Crazy question from the family table

[krulle]

Hi

Yesterday at the family table, the conversation turned to scuba diving, since the boyfriend of my wifes cousin occasionally dives as well.

Soon the craziest questions popped up from non divers. And on one question, I had absolutely no clue what to answer. But I’m curious for the answer myself!

The question: “How long does it take to go up after being down for three hours at -60 meters?”

Let’s translate the question: “If one dives a rebreather 15 min down to -200ft in the sea, and stays there for 2h45mins, how should a safe ascent look like?”

Does anyone know how to calculate that? Probably it depends on the type of rebreather, but let’s assume a common CCR.

I’ve no intentions to try such dive myself, as it would be way beyond my diving capabilities; but the question made me curious.


Thanks in advance!

Kristof

 

[lv2dive]

It doesn’t depend on the type of rebreather, it depends on
a) the partial pressure of oxygen the diver is breathing and
b) the chosen decompression algorithm

You will get widely varying responses based on those two variables.

 

[krulle]

OK, guess that both choices depend on balancing advantages and drawbacks.

What would you chose?


KR

Kristof

 

[lv2dive]

***do not try this without proper deco training!!!****

One example, running 1.0 and 10/65 at depth and 1.3 and 50% at 70 feet and up (probably a little rich but I’m just throwing it together), with GF 50/85, 10/65 dip would be 491 minutes, including 370 minutes of deco. Yep, that’s over 6 hours of deco.

 

[lv2dive]

Everything else the same, but change the decompression algorithm by bumping up your GF to 95 across the board, and deco time goes down to 305 minutes (total run time 426 minutes). NOT RECOMMENDED- included for comparison purposes only.

Again, pls get depth appropriate decompression training before attempting ANY decompression dives

 

[krulle]

sorry for the many questions. But how should I read above info?

number1: number2 number3' Code

number1: not a clue
number2: number of minutes of ...
number3: number of seconds of ...
Code: gas mixture?

 

[lv2dive]

I wouldn’t do a two hour bottom time on the ocean at 200’ without a chamber on the boat - too much could change on the surface in that much time.

I have done 4+ hours deco in a cave... running 1.0 on the bottom and 1.3 for deco. It really sucks to move from your 20 foot stop to your ten foot stop, and still have a triple digit deco obligation (100+ minutes)! And that is short compared to some of the exploration dives going on.

 

[lv2dive]

I deleted all of that - it was the deco schedule and I realized it was better not to publish it.

sorry for the many questions. But how should I read above info?

number1: number2 number3' Code

number1: not a clue
number2: number of minutes of ...
number3: number of seconds of ...
Code: gas mixture?

 

[krulle]

so bottomline, it would be at least 5-6 hours back up, so total of almost 9 hours under water?

 

[lv2dive]

The first option I posted, with more than six hours of deco alone, is the more “conservative” deco schedule than the second option, with about five hours of deco. There is only one instructor I am aware of who is a proponent of such high gradient factors (95/95) and I’m not sure he is teaching right now.

I used those two examples to show you that changing the decompression algorithm can significantly change the decompression obligation by an hour. I could have also changed the partial pressure, lowering the amount of oxygen in the gas being breathed, and that would have had a similar result in terms of adding time to the decompression obligation.