First dive at 40 meters - Newbies recreational

[David Novo]

Connected doubles is better than a pony in terms of gas, even if the pony and back gas tank have the same amount.

If you have a freeflow, or a burst hose, etc, with single tank, you are left with the gas from the pony.

If it happend with connected doubles, you would close the valve of the tank with the problem and still have the gas from both tanks.

 

[David Novo]

OOA at 40 m plus down current, this is Murphy’s law. If both buddies have OOA and down current, it is a message that someone up there wants to meet early . But I get the point.

So will you abort the dive everytime you feel a downcurrent because you would not be able to face an emergency in that situation?

Probably one OOA would spell trouble already, without an adequate gas reserve.

Just plan your gas conservatively so there is room for surprises.

 

[KenGordon]

Good one. I did not think about a free flow. Would such a problem arise, I think that the best solution would be buddy breathing and ascending to 5 meters for the tank at the safety stop or worse (straight to the surface). With 150 bars for two, a controlled ascent could be possible, right?

There are a couple of unknowns in a plan for an ascent from 40m. The ascent rate, including delay at depth, and the rate of gas use.

Divers will often assume that they ascend at 10m/minute for planning but completely fail to Do so in the water. Achieving such a rate takes practice as divers have generally been indoctrinated to come up too slowly. Your actual gas consumption rate is another variable. Given low air consumption is an obsessive subject people like to lie to themselves about what they really use, then when stuff gets complicated and they stop paying attention to breathing it goes up, add actual stress and it goes up even more. Practice and experience help. If you can look back on how much gas you used while doing some tricky thing like putting up a DSMB or shutting down a twinset then you can gauge what elevated gas consumption to use. Given a proper plan you know you can safely get to the surface and so be more relaxed in the execution and so indeed more likely to succeed.

Assuming a not too panicky combined 50l/minute and a vaguely realistic 6m/minute ascent plus a couple of minutes to sort stuff out at 40

250l/minute for 2 minutes = 500l
Average depth of 20m for ascent => 3 bar for 40/6 = 6.6minutes so 6.6 * 3 * 50 = 1000l

so 1500l out of your 11 or 136 bar.

if we are generous with your gas consumption at 15l/minute and your descent takes 3 minutes that is 15*3*3 = 135l or 12 bar, so you have 210-12-136 = 62 bar or about 680l on the bottom, at 5 bar and 15l/minute you are using 75l/minute and so have about 9 minutes gas at 40m.

You can try these calculations with different assumptions and see that the biggest unknown (actual gas consumption on the air sharing ascent) has the biggest impact of the safety of the whole exercise. You might see that taking a bigger cylinder makes everything much less marginal.

However, despite the best planning what will kill the OOA diver is being too far from their buddy and bolting to the surface.

 

[Storker]

With 150 bars for two, a controlled ascent could be possible, right?

You should be able to answer that question yourself.

Didn't your deep dive class cover calculation of proper reserves? If it didn't, I strongly recommend that you learn it now.

 

[Dody]

You should be able to answer that question yourself.

Didn't your deep dive class cover calculation of proper reserves? If it didn't, I strongly recommend that you learn it now.

It was a rhetorical question.

 

[David Novo]

It was a rhetorical question.

Based on this thread, it did not read that way. The answer depends a lot on your combined (yours and your buddy's) SAC during an emergency and the time you take to ascend.

 

[Dody]

Based on this thread, it did not read that way. The answer depends a lot on your combined (yours and your buddy's) SAC during an emergency and the time you take to ascend.

Yes, it did. I made the calculation a dozen of times with different assumptions. Anyway, in an ascent with my buddy's octopus and a potential air shortage coming, we would rather use the old rule 18m/ min than 9. This dive was the closest that we have had to an emergency so we don't really have relevant emergency SAC values. 50 seems a bit high though.

 

[David Novo]

Yes, it did. I made the calculation a dozen of times with different assumptions. Anyway, in an ascent with my buddy's octopus and a potential air shortage coming, we would rather use the old rule 18m/ min than 9. This dive was the closest that we have had to an emergency so we don't really have relevant emergency SAC values. 50 seems a bit high though.

If your gas planning requires ascending at 18m/min, you should revise it.
50 l/min for both divers is not a lot. I would use 60l/min (30 each), so @KenGordon was not too conservative.

 

[Raphus]

18m/min is no joke from 40m. Fast tissues might not like that.
You are still in NDL, but the calculation is done based on a slower ascent rate.
And its really hard to hold that ascent rate.

I think 50l/min for 2 divers in an emergency is not high at all.

 

[Dody]

If your gas planning requires ascending at 18m/min, you should revise it.
50 l/min for both divers is not a lot. I would use 60l/min (30 each), so @KenGordon was not too conservative.

18 m/ min is not no gas planning. This is a freaking emergency.
I meant 50l/ min per person as I divided 150 bars by 2.
This spreadsheet helps.