Extending bottom time with stage question

[OJSimmonds]

Hello all,

I have been doing some thinking and want to ask people’s opinions of it.

Let me set the scene.

Diving a 30-35m Wreck (e.g the M2 – 35m seabed)
Using Nitrox 32, so about 30mins no-stop time at 30m.

So if diving with a single 15 (and assuming 25l/min SAC) would give absolute max of 20mins at 30m
30m – 4bar abs, 4x 25l/min SAC = 100l/min – keeps the number simple
Actually less to account for ascent and descent but I am only doing quick sums to make a point.

This is obviously less than the No-stop limit.

If the person were to take a 7l stage also with 32% in it (an extra approx 1400litres or 14mins at 30m)

They could drain the 7l first, then switch to the 15l and still keeping it a no-stop dive easily get the full 30min no-stop time, without any deco obligations (apart from a 3-min safety stop). They would actually have more gas reserve in the 15l than conventional thirds as still avoiding deco

Does this sound correct? And make sense?

Potential issues:
Complete Failure of 15l for some reason – could breath off buddies tank (in the case I am thinking of buddy has a 12l twinset, so plenty of gas) and ascend with no deco
Issue with 7l - just switch to 15l and ascend again no deco
Any others I am missing??
(Note person is more than happy carrying a 7l Ali stage)

I would appreciate any helpful and constructive comments about this plan.

Many thanks

Olly

[Divingminxsy]

I would appreciate any helpful and constructive comments about this plan.

Time to get a twinset

[Scuttler]

If you're going to plan it then you need to decide if your max depth is 30 or 35 (you can't have your cake and eat it)! always plan for the worst

From V-Planner (+2 Conservatism):

35m 20 minutes @ 25l/min = 2696l (you'll have a deco obligation of 3'@6m)

230x15= 3450l so enough gas

If you end up +3' then the plan is still ok for you on your own at 3159l but you now have a 7' stop which means it's tight for your buddy if he has a catastrophic gas failure!!! Adding the 7l might be kind for him.

[OJSimmonds]

If you're going to plan it then you need to decide if your max depth is 30 or 35 (you can't have your cake and eat it)! always plan for the worst

From V-Planner (+2 Conservatism):

35m 20 minutes @ 25l/min = 2696l (you'll have a deco obligation of 3'@6m)

230x15= 3450l so enough gas

If you end up +3' then the plan is still ok for you on your own at 3159l but you now have a 7' stop which means it's tight for your buddy if he has a catastrophic gas failure!!! Adding the 7l might be kind for him.

Thanks for that. It seems it might be a possible plan then.
Something to think about and plan out properly - I must emphasis most of those numbers were off the top of my head and I would of course plan it properly.

Olly

[dynarob]

A couple of times I have tried an ali7 side slung (for accelerated deco in my case) with a 15L single and personally I felt really uncomfortable and lopsided - spent the whole dive trying to correct it.

[OJSimmonds]

A couple of times I have tried an ali7 side slung (for accelerated deco in my case) with a 15L single and personally I felt really uncomfortable and lopsided - spent the whole dive trying to correct it.

I have found the use of trim weights can correct this a bit, but thanks for your comment.

Olly

[Scuttler]

Thanks for that. It seems it might be a possible plan then.
Something to think about and plan out properly - I must emphasis most of those numbers were off the top of my head and I would of course plan it properly.

Olly

No problem. To be honest the M2 on a single 15l with a 7l stage isn't an unreasonable undertaking because you will be able to see most of the wreck in the time you're thinking and you'll spend most of it in the 30m region. Like you say you need to just make sure the sums stack up. I'd also allow for a short fill and assume you've only got 220 bar in the cylinders.

I've assumed you were using tables which can be on the aggressive side. Certainly if I switch to a zero conservatism on V-Planner then you have a 1 minute obligation with 20'@35m.

[pieater]

I have found the use of trim weights can correct this a bit, but thanks for your comment.

Olly

Sounds like you might be compensating for the weight of the cylinder at the start of the dive by putting lead on the other side.

By the time you've emptied a very floaty 7l you could have made the problem a lot worse - unless you swap the side the cylinder is carried on.

If you don't want to do that, just mount the thing diagonally across the chest.

[Mark Chase]

Theres nothing wrong with your train of thaught. Id empty the 7ltr first then move on to back gas.


Getting down the shot costs the most gas on any dive so the 7's weight will help you get down and breathing 100bar off it will help you carry it more easily. Id use a steel so t didnt get too boyent if you drain it to 20bar.

Then switch to back gas.


If your going to do it the otehr way arround it would make more sense to have 32% in the back gas and say 40% in the 7ltr stage.


Id empty back gas to 20bar then switch to the 40% at 30m for the ascent.


On your SAC, 29mins on the bottom 1 @ 9 and 6 @ 6 for the ascent.

This would only be safe on a dive where an imediate ascent was posable as all the reserve gas is in the 7ltr

Many will say that havig no back gas left is unsafe but thats their beleife not my own. If my 7ltr is leaking ill know.

ATB

Mark

[pieater]

Some big differences there between the max-min depth and the max-min time.

Using the minimum depth and the minimum time we have:
4 ata x 20 mins x 25lpm you'll need 2000l (plus some for a minimal stop)

Using max depth and max time we have :
4.5ata x 30mins x 25lpm and you'll need 3375l (plus more for stops)

Just goes to show how 5m and 10mins can make a biiiiiiiiiiiiiiiiiig difference.