Modifying rule of thirds

[Gary]

One of the things thirds doesn't cover is time. In addition to adequte reserves it is necessary to have some extra time. If something goes not according to plan and you need 10 or 15 minutes to sort it out you should have that without having to worry about what is happening to your reserve. If you are on a rebreather and you get hit with a CO2 spike and you are breathing like a freight train and you need 5 minutes on OC to get your breathing down and still have enough gas to swim out safely on OC.

It'd be nice to think that if a rock fell on your leg and you were trapped you (and/or your buddy) had time to try several different things to get you free and still be able limp out in great pain, breathing like crazy.

As thing get deeper time gets shorter. If thirds is 10 minutes than things are getting tight and thirds isn't enough anymore.

[BgDadddy]

When I solo dive, I'm more conservative with my gas, not less. Typically I will dive to 4ths.

Rick

Edited to add: Oops, I posted that after reading the first page. I had no idea what was coming...

[Meister481]

Without question, the way that gets you out of the cave alive every time IS the right way. That being said, the "What ifs" begin!

I would want to have enough gas on my person (SOLO) or on the team to cover 2 major failures.

Deco gas stolen or unusable.
Zero vis-3 light failure-siltout-2 mask failure-Makes scooter(s) useless.
Blown burst disk on back gas or stage-fraction of gas lost.
Broken Scooter at Max Penetration.

Do these algorithms work when Murphy craps on your head? Does everyone plan for scooter failure at Max P and lost deco gas and additional deco gas for longer exit time? I know thirds aren't enough when you get into big scooter dives. There's alot more planning I want to do than "hey that there thirds rule will work!"

I'm just curious on what planning everyone is saying is enough gas to get out. The topic is a wonderful example of what this forum was created for- the discussion of the mechanics and techniques of cave diving. I'll be taking a class next month, so this will be an interesting topic to discuss at length with my instructor.

[MengTze]

Break the dive up in segments and plan accordingly. Always have enough gas to get out and add some for emergencies. Then you reflect that against your comfort level and acceptance of risk. Adjust if needed and you end up with a gas plan that should suffice.

It is not rocket science and a very personal thing as we effectively calculate our acceptance of risk when we plan a (Cave) dive.

[gsulliva]

Andrew I hope your algebra is better than your ability to post a URL (teasing of course).

Here is a working link:

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


Gareth

[aainslie]

Andrew I hope your algebra is better than your ability to post a URL (teasing of course).

Here is a working link:

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


Gareth

Oops - thanks (and greetings!!), Gareth.

[atedeschi]

Ok so I have been playing with this spread sheet, and I think it can be used for downstream and with help making 3rds more conservative for different situations. would like to know what percent or psi you are putting in reserve for:
just some normal reserve
low/no flow
restriction(each one)
narrow
silty
unknown
deep
exploration
long
survey
did i miss anything? you can add scooter but i dnt scooter so im not sure about all the rules etc.

[aainslie]

Hey Ant,

I don't think I'm qualified to make those calls. My math is better than my diving skills! However, I feel that experience in these types of dives will inform you. Start conservative, and then reduce as you gain experience under each set of circumstances.

[atedeschi]

yeah thats what i figured but i was just wondering what some do or if anyone has like rules of thumbs for certain situations. Hopefully someone will chime in.

[aainslie]

Andrew I hope your algebra is better than your ability to post a URL (teasing of course).

Here is a working link:

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


Gareth

Ant - aka ateseschi - pointed out an error in my calcs. I'd used 44 instead of 33 as a conversion between depth and pressure - my bad. I've corrected it and uploaded a new version. it only affects a couple of calculations, but I'm glad he picked it up. Thanks Ant!