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

[wrybosome]

1/2 + 200!!!! :d

What's the 200 for?

 

[Jax]

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?

Good thing that is each person's personal choice to make.

That said, I won't lie and say I've never puttered around 15' and played with the fish and such after my safety stop. That last 1000psi is 29cf for me!

 

[redacted]

Good thing that is each person's personal choice to make.

That said, I won't lie and say I've never puttered around 15' and played with the fish and such after my safety stop. That last 1000psi is 29cf for me!

Yes, it is to each their own.

For me, many of these dives are costing me about $1 per minute of dive time. So giving up good dive time has a direct cost that I don't chose to give up unless I see some payoff. To make it worse, that $1 per minute includes ascent, descent, and SS and the dive time that has to be unnecessarily sacrificed to the rule of 1/3rds is the cream of the dive, bottom time at the target.

 

[Blackwood]

1/4 and 1/4 leaving 1/2 researve doesn't make any sense. Rule of thirds does.

I agree with you that 1/4, 1/4, 1/2 reserve doesn't make sense. But I'm having a hard time seeing why 1/3 out, 1/3 up, 1/3 reserve does make sense.

What are the assumptions that lead one to 1/3 up?

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

It's something that got a lot of cave divers killed back in the day.

Same path in as out, therefore same gas out as in. Turn 200PSI before half your gas for contingency (e.g. on 3000PSI tanks, turn pressure using 1/2+2 would be 1700), and you should get out. Turns out it often wasn't enough.

And as it turns out, its successor (1/3 in 1/3 out 1/3 for emergencies) is mega aggressive given buddy diving (1/3 in, sharing gas at max penetration = 2/3 out. No contingency. Better be as fast getting out as you did getting in).

 

[Blackwood]

The only reason I can see diving thirds for OW solo diving (something I have done) is simply being cautious: 1/3 out, 1/3 back, 1/3 for a f***up (entanglement, lost, etc). And I had an AL40 for a pony, which was a little less than half my backgas (was diving single LP95s at the time.)

As I understand it, Cruisin Home and Jax are talking about starting the ascent with 2000 PSI remaining (1000 out, 1000 up, 1000 reserve).

Maybe I misunderstood.

 

[Jax]

As I understand it, Cruisin Home and Jax are talking about starting the ascent with 2000 PSI remaining (1000 out, 1000 up, 1000 reserve).

Maybe I misunderstood.

Bottom line is that you plan your dive with 2/3s of your gas. If you have 3000psi, you return to the boat with 1000psi. If you look at regular buddy diving, you return with 500psi, so solo diving "wastes" only 500 extra.

The "out and back" is just a 'verbal shorthand'. If we are anchored in shallow (20fsw) water, I have no compunction of puttering around the anchor to use up that last 500. But that's me.

 

[ScubaSteve]

Then it is a gas plan and not rule of thirds. Technicality or not, it mkaes a huge difference.

 

[Blackwood]

Bottom line is that you plan your dive with 2/3s of your gas. If you have 3000psi, you return to the boat with 1000psi. If you look at regular buddy diving, you return with 500psi, so solo diving "wastes" only 500 extra.

The "out and back" is just a 'verbal shorthand'. If we are anchored in shallow (20fsw) water, I have no compunction of puttering around the anchor to use up that last 500. But that's me.

Got it. So if it only takes 500 to get up, you'd start your ascent at 1500?

I was thinking you meant you start your ascent after consuming 1/3 (which in reality means you're reserving significantly more than 1/3 of your gas for emergencies).

It's still a little arbitrary for my tastes, but that's fine.

 

[Jax]

Got it. So if it only takes 500 to get up, you'd start your ascent at 1500?

I was thinking you meant you start your ascent after consuming 1/3 (which in reality means you're reserving significantly more than 1/3 of your gas for emergencies).

It's still a little arbitrary for my tastes, but that's fine.

Correct. It take me about 6cf to come up and safety stop from 60', so I always figure on 250psi. 2000psi on my 100cf tank lets me have the same down time as people sucking up 2500psi from an AL80. Yes, it is ultra conservative, but when you need the emergency gas, you need it!

 

[Jax]

Then it is a gas plan and not rule of thirds. Technicality or not, it mkaes a huge difference.

:huh:

Could someone help me out with this statement?

While I've been given to understand the rule of thirds originated in cave diving, I've heard 'the rule' used in way too many other situations . . .