Downstream and Thirds

[aainslie]

Hey all,

How do YOU modify thirds when diving a cave with serious flow, downstream?

I just worked through the algebra but I'd be interested in other peoples' opinions before trusting my solution.

I'll post my solution once I've heard a few suggestions. I couldn't find a simlpe rule of thumb that was safe but I might be missing something. And my solution requires firing up a spreadsheet before each dive, which sucks.

Regards,

Andrew

[Sludge]

Until you're very familiar with it, I'd say sixths AND a buddy bottle.

[FW]

Sheck Exley did some tests at Manatee, and came up with fifths as a minimum, but that flow isn't all that high, except in a few spots.

Like Slüdge said, get familiar with the cave. Make short dives, and record your gas consumption both directions.

You can do the same in an upstream dive, then reverse the numbers. You will need someplace with about the same current.

The problem with using math is, the flow is never constant, and things like restrictions can add a lot of time, and gas, to negotiate.

[Jerry]

I agree with the ideas above; in an unknown start out conservative. Sixths and a buddy bottle can always be changed on subsequent dives. This is how we did downstream Hart from Black Lagoon years ago. We finally settled on fourths. We did continue to carry a buddy bottle.

Jerry

[Gary]

IIRC the old CDS cavern manual said to start with sixths until you do a dive to compare air use in vs air use out.

So if you used 25 cuft going in and 50 cuft going out then your total with reserve would be 25+50+50 and your useable gas would be 25/125 = 1/5.

[Line Squirrel]

Like Slüdge said, get familiar with the cave. Make short dives, and record your gas consumption both directions.

The problem with using math is, the flow is never constant, and things like restrictions can add a lot of time, and gas, to negotiate.

Agreed & Agreed.

I dive sixths first time in, adjust by small amounts on subsequent dives.

[aainslie]

OK, I did some algebra and found that in a 25 fpm flow, if you swim at 50 fpm, 6ths is not conservative enough!

I'll post n Excel sheet later. At 3600 PSI your turn at the conditins above s 3100 psi! I was shocked.

[aainslie]

Hey all,

I couldn't post an Excel sheet so here is a link:

http://www.andrewainslie.com/spreads...am%20calcs.xls

Some comments:

1) A good mathematical answer is always better than a series orandom approximations hoping one day to get it right. This problem is HIGHLY amenable to a bit of thought.
2) Only three things are needed to determine average swim speed and flow speed: a) known distance; b) time to swim in; c) time to swim out. If you don't have a known distance but you know your swim speed from other dives, only TWO things are needed: a)time in and b) time out.
3) Sixths can be an unsafe choice. See the spradsheet for any flow rate over 20 fpm.

What REALLY starts getting interesting is thinking about safe reserves swimming into flow. I've set up a chart of safe reserves there too. In a KNOWN flow of 20 fpm you can safely go below halves.

[Gary]

What REALLY starts getting interesting is thinking about safe reserves swimming into flow. I've set up a chart of safe reserves there too. In a KNOWN flow of 20 fpm you can saely go below halves.

Perhaps, but there was a guy who died at Little River right after making such a proclimation - and the flow back then was a lot higher then it is now.

It's best not to get too inventive when calculating safe turn around pressures. Thirds is the least conservative anyone should plan on. If in an accident the flow makes a net benefit great - but don't plan a dive that way.

[Kelly Jessop]

1) A good mathematical answer is always better than a series orandom approximations hping one day to get it rght. This problem is HIGHLY amenable to a bit of thought.
.

Thanks for sharing your spreadsheet.

A mathematical answer is great,but there are so many other variables that you can't plan for ie. percolation that impairs visibility and speed,restrictions that are more difficult on the exit side,plus Murphy to name a few. Typically I would use progressive penetration to supplement any algorithm,and correlate the two together. I reminded of a very experienced cave diver who dove a syphon system for the first time,and planned things well,but exited the cave with only 500psi because of the other unforeseen variables.