What kind of profiles are requiring a 80cf of o2? And..

[211ratsbud]

Why did you reserve 2/3? Why that number and not 1/2? Or 3/4? Or 1/3? Or some other number?

In class we often practiced 3/4 reserve. (More was encouraged) . Contingency for buddy lost gas and +t/d. That must've been purely situational as we were making our first deco dives and planning the extra o2 was a buffer.

Planning my own dive I was referring to 2/3 being less conservative than I was taught but something I was ok with.

I'm going to consider dive profiles which allow me to carry out 1.5-2x gas in the 40. (More like 2x)

When you get instruction you dont get a speech on how the other half does it. You just get a speech on why we do it this way.

 

[stuartv]

But if you’re sharing the deco gas... why do you need to double the stop time? If you aren’t doubling the stop time, why reserve double the gas?

You’re still offgassing when on backgas, just not as fast as with oxygen.

Yes, if my buddy loses his deco gas and shares mine, he should not need as much as me, since he’ll off-gas some on back gas while I’m breathing deco gas. That just means my planning is simpler and has extra safety margin.

 

[michael-fisch]

Presuming we’re talking about a dive where I’m only carrying 1 deco gas...

If my plan requires up to 20 cu-ft, I’ll carry an AL40. If it requires more, I’ll either shorten my planned bottom time, or carry an 80.

Half my bottle is for me. Half for my buddy, in case he loses his deco gas.

So, half the bottle is contingency gas. Backup contingency plan is remaining back gas.

If **** happens where my buddy and I use ALL our back gas just to get to the gas switch, and one of us has lost our deco gas, and we suck down the deco gas quicker than planned...well then, I guess we are screwed (meaning surfacing with omitted deco).

You haven't learned O2 Deco correctly. If one diver of a 2 man buddy team loses his O2, he breathes his next deeper gas for 2/3 of the O2 stop while his buddy with O2 breathes O2 for 2/3 of the stop. The O2 moves to the other diver who now breaths O2 for 2/3 of the O2 stop while his buddy breaths the next deeper gas.

End result of not having your own O2 on a 30 minute O2 Deco is each diver breaths O2 for 20 minutes and the next deeper gas for 20 minutes. Gas wise your reserve next deeper gas has to cover 20 additional minutes at 6M and the working O2 bottle has to cover 40 minutes instead of 30 minutes.
40 minutes at 0.5 cuft a minute (deco, your'e not doing anything but breathing out) works out to be 20cuft. Assuming that you started the dive with your AL40 half full of O2, even if the sh*t hits the fan you will come out with 750 psi in the tank.

Michael

 

[rjack321]

In class we often practiced 3/4 reserve. (More was encouraged) . Contingency for buddy lost gas and +t/d. That must've been purely situational as we were making our first deco dives and planning the extra o2 was a buffer.

Planning my own dive I was referring to 2/3 being less conservative than I was taught but something I was ok with.

I'm going to consider dive profiles which allow me to carry out 1.5-2x gas in the 40. (More like 2x)

When you get instruction you dont get a speech on how the other half does it. You just get a speech on why we do it this way.

Can you post the depth and time you are planning for? I am having a hard time understanding what you are trying to deco out from.

5mins over is no big deal at 140ft (for instance) and does not justify bringing an 80 of O2. As a single deco gas dive O2 would not be my choice anyway. The only dives I would bring 80cf of O2 on are long shallow cave dives where I don't have 50% because the max depth was 110ft and the average was 90ft (for example). Those are doubles+ a stage cave dives and the O2 is dropped. If the dive is deeper and/or in the ocean I have 50%.

For me on OC I first upsize the 50% from a 40 to an 80.

I have never needed 80cf of O2 in the ocean, even 240ft dives don't require that (20 mins at 240 on 15/55 is still only 34 mins on O2 with GF 50/70). You have backgas, and deeper deco gases. 80cf of O2 on top of the 80cf of 50% is a brick and going to cause you more problems than it solves.

 

[stuartv]

You haven't learned O2 Deco correctly. If one diver of a 2 man buddy team loses his O2, he breathes his next deeper gas for 2/3 of the O2 stop while his buddy with O2 breathes O2 for 2/3 of the stop. The O2 moves to the other diver who now breaths O2 for 2/3 of the O2 stop while his buddy breaths the next deeper gas.

End result of not having your own O2 on a 30 minute O2 Deco is each diver breaths O2 for 20 minutes and the next deeper gas for 20 minutes. Gas wise your reserve next deeper gas has to cover 20 additional minutes at 6M and the working O2 bottle has to cover 40 minutes instead of 30 minutes.
40 minutes at 0.5 cuft a minute (deco, your'e not doing anything but breathing out) works out to be 20cuft. Assuming that you started the dive with your AL40 half full of O2, even if the sh*t hits the fan you will come out with 750 psi in the tank.

Michael

I carry too much O2. Noted.

Thank you for your input.