First dive at 40 meters - Newbies recreational

[Dody]

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

My calculation has been much more conservative with 300 bars. First, because of the compressibility factor (almost 20%) that I don’t use at 200 bars. Then I rounded up. And that’s right, the result is pretty close to 200.

 

[Edward3c]

Providing that my equipment (regulator, cylinder) allow 300 bars (I currently would not exceed 230 because of the air compressibility factor), I would say that I want to surface with at least 50 bars so my safe dive plan should only account for a 250 bars air consumption, 3000 liters. Then I divide by the compressibility factor which is greater than 1,1 at 300 bars. A fair estimate would give 2500 liters.

The point I wanted you to discover was that compressed air above, say, 260 is no longer linier with volume. Well done.

Its was quite unnerving when I started with 300bar cylinders and seeing the pressure drop to around 250bar in not time.

It wasn’t my intention to start a 300bar planning run. However, @David Novo is right, to use an arbitrary formular for all dives is misleading. Each dive should be planned on its own merit.

 

[David Novo]

My calculation has been much more conservative with 300 bars. First, because of the compressibility factor (almost 20%) that I don’t use at 200 bars. Then I rounded up. And that’s right, the result is pretty close to 200.

Would you also divide it by a third with 200 bars? Why?
My point is related to how you calculated, not the result.
If the goal is to be as conservative as possible, for no reason, you can sit on the boat.

 

[Dody]

Why do you even calculate it diving by a third?

Training bias ? This calculation is quick and easy and gives me big picture. Then I do a more precise calculation/ simulation with a real diving plan.

 

[Storker]

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

Yes.

I really dislike the rule of thirds. For open water diving, it's seldom a good rule. It's either too conservative (in the shallows) or too liberal (at depth).

Do a proper min gas calculation and compare the output to the rule of thirds. The former depend heavily on depth, the latter not at all. Which makes no sense, since depth is one of the only two factors determining what your reserve should be.

 

[David Novo]

I really dislike the rule of thirds. For open water diving, it's seldom a good rule. It's either too conservative (in the shallows) or too liberal (at depth).

Do a proper min gas calculation and compare the output to the rule of thirds. The former depend heavily on depth, the latter not at all. Which makes no sense, since depth is one of the only two factors determining what your reserve should be.

It depends on what you do with it.
I would not use the rule of thirds, halves, etc to calculate a reserve but for turning pressure it is useful depending on the context.

 

[Dody]

Would you also divide it by a third with 200 bars? Why?
My point is related to how you calculated, not the result.
If the goal is to be as conservative as possible, for no reason, you can sit on the boat.

Yes. That’s what I do by training and I now better understand the limitations of this methodology.

 

[Storker]

It depends on what you do with it.
I would not use the rule of thirds, halves, etc to calculate a reserve but for turning pressure it is useful depending on the context.

Gotcha.

My dives are generally triangular profile, almost never following the same route back as the one I follow out. Only exception is wall dives, but then I go back at a shallower depth, so there's still new things to see.

I basically drop fairly fast to my max depth, stay there no longer than to my ascent pressure and then start my slo-o-ow ascent tracking back to the boat or the entry site. In which case "turn pressure" becomes rather meaningless. Or, in the case of a wall dive, "turn pressure" is equal to ascent pressure, because when I start my slow ascent it's time to turn around.

 

[Dody]

The point I wanted you to discover was that compressed air above, say, 260 is no longer linier with volume. Well done.

Its was quite unnerving when I started with 300bar cylinders and seeing the pressure drop to around 250bar in not time.

It wasn’t my intention to start a 300bar planning run. However, @David Novo is right, to use an arbitrary formular for all dives is misleading. Each dive should be planned on its own merit.

See . I am making progress

 

[Dody]

One month ago, I asked my instructor why people was making so much fuss about being separated from a group at sea if you don’t have any other problem. Just ascend and you will be safe at the surface, I said. Then I read all those stories about people not, never recovered and those who survived several days. I never imagined that it was so easy to get lost and so difficult to find a drifting diver at the surface. I completely overlooked thirst, cold, exhaustion effects. Still have a lot to learn but that’s fascinating .