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

[Tortuga68]

A couple of times I have taken my BC off,inflated it and paddled back lying on top of it

What BC do you use?

 

[redacted]

First off, Thal, your reputation and experience in the dive industry are beyond reproach, more respectful I could not be.

As an SDI Solo Diving Instructor I thought I would jump in and attempt to answer your specific question;



First off, I don’t think the actual math behind the rule of thirds in ever explained in the course, unlike the “Rock Bottom” calculations that are explained in other courses by other agencies.

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 – solo divers must still be configured with a complete redundant gas source to safely manage the unlikely event of catastrophic failure somewhere in our primary gas system.” (Copyright credit to Scuba Diving International)

Frankly, I would agree that the rule of thirds is ridiculously conservative for an open water dive, however, it is for that very reason that is becomes most appropriate for the Solo Diver. Because you are entirely self reliant on only one set of brains to both recognize and solve any problems that crop up, it behooves the solo diver to be ultra conservative in every facet of their dive planning especially in gas management as one does not have a handy backup gas supply swimming nearby.

That rational would suggest that your redundant gas supply should be sized to be at least 1/3rd of your primary supply. Is that the current SDI policy??

When I dive solo, making it back to an upline or other specific point is not necessary for safety purposes. If it were, then it seems to me the dive should be managed like any overhead restricted dive as access to the surface is not open.

I manage my gas for solo dives with backwards planning. I plan to arrive at the rest stop with at least 500 psi (Al80 or HP100 cu ft tank). On a typical 75 to 100 ft dive, this puts me with a planned start ascent of at least 750 psi. To give me a little room for navigation errors, I plan to have the start ascent point in view by 1000 psi. In all cases, the ascent could be safely done at any point in the dive but any ascent other than the planned ascent route is subject to result in a dingy ride and counseling session. A typical dive will involve at least one navigation out to a planned target, some time period at the target, and then the navigation back to the start point. The exact navigation plan is somewhat open and dependent on available targets and conditions at depth. The rule of thirds would seem to do little to further safety and would unnecessarily shorten the time on target. I don't believe the application of my planning technique has ever resulted on using more than 1/3rd of back gas for the trip out to the first target so hazards are pretty much known while you still have 2/3rd of you back gas remaining. I just don't see any worthwhile advantage of the rule of 1/3rds over my technique. I'm sure one could hypothesize some advantage but not one that makes 1/3rds a better solution than 1/4ths out and back and 50% reserve or even a more conservative approach.

 

[Dyno Bill]

What BC do you use?

I use an older Dacor rig, it's an integrated weight rig with a really large 75 lb lift bladder so I imagine it would hold a couple up. Just lie all the hoses etc in the upturned rig and climb on with tank hanging below. Don't even need a good snorkel for that one.It does take a little practice though. I saw another diver do that years ago during a really long surface swim and it seemed a good alternative to the snorkel.

 

[idive2]

That rational would suggest that your redundant gas supply should be sized to be at least 1/3rd of your primary supply. Is that the current SDI policy??

I'm not sure what their official policy is but 1/3 the size of your air supply is what
they teach in the solo diver class. I had my own 13cf pony and they were quite
insistant that I carry the 40cf pony which they provided. I was diving with my own
steel 100cf tank so a 30cf is just shy of 1/3 so the 40cf was the recommended size.

 

[mdax]

1/4's in and 1/4's out 50 % for problems is how we dive quite a few caves..I view 1/3's as more of a minimum starting point...work out the math for max penetration catastrophic loss and it's a bit too close for comfort for a 2 person team.
Since you're solo you can do whatever you'd like, 1/3's sounds pretty reasonable, no one ever died from coming back with too much gas.

 

[WetDawg]

That rational would suggest that your redundant gas supply should be sized to be at least 1/3rd of your primary supply. Is that the current SDI policy??

Your redundant gas supply is only for a catastrophic primary system failure or an actual OOA situation. You don't use your redundant gas supply in factoring thirds.

And to answer previous questions, there is no official policy on what type of redundant system to use. Upon request, I teach setup and use of doubles, ponies and sling bottles and let the diver choose whatever suits their dive style and/or budget. Keep in mind that most divers that take the Solo course are not really looking to go jump in the ocean and cruise around by themselves every dive. Most come to me after they have seen or had bad “insta-buddy” experience and realized that they need to be more self-reliant.

Oddly enough, the end result is that with increased self reliance skills, situational awareness and a conservative attitude towards gas management, Solo divers tend to make excellent buddies

 

[Jax]

1/2 + 200!!!! :d

Okay, I'll bite -- 1/2 gas amount + 200 psi?

 

[Jax]

I am also not a solo diver but know many divers who like to dive solo (especially on commercial dive trips). I have noticed that all of them have an octopus on their main cylinder. Given that they are diving alone (and as solo divers have an independant alternative air source!) who is the octopus for? Could it be that SSI are using the rule of thirds so that the solo diver can still safely assist another diver? (i.e.planning for the unexpected OOA from your "insta non-buddy")

If my primary quits, I go to my secondary. If the first stage goes or the burst disk goes, I go on my pony.

And yes, that last third in my back gas is for anyone's emergency, my own or someone else's.

 

[Jax]

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

WetDawg alluded to that:

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 – solo divers must still be configured with a complete redundant gas source to safely manage the unlikely event of catastrophic failure somewhere in our primary gas system.” (Copyright credit to Scuba Diving International)

However, they do not care if it is twins or a pony. I certified with a rented 19cf because my 30cf was in hydro.

 

[wrybosome]

Thirds for OW NDL solo dives is a lot more conservative than anyone I know of here. Similar to AWAP, I plan on being back at the anchor with ~1000 psi. In the event of entanglement or equipment failure I have another 40 cu ft in a pony. In the event of getting lost it's not critical to ascend at the anchor, just embarrassing to lose it. A safe ascent could be made from any point on the wrecks out here.

I've never had a single person out here tell me they dove thirds when they weren't planning a penetration. It's just excessively conservative imho. In a solo OW no deco dive, what is the scenario where not following thirds on my main tank gets me hurt, bent, or drowned?