Split from A&I Yukon thread: Gas Rules in OW Solo Dives

[ScubaSteve]

The rule of thirds in general terms:

You plan to use 1/3 of your gas on the way out, one third to return and one third is held in reserve. This implies a certain dive profile which is less typical for OW dives.

I think what is being missed on why people have issues with the rule of thirds for OW dives is the fact that most OW dives the diver does the following: (direct) Descend, Dive, (direct) Ascend. The rule of thirds, as used by cave divers (because there is no "(direct) ascend" option, is good because they usually follow the same path (or similar) as they did to get into the cave meaning they should need basically the same amount of gas if nothing goes wrong. Rule of thirds could be more applicable to an OW dive if the profile was: Descend, dive bottom to a point, turn and dive bottom to where you descended to, Ascend. This still is an overkilled gas plan IMO because the diver still has direct access to the surface but it is better than a drop, dive, ascend profile.

 

[DevonDiver]

First off, I don’t think the actual math behind the rule of thirds in ever explained in the course....

If it's not mentioned in the course...and an SDI instructor, trained to teach the course, doesn't know why, then it does look suspiciously like someone decided that procedure arbitrarily, based on existing tech/cave procedures.

In fact the quote straight from the SDI Instructor Guide is this: “We have settled on the rule of thirds as the appropriate reserve volume for the purposes of this course. However, even though this would give a diver ample volume for most emergencies – such as emergency decompression or losing up lines....

SDI consider 'losing up lines' to be an emergency? No wonder they demand such high reserves. It seems they are suggesting that no up line = no ascent. So they plan reserves as though the whole dive has a 'ceiling' and that a nav error prevents ascent?

Not sure why even emergency decompression comes into this... I assume that you have to be a relatively 'responsible' diver to be awarded a solo diver certification.... so to encounter so much emergency deco....requiring 1/3 gas.... would mean a monumental, unforgiveable, virtually suicidal screw-up.

I just don't see how a recreational solo diver, with no overhead ceiling, who plans his dive properly, could ever reasonably require 1/3 gas reserve.

 

[DaleC]

I don't use any "rules" when I solo dive; I calculate my rock bottom values for max depth and begin ascending from there.

I calculate my rock bottom to allow for some time solving a problem at depth and to ascend with all stops needed for that particular dive.

If direct ascent is possible that is accounted for. If returning to an anchor line or some other form of indirect ascent is needed that is also accounted for in my rock bottom calculations.

My redundant air source needs to hold my max rock bottom value for that dive.

If an OOA diver needs gas they can use my redundant air source. At any point in the dive both they and I will have enough gas to get up.

I also have a sense of the rock bottom volumes for the next couple of plateau depths so I know when to ascend from those depths.

 

[WetDawg]

DD,

How about settle down there and re-read the post...I simply quoted the SDI Instructor Guide.

If it's not mentioned in the course...and an SDI instructor, trained to teach the course, doesn't know why, then it does look suspiciously like someone decided that procedure arbitrarily, based on existing tech/cave procedures.

Before you go off half-cocked, why don't you enlighten everyone on where the original idea for the rule of thirds came from and give us all a history lesson?

 

[Thalassamania]

DD,

How about settle down there and re-read the post...I simply quoted the SDI Instructor Guide.

Before you go off half-cocked, why don't you enlighten everyone on where the original idea for the rule of thirds came from and give us all a history lesson?

We all know where it comes from and what it's utility is, what we are wondering about is the reason(s) that SDI makes this recommendation for solo openwater diving.

 

[DevonDiver]

and for those that didnt, a quick Google will direct them to fascinating articles about Shek Exley, Blue Print for Survival and Accident Analysis.

So, Shek did his research on cave incidents..and the resulting procedures were intelligently deemed to apply to all overhead/ceiling environments - so they were consequently adopted for wreck penetration and decompression diving. The common denominator being the inability to return directly to the surface.

That doesn't apply, in any way, to solo diving. I've even been reading some interesting discussions recently amongst the cave community that discusses lowering the 1/3 rule for solo cave divers.

 

[muddiver]

What is the SDI recommendation for solo diving - twins, pony?

Unless I totaly forgot, the SDI Solo Diving Manual stated that any adiquate redundant gas system was ok. I even think SpareAir was mentioned, but the instructor I was working with laughed almost as hard as I did when I pointed that out.

 

[Jax]

and for those that didnt, a quick Google will direct them to fascinating articles about Shek Exley, Blue Print for Survival and Accident Analysis.

So, Shek did his research on cave incidents..and the resulting procedures were intelligently deemed to apply to all overhead/ceiling environments - so they were consequently adopted for wreck penetration and decompression diving. The common denominator being the inability to return directly to the surface.

That doesn't apply, in any way, to solo diving. I've even been reading some interesting discussions recently amongst the cave community that discusses lowering the 1/3 rule for solo cave divers.

What would "lowering" the rule consist of?

 

[muddiver]

It's not locked into a third out and a third in . . . you plan your dive and the gas to it. But yes, they recommend planning use of only two thirds.



Don't know how that could happen . . .

Is that the new book, 2009, I think? I was told mine was a new book.

The company IT system won't let us use spellcheck through web based programs. And I am an engineer, not an english teacher.

I did the course in 2009 so it is probably the 2009 book.

 

[DevonDiver]

What would "lowering" the rule consist of?

Here's an example: Modifying rule of thirds