Solo Sidemount Gas Management

[Nitrogenius]

Being able to see where and what the leak is a pretty big advantage, IMHO.

No doubt about that. But being able to preserve gas in case of a leak (manifold) etc. is also a big advantage.
See what I mean? Those arguments are all in the "mitigation"/"detection" category and as such only somewhat related to catastrophic failure risk assessment. The latter is pretty black and white and in that perspective Solo SM (or even BM indie) to thirds offers the same risk profile as a team of two hogarthian on thirds..
That was where my comment of same advantages/disadvantages was aimed at..

The assessment of all other risks in such comparison becomes much more grey and matters of taste kick in fairly early..

[Jack Hammer]

I deliberately said hogarthian BM vs SM. Independent BM offers the same "advantages and disadvantages"
If you think that a hogarthian BM team of 2 is safer from a worst case critical gas failure perspective, then I kindly suggest you to think it over..
A common misconception and widely spread misleading safety in the hogarthian team of two diving to thirds approach..
Surely in any other situation than a full loss of gas of one diver at max penetration the manifolded double offers advantages to the team vs and independent approach, but then again it really depends on the focus of your risk assessment.

I'm not for or against SM or BM. Both have advantages that have been hashed out repeatedly and don't need to be drug up again. Im talking anout Hogarthian BM. Simply put, if solo in SM and you have a loss of access to one tank (real world most likely failure is a reg, other failures possible) you have less gas than a team of 2 BM divers with a similar failure. You have the advantage of only one potentially stressed diver sucking through gas compared to possibly 2. Barring an unheard of isolator loss in BM, a BM diver will still be able to salvage a portion of their gas and still has the reserve in the 2nd divers tank (unless separated).

It's not the same risk.

- Jeff

[Nitrogenius]

It's not the same risk.

It absolutely is the same risk from a rock bottom perspective.
All the arguments you mention are valid arguments, but they all play in the "convenience" zone and not rock bottom critical failure.
Surely there is a rationale that the doubles with manifold isolator became the preferred setup over independent doubles in a team diving approach. The reason lies that it is much more versatile in case of "minor" events with a much higher likelihood to occur than the catastrophic event. In regards to catastrophic failure risk management though it is the same and after all catastrophic failure risk management is the major one to be concerned of.
The rest is not neglectable, don't get me wrong, but in that area there is many many ways to skin a cat and individual taste kicks in as well in my opinion..

Be reminded that this came up do to the "outcry" of DA as to how so many divers can deem solo SM thirds as safe cave diving practice. And from a critical failure risk management perspective to me that is fully understandable (I still do it differently as my risk appetite is lower)

As for the "convenience" failure area there are mitigation so to address your concerns as well (spare reg/tools etc.) this combined with the advantage raised by bent, to have the valves fully accessible opens other ways of mitigation than the obvious, but it does not make sense to get into that area I think as it will be very opinionated and potentially preference biased, while catastrophic failure risk assessment is as stated earlier simply black and white..

[crookshanky]

Too many variables. I start with thirds, but add gas for each restriction, or anything else that might slow me down. I never dive all the way to thirds, even in big easy passage. Remember if you loose a tank, a third of that will just barely get you to the entrance, not even enough spare gas for a safety stop.

Forrest, do you calculate your turn pressure before you descend or underwater at the cave entrance? I was always taught to calculate at the cave entrance. Using thirds from the cave entrance and having a failure at the turn would leave just enough gas for a return to the entrance and a CESA.

[FW]

Forrest, do you calculate your turn pressure before you descend or underwater at the cave entrance? I was always taught to calculate at the cave entrance. Using thirds from the cave entrance and having a failure at the turn would leave just enough gas for a return to the entrance and a CESA.

Before I descend, during the predive check. That way, you shorten your penetration by the amount of gas getting to the entrance. If you calculate at the entrance, you can get farther, and then have less gas to get out. Calculating before you descend may give you an extra couple of minutes, which may make all the difference. Then as I dive, I add to the turn pressure for things like restrictions, etc. I also don't count on the current, i.e. I never extend penetration hoping the current will get me out in an emergency. Current will actually slow you down in a siltout, light failure, etc.

[crookshanky]

Before I descend, during the predive check. That way, you shorten your penetration by the amount of gas getting to the entrance. If you calculate at the entrance, you can get farther, and then have less gas to get out. Calculating before you descend may give you an extra couple of minutes, which may make all the difference. Then as I dive, I add to the turn pressure for things like restrictions, etc. I also don't count on the current, i.e. I never extend penetration hoping the current will get me out in an emergency. Current will actually slow you down in a siltout, light failure, etc.

That makes perfect sense. Do you have a standard rule for restrictions? Add a multiple of the gas used to get through the restriction to the turn pressure, perhaps?

[FW]

That makes perfect sense. Do you have a standard rule for restrictions? Add a multiple of the gas used to get through the restriction to the turn pressure, perhaps?

I add 50 to 100 psi. If the restriction is tight enough to warrant more, I have to think hard to decide if it is worth risking my life to go through it.

[Kelly Jessop]

. Do you have a standard rule for restrictions?

I hate "standard rules" because they have failure written on it. Unfortunately restrictions can be easy from one direction and coming out, be very challenging. If there is heavy flow and the area is clearing out quickly, then a small gas buffer. Very silty with low flow ,and the viz will still be obliterated upon return, I may decide to stop the dive at that point, and test the exit of the restriction with full gas available. The other contingency is depth, with gas being depleted faster, so that is why I stay away from standard rules. I have significant buffers built into the plan, and would rather turn early and come back another day if there are too many variables when it comes to restrictions.

[inverted_bear]

2) I'm appalled by the number of cave divers in the poll who've selected "thirds" as an acceptable gas plan for a solo dive (6 at this point, with 2 voting for fourths, and no one opting for sixths). I guess having the half plus 200 option makes thirds look "conservative".

Calm down...nobody likes the scuba police....especially when they don't know the thought process of respondents to a vague poll. The poll clearly specified "a typical full cave dive at Ginnie." Everybody that picked thirds was responding to a prompt that stated an average florida cave dive...that would be a spring of moderate size and viz. Do you really think that thirds is extremely dangerous going up the gold line at ginne? You have so much flow in big cave that you can still exit on less than your second third in lights out. That would be a clear example of a situation where thirds is more than conservative enough. The poll made no mention of grinding past numerous no mount restrictions or diving into a siphon...that would clearly warrant different gas planning.

[FW]

Calm down...nobody likes the scuba police....especially when they don't know the thought process of respondents to a vague poll. The poll clearly specified "a typical full cave dive at Ginnie." Everybody that picked thirds was responding to a prompt that stated an average florida cave dive...that would be a spring of moderate size and viz. Do you really think that thirds is extremely dangerous going up the gold line at ginne? You have so much flow in big cave that you can still exit on less than your second third in lights out. That would be a clear example of a situation where thirds is more than conservative enough. The poll made no mention of grinding past numerous no mount restrictions or diving into a siphon...that would clearly warrant different gas planning.

Actually the title of the poll is "What gas management do you use for sidemount solo?". Not to mention the title of the thread is "Solo Sidemount Gas Management".

Thirds *might* be OK in Devil's Eye with a team, but not solo.