you'd better be doing with thirds also.

It's another reason I hate thirds. it's such a simplistic rule that people think it's good enough. It isn't. It's frequently too liberal.

Andrew, I haven't looked at your spreadsheet so I don't have an opinion on it either way. I have noticed that this discussion comes up from time to time. You are in a no win situation. The % of trained THINKING divers is small in the real world, here in cyberspace it is even smaller. Look at some of the answers in respoinse to the original posed question. They show no understanding of the contingency being planned for when soloing. For ex: "I'll dive to sixth's so I don't need a buddy bottle!!!!". Give me a break. Some of the more experienced have correctly stated that more complex gas planning is involved in more complex dives. Essentially, have enough alternatiive gas to swim out and make a safe ascent from deepest penetration allowing for as many critical failures or situations as you can invision.(For example loss of all backgas, DPV, Complex navigation in Zero viz). All of this gas does not necessarially have to be on your person at all times. Some can be staged and placed along a planned escape route.
Since the training agencys don't take a position on Solo diving, little contingency gas planning is being taught in the normal course of cave training. (DPV class is an exception to this). This is why you see chest thumping and Rule of Thirds shouted at anyone who suggests anything else. (Many would not even notice that a more conservative approach might be the result, That would require THOUGHT). IT'S TIME FOR THE MAJOR TRAINING AGENCYS TO CHANGE THIS. With the tremendous growth in CCR diving a need for this planning has now become comonplace.. An OC diver with a CCR diver is essentially diving solo as far as gas planning. Can it really be within a training Agency's mission to ignore this? I hope not.

Take care,
John

[Ken Hill]

Some points to ponder along with the finer points already made.

1. Isolation - takes at least 8 seconds

2. Burst disc failure - takes about 54 seconds on a pumped set of 104/130's to be rendered useless. At 3500PSI you would lose 500PSI in your attempt to isolate and then have 1500PSI left in the remaining tank. But hell you'd be at the front of the cave.

3. Rule of thirds is the minimum recommended safety factor. Margins of safety are realized when a 3500PSI tank is computed at 3300 making the turn 2400PSI. Just shy of 25%

4. If forced to isolate at 2400psi you would lose 500PSI, and have half of 1900 or ~ 950PSI to exit. But now you'd be at the back of the cave.

5. Exiting in high flow causes other things to break, light cords to snag and one time a small guy leading a big guy leading through the narrow part of the lips sat there for seven minutes because the big guy couldn't go forward.

6. Solo diving with out a buddy bottle ... not me!

My 02 cents. Great thread if it gets you to plan more conservative that you normally would. /Ken

[jj1987]

Some points to ponder along with the finer points already made.

1. Isolation - takes at least 8 seconds

2. Burst disc failure - takes about 54 seconds on a pumped set of 104/130's to be rendered useless. At 3500PSI you would lose 500PSI in your attempt to isolate and then have 1500PSI left in the remaining tank. But hell you'd be at the front of the cave.

3. Rule of thirds is the minimum recommended safety factor. Margins of safety are realized when a 3500PSI tank is computed at 3300 making the turn 2400PSI. Just shy of 25%

4. If forced to isolate at 2400psi you would lose 500PSI, and have half of 1900 or ~ 950PSI to exit. But now you'd be at the back of the cave.

5. Exiting in high flow causes other things to break, light cords to snag and one time a small guy leading a big guy leading through the narrow part of the lips sat there for seven minutes because the big guy couldn't go forward.

6. Solo diving with out a buddy bottle ... not me!

My 02 cents. Great thread if it gets you to plan more conservative that you normally would. /Ken

Ken, as someone who's very new, I've always adhered to, but thought the 1/3 rule was very constrictive in a higher flow system (especially for me, still at cavern), but this gives me another way to think about it. It's always nice to KNOW the rules, but appreciate when someone gives their time to explain them.

[OneBrightGator]

I still don't get why it's reckless. Care to explain?

Sure, just keep in mind this is my opinion based mostly on a lackadaisical attitude toward diving (except the safety aspect) and gut feelings. It relies little on your fancy "math"

Basically, the Rule of Thirds is farm animal simple to remember and understand. Hmmm... if it takes this much to get one person out, then it will take twice to get us both out. ba-da-bing! Now, instructors should teach and show (lights out air sharing exit is a good way) how liberal that rule really is and explain that just because you can use a third doesn't mean you always have to or even should.

You're trying to nail down, with math and logic, how to safely go inside an underground, water-filled cave (which isn't very logical in the first place). We're not constants, neither are the caves. On top of that, in the example pre-loaded on your spreadsheet you're only getting an extra 100psi! whoopie. If there's something you really want to see 100 psi further, work on your technique, if it's past that use a(nother) stage, scooter, 'breather or buddy with bigger legs then yours.

What it basically boils down to is just because you do it, doesn't mean you should talk about it on the internet. Stuff like this, how to find a hidden jump, how to get access to a closed site, etc. is best reserved for spring-side chats.

Oh, and I don't cave dive solo and don't really care to or am knowledge enough to talk about it.

[Moonfuzzy]

Basically, the Rule of Thirds is farm animal simple to remember and understand. Hmmm... if it takes this much to get one person out, then it will take twice to get us both out. ba-da-bing!

On top of that, in the example pre-loaded on your spreadsheet you're only getting an extra 100psi! whoopie.

My thoughts exactly. Most people have enough going on in the water already (remember task loading). It is easier to use the rule of thumb and modify it as experience dictates than to plug numbers (many are probably assumptions, right?) into a spreadsheet and remember the details the next day... unless of course you are gearing up for a trimix dive ; )

And all that trouble for almost no gain, I'll stick with thirds.

BTW - to answer the original question I do reserve more gas if the dive is more complex or if I am doing a circuit and part of it will be slower going.

However, if there are multiple bottles in the dive plan there is a reserve built in because losing one only affects a smaller and smaller portion of your total gas. (Ex - on a double stage dive losing one stage at max penetration would only kill 1000psi of my planned usable gas, I have a second stage that would cover that w/o touching backgas, and my buddy has two also.)

If I am using 1/2 +200 on stages I always reserve the ghost 1/3rd in my doubles, then calculate thirds.

[aainslie]

Sure, just keep in mind this is my opinion based mostly on a lackadaisical attitude toward diving (except the safety aspect) and gut feelings. It relies little on your fancy "math"

Basically, the Rule of Thirds is farm animal simple to remember and understand. Hmmm... if it takes this much to get one person out, then it will take twice to get us both out. ba-da-bing! Now, instructors should teach and show (lights out air sharing exit is a good way) how liberal that rule really is and explain that just because you can use a third doesn't mean you always have to or even should.

You're trying to nail down, with math and logic, how to safely go inside an underground, water-filled cave (which isn't very logical in the first place). We're not constants, neither are the caves. On top of that, in the example pre-loaded on your spreadsheet you're only getting an extra 100psi! whoopie. If there's something you really want to see 100 psi further, work on your technique, if it's past that use a(nother) stage, scooter, 'breather or buddy with bigger legs then yours.

What it basically boils down to is just because you do it, doesn't mean you should talk about it on the internet. Stuff like this, how to find a hidden jump, how to get access to a closed site, etc. is best reserved for spring-side chats.

Oh, and I don't cave dive solo and don't really care to or am knowledge enough to talk about it.

I love that you're pretending to be dumb here. I've read enough of your posts to know otherwise.

Do me a favour. Next time you swim something seriously high flow, take those 3 measurements nad shove them in the spreadsheet. You'll find that the difference is more than 100 psi.

Anyway, I've had my rant. It's been fun.

Oh, and jj1987, do me a favour and listen to the others. It's definitely the safer thing to do. Even I'll concede that.

[OneBrightGator]

I'm not pretending to be anything. I understand what you're saying and I've done plenty of dives where my egress gas was half of my ingress gas, I'm fine with that. I don't agree that it's safe to out-calculate a the basic idea of what goes in must come out. In high flow caves there's a built in safety, in low/no flow caves a diver must pad for safety. Simple, basic stuff... like me

[novadiver]

RULE OF THUMB , is simplistic at it's best. such as the rule of thirds. While Andrew and Ben can disagree about GAS RULES , The basic rule of having enough gas to egress the system is the only rule that needs to be justified. Do you really need a third of your gas if you're 300 feet back ?


The rule of thirds is a built in safety margin , not really a gas plan that pushes the envelope.

Do you really want to push the envelope ? Maybe that's not a good Idea. Maybe penetrating to thirds and goofing off on egress is a better plan , while penetrating to thirds and surveying on egress would suck.

The plan makes all the difference.

[Serota]

Ken, as someone who's very new, I've always adhered to, but thought the 1/3 rule was very constrictive in a higher flow system (especially for me, still at cavern), but this gives me another way to think about it. It's always nice to KNOW the rules, but appreciate when someone gives their time to explain them.

Isn't the rule still 1/6s for cavern and intro.?