Downstream and Thirds

[aainslie]

EDIT - I changed this a few times as I checked my results. Last edit was noon, Florida time.

Gary,

I've heard that story. I suspect his model was incomplete. Little River is a classic example of the average flow changing, AND of the probability of a silt-out increasing as you go further. But your comment, plus Kelly's, have made me add something to the spreadsheet.

Kelly,

I understand all that, but any of these variables can be accommodated. Plus progressive penetration i definitely important, after all how do you know how conditions are going to change? But, thanks for the suggestion, I've changed the spreadsheet to help accommodate some of your concerns. Let me explain.

Let's start with a mental baseline. Almost everyone dives thirds, regardless of flow. So, if you're in a no flow cave, and you get percolation/restrictions/Murphy's, NOW what? The answer is, a GOOD diver adds a safety margin. But many divers blithely use thirds regardless of conditions. THAT is extremely unsafe.

Now, in a downflow cave, what exactly would you add a safety margin to? Most people don't know so they use sixths. Sometimes that's enough, sometimes not.

I've added a piece to the spreadsheet to accommodate the percolation/restrictions/Murphy's idea - I changed the spreadsheet name to make it clear its a different version. You simply add the % of the your gas that you want to hold as a Murphy's reserve. You might add 9%, say, in Ginnie where there's not much to go wrong (but you still have to change stages, recover spools etc), and 60%, say, in some squirrely little sidemount cave with 10 year old line. Again, a bit of math gives you a better sense than just guessing. This is done by simply reducing the tank volume by that percentage, i.e. we're saying "I'm keeping 9% of my gas aside as unusable, for Murphy's".

Just for interest sake, let's take a no-flow situation. With a 9% safety, the turn p becomes 2600 instead of 2400 for a 3600 tank. Now that I've done this calc, I would argue that anyone turning on thirds in Mexico (I'm not using Peacock because I want to exclude arguments about alternate exits) should, at the very least, use this turn pressure, NOT 2400. Now let's take a squirrelly little cave like Ruth's, say. With a 60% allowance, the turn P becomes 3200 PSI!!!

Now seriously, if you hadn't done the math would you have thought that keeping 60% of your gas for emergencies would drop your thirds from 2400 psi to 3200 psi? It's a massive drop, isn't it? Again, it's LESS THAN SIXTHS! By the way, if your intuition is that using sixths is like reserving 50% of your gas for Murphy's you'd b exactly right.

Another interesting calculation - let's say Ginnie's average flow along the Gold line is about 10 fpm (I'm guessing here but it would be easy to get an accurate number). What is the implicit reserve past thirds? The answer is about 22% - you'd be treating 22% of your gas as inviolable ON TOP OF THIRDS! That's a healthy margin. But in Mexico it would be zero, and that's NOT healthy.

Of course, I may be wrong here, so I'd REALLY appreciate some people checking my math and my assumptions. Doron Nof independently checked my baseline calculations and came to 6/7 as the TP with a flow of 25 fpm and a swim speed of 50 fpm - exactly what I came to. But I'd like somene to check my Murphy's adjustments.

I am NOT suggesting that this should be introduced in an intro class. But for anyone doing serious exploration, I suggest that a bit of math would end you up with a more consistent safety margin. And this sometimes means using LESS than sixths... and sometimes POSSIBLY using more than thirds.

In particular, a) anyone using thirds in a no-flow cave is NOT being safe and b) using sixths in a downstream cave or in a squirrelly unexplored no-flow cave may be insufficient, depending on what safety margin you want to use.

Here's the new spreadsheet - look for the input for Murphy's:

http://www.andrewainslie.com/spreads...s%20Murphy.xls

[Gary]

The final numbers in your sheet are values not equations and aren't changing. At least in the Murphy I downloaded.

I hope though you are suggesting this just to stress me out and don't ever plan to actually violate thirds. It's not a good idea. Add as much safety as you want but stay away from justifing any reason to be less safe then thirds. It's bad reasoning regardless of the numbers.

[lamont]

In particular, a) anyone using thirds in a no-flow cave is NOT being safe[/url]

Sheck nearly died once diving 1/3rds when at max penetration his buddy lost almost all his gas and they had to share back to their stage drop. Story is in _Caverns Measureless to Man_ somewhere... Cave in europe, IIRC...

[aainslie]

Hey Gary,

I've updated the sheet. The areas in YELLOW are for user input, BLUE is the key output data. It's pretty rough, I'm still fine-tuning. The algebra is at the top right. The key equation is in cell E20. M - Murphy's allowance, in = speed going in, out = speed going out.

And just to freak you out, I am going to claim that if you're diving Ginnie on the main line, you'd be as safe going down to 2100 psi as you would going to 2600 psi in Peacock. Both would be safer than sticking to the rule of thirds in Peacock. The rule of thirds is a BAD idea in a no flow cave.

[Bob Cree]

Andrew, I agree. Being a physicist - the math works...and your recent spreadsheet update shows you realize it is much more complex than calculation can absolutely allow for. In addition to the time-motion mechanics of the dive, you have to take many other esoteric effects into account. Configuration, solo/buddy/number of buddies to name just two of the more obvious ones. You obviously cannot do this 100%. I look at your analysis as a very valuable study of minimums - a bit akin to using deco software to look at various scenarios and do worst case planning.

I (and I think what Gary was trying to say) would hate to see someone who does not think things through quite so thoroughly and who primarily does the Ginnie type dive to start planning dives with turn pressures lower than 1/3rds (and you have to admit, if some folks thought it was ok - they would do it, oblivious in thought as to why).

Certainly sidemount gives you the independent gas as does isolating, but isolating takes time. Buddies have their drawbacks, but do sherpa extra gas for you. Granted in the sidemount/solo world, these become a bit more defined and your calculations may fit this reasonably well for you...but, for most...in a respectable flow upstream dive, Gary is right - turn on thirds, then take the time and exit advantage you so rightly point out as an opportunity (note: not at max penetration, only as you gain advantage by getting closer to the exit) to take your time exploring a bit and maybe concentrating on skills development.

In downstream diving, you make a valuable point on why these dives require MUCH more planning than the typical upstream dive. Maybe more than some might have thought.

In low/no flow caves, perhaps the solo/buddy question begins to play a more crucial role in gas volume safety. I agree with your concerns that 1/3rds in these systems may not be conservative enough - the sherpa can help you and does add safety margin (not wanting to hijack, please lets not get into the solo/buddy debate regarding non-gas related buddy issues).

[Al]

Another thing to consider is rate of gas consumption. I believe that in addition to slowing you down, swimming against the flow makes most people use more gas per minute. This of course depends on a diver's fitness level and the type of propulsion used in a particular environment.

[aainslie]

addexdiver,

Thanks for the points and comments. I'm essentially trying to create an informal form of peer review here and it's great that you checked the math and added your feedback. Thanks!

I REALLY agree that the rule of thirds is a good, conservative rule in any cave with appreciable flow. I also believe that there's a serious amount of "caveat emptor" (even though it's free) for anyone even vaguely considering using my sheet to go below thirds in a high flow cave. One would be NUTS to do it without a huge amount of understanding of the environment on is entering, and the math behind what I've suggested here.

al,

Great point! I'll mess with that and add it. Thanks!!

[Sludge]

There are two incidents that came to mind, and have stayed in my mind ever since this thread began.

The first was a dive I made last summer at the St. Andrews jetties. What we like to do is get in on the beach side an hour before high tide, swim out around the tip, and drift all the way to the Kiddie Pool. But it was after high tide, so I thought I'd just swim out to the shell pile and goof off for an hour and then swim back to the beach. But the current was absolutely not what I was expecting. Halfway to the tip, where it starts going from 22' to 50', a monstrous current pulled me to the tip. I sort of freaked out, and decided to swim back up the hill. It was impossible. I dumped all my air and digged and clawed all I could, and the best I could do was hold my ground. My breathing rate went through the roof. (Keep in mind all I had to do was inflate my BC and surface.) But this is one of those situations where what you expected and what you got were two different things. I changed my plan and went ahead and drifted to the Kiddie Pool. But after high tide, the current was not supposed to be this way.

The second incident, I admit, is one I am sure I don't have all the information, just what I heard third-hand. An open-water diver at Catfish Hotel supposedly went into the downstream tunnel, got caught in the flow, and was unable to swim back up the hill before he ran out of air and drowned. When I heard about it, I thought about that jetty dive, and what a horrible feeling that must be, seeing the surface of Catfish Hotel, and not being able to reach it. In my case, I had over 100 cubic feet of gas, and I could easily save my life by inflating my BC. But this diver was doomed, and he spent his last few minutes fully aware that he was doomed.

So back to the subject at hand. You can't have too much gas. Imagine the flow being even higher than you had guessed. Diving sixths with a full buddy bottle still may not be enough.

[aainslie]

OK, I've added something called "Breathing delta" in C12. Let's say you think you have a SAC of 0.5 when going with the flow, and 0.65 agaist the flow. That means you breathe 30% more heavily against the flow than with the flow. You would then enter a breathing delta of 30%.

I only allow this for downstream calcs, i.e. I do NOT let you use this to reduce gas reserving requirements when swimming upstream. That would be too aggressive.

Thanks Al! Nice Suggestion!

Note - now on a 25 fpm flow downstream cave, with a 9% Murphy's reserve, and a 30% breathing delta, your TP would be 3300 psi, or TWELFTHS!!! I suspect that's the sort of situation that ends people up with 500 PSI after diving a siphon. You need to be VERY conservative with downstream flows.

[Gary]

And just to freak you out, I am going to claim that if you're diving Ginnie on the main line, you'd be as safe going down to 2100 psi as you would going to 2600 psi in Peacock.

Bad example. I'd have half a dozen different exits to choose from at Peacock. Probably only need about 40cuft of gas to get out from almost anywhere and would have a tremendous amount of time to deal with any problem.

I calculate something with spring and siphon numbers on it recently. Even a 20'/min flow makes a huge difference in air use (almost double in vs out). But for safety reserve I'd stick with thirds at a minumum. (more below)

Math check:
"10% Murphy ... 3600 PSI, then the program only uses 3260 PSI"
3240?

For Use Pressures should only calculate usable to the hundreds place. That would correct the Turn Pressure properly and remove the wierd rounding method you are using (which I don't think is mathematically correct but does produce the right answer).

Assumptions:
Divers don't dive at a single rate regardless of the flow they are in (although scooters can be depended on to do so). They swim slower going with the flow and faster when fighting against it. That is a good thing actually because it is more efficient that way as long as they don't overdo it the SAC vs speed (gas used per distance traveled) is in their favor swimming somewhat harder into a current. (Tracking SACs vs swim rates is a fun project BTW).

Also divers may use pull and glide going in and drift out swimming at a much lower speed. Even if they pull and glide out the rock is stationary with respect to the flow and not accounted for in your flow assumptions. They are not getting the same advantage pulling a stationary rock on the way out with the flow as they got doing pull and glide when swimming against it.

Your equations and assumptions may be good for calculating an initial safe ratio (instead of just guessing) but it may be better to calculate turns based on actual air use once you've dove the system.

Safety:
If you compare a no flow cave to a high outflow cave then "yes", at max penetration the person in the high outflow is in a better situation BUT comparing turning on thirds in a high flow cave vs turning past thirds in a high flow cave the person turning on thirds is safer.

For people figuring out some justification to go beyond thirds vs those actually doing so: the ones going past thirds have a much worse survival rate.


Shorting yourself on thirds is just a bad idea. Bring more gas instead. Fine to add conservatism to dives, bad to remove it. If solo diving it is especially bad because no one else is there to refuse to dive. If you are solo you have to reject your own crazy ideas.

A mathematical answer is great,but there are so many other variables that you can't plan for.

I'm with Kelly on this one.

Sure, I love math and I agree that your math is basically correct. There are some problems with your assumptions but still a reasonable model. However, the model is way too simple and doesn't take everything that can happen into account. "Thirds" is an incredibly blunt instrument but has proven to be largely safe in a wide array of different situations.

If there ever was a good rule in cave diving it would be: never short on thirds.

Another good rule would be always allow yourself enough time to handle any problem. Shallow one might have an hour or more to take care of anything. The deeper you go the less time the gas lasts. At 300' one may only have 10 minutes worth of gas to spare even without having a gas failure.

Divers with hours worth of gas respond to problems much better then divers aware they have dove past thirds, have only a few minutes of gas to spare and are fighting a tangled reel in high flow. While their mathematical model may assume they are swimming out peacefully with the flow they are actually working harder then on the way in. The flow is making things worse, not better.


It depends too much on the situation to assume the flow will always help you on the way out. You can't even assume zero flow for the way out because sometimes it will work against you. You can probably be sure that the outflow won't penalize you more on the way out then it did on the way in.

Solving for that equation gets you "thirds".