First dive at 40 meters - Newbies recreational

[David Novo]

If you have an adequate amount of gas, you do not need to ascend at 18m/min in an emergency.

For a dive to 40m and following PADI's ascent profile, I would calculate my gas reserve as follows:

Average pressure = (5+1) / 2 = 3 bar
Ascent time = 40m / 9m/min + 3 min (safety stop) + 1 min to solve issues at bottom = round to 8 min
Combined SAC = 30 l/min x 2 = 60 l/min
Reserve gas = 3 bar x 60 l/min x 8 min = 1440 litres = 120 bar (assuming a 12L steel tank - aluminium "12L" tanks just have 11.1L)
Usable gas (assuming 200 bar in the beginning) = 200 bar - 120 bar = 80 bar

This assumes direct ascent from the bottom.
If you still need to return to where you started (desirable but not mandatory as in an overhead environment), your usable gas is 40 bar each way. This means that you should turn back and start ascending at 160 bar.

P.S.: Were you taught to plan gas as shown in the spreadsheet? A 50 bar reserve is not appropriate for dives to any depth.

Return at 230 bars. Start ascent at 160 bars.

How did you reach these numbers?

I am sure that you will say that 160 bars is not enough to safely ascend with a buddy having trouble.
That would mean that it is not safe to do a 40 meters dive with a 12 l tank (let alone a 80 cu ft) at 200 bars and that you definitely need a second tank.

I think 160 bar would be enough to ascend, even 120 bar as per my post above.

Would I dive to 40m in a single 12L tank with 200 bar on air? No. What for? I just have 40 bar each way, low NDL, narced and without redundancies (except my buddy).

P.S.: Numbers above for a 200 bar tank.

 

[Storker]

at what pressure would you start to ascend?

With my 10L 300 bar and 16-18 L/min: 100-120 bar remaining at 30m. Closer to 120 than 100. At least 150 bar remaining at 40m, preferably more because I'll be narked and less capable to deal efficiently with an abnormal situation.

 

[Dody]

How did you reach these numbers?



I think 160 bar would be enough to ascend, even 120 bar as per my post above.

Would I dive to 40m in a single 12L tank with 200 bar on air? No. What for? I just have 40 bar each way, low NDL, narced and without redundancies (except my buddy).

I did a very quick calculation. 300 bars with the air compression factor gives me 260 bars of usable air. I took 50 bars out, it gives me 210 bars. 70 to go, 70 to return, 70 to surface. 300 - 70 = 230. 230 -70 = 160.

 

[David Novo]

I did a very calculation. 300 bars with the air compression factor gives me 260 bars of usable air. I took 50 bars out, it gives me 210 bars. 70 to go, 70 to return, 70 to surface. 300 - 70 = 230. 230 -70 = 160.

How did you calculate the 70 to surface? A third?

 

[Dody]

How did you calculate the 70 to surface? A third?

Yes.

 

[David Novo]

Yes.

So basically you get the same result, whether your dive is to 5m or to 100m. Does this make sense to you?

 

[Dody]

Now, I would have double checked with the SAC and ascent time to prove it is realistic.

 

[Storker]

300 bars with the air compression factor gives me 260 bars of usable air.

270. Ish.

Except that talking about pressures when you consider compressibility factors is bad practice. A 10L 300 bar tank at rated pressure is at 300 bar, but the amount of gas is around 2700 surface liters. Incidentally, about the same amount of gas as a 12L 232 bar and about 10% less than a 15L 200 bar.

And the compression factor only plays a role in the top 80-ish bar from 300. When you're down to 200-230 bar, the compression factor is basically 1. So it only plays a role when you calculate tank capacity and total run time, not for min gas pressures.

 

[Storker]

basically you get the same result, whether your dive is to 5m or to 100m. Does this make sense to you?

It sure doesn't to me. While I might be rather conservative with my ascent pressure at depth, I have no issues breathing my tank down to 30-ish bar if I'm mucking around in the shallows (5m or shallower) just to get the most out of my tank. At least as long as I'm not exceeding the max run time given to the dive leader.

 

[David Novo]

Now, I would have double checked with the SAC and ascent time to prove it is realistic.

Why do you even calculate it diving by a third?