using a pony bottle to extend bottom times?

[Yoyoguy]

first off, it is no longer a pony bottle if you're intending to use it, it is a stage bottle. If you want to dive with a stage bottle, then that is perfectly fine, just understand how to use them and proper gas management.

I agree with 100ft on an al80 may not be the smartest idea, but such is life.

Why are you doing a 3 minute stop at 50ft? You are still ongassing with that profile btw, certainly not helping your offgassing and wasting about 150-200psi/4-5cf of gas for nothing. I'm not getting into a deep stop discussion, but half depth is idiotic especially when you are well within NDL's since you are still ongassing. Half pressure is fine, but half pressure of 100ft is 30ft ish, not 50.

Other than that, listen to what was written by the above posters about making your sac rate better by being more efficient. Quick mental math puts your SAC rate at 0.8 give or take, which is very high. If you can get your SAC rate down to 0.6, you will hit your NDL's on an AL80. 0.6 is still pretty high unless you're kicking around all over the place in which case it is "normal". If your computer tracks average depth you need to be tracking your sac rate from every dive. The 0.8 comes from 12 minutes at 100ft, burning approximately 1500psi which gives you 400psi for descent which is a little conservative so it very well may be 0.9cfm. Point still stands that slowing down and learning to be efficient is going to fix most of this problem for you.

I would recommend doing a modified breast stroke kick as your only kick. Does two things, prevents you from silting the place up, but more importantly it uses small muscles. When you flutter kick you use the 3 biggest muscles in your body, glutes/hamstrings/quads, with a frogkick, you use your calves, and the small muscles in your hip. Smaller muscles burn less oxygen, so combine a more efficient kick with using more efficient muscles, and your breathing rate goes down.

the whole 50ft thing was a "suggestion" for the wreck we dove on that day. it is not something I normally do. Everyone else in the group did the stop as well (only person I knew was my father, others were part of the group in boat we hired)

 

[CptTightPants21]

You also have an elevated SAC rate.

Some quick math from the info you provided

1900psi/3000 x 77.4 is equal to 49 cu ft of air consumed.

49/13 minutes = 3.78 cuft of air used per minute

3.78/4 ata =.94 cuft of air used per minute on the surface

Any non-stressed/hard working SAC rate below .5 means you are a woman or a scuba god
.5-.7 means you are an average competent diver, anything greater than .7 is elevated.

Some people are just bigger and consume more air, but a SAC rate above .7 usually indicates someone who is new/not comfortable in the water or out of shape.

 

[Steve_C]

I will admit I NEED to SLOW WAY DOWN!!! .

Problem solved. Go slower. Relax. Kick less.

I do a flutter kick most of the time. On an easy dive I am 0.4. Goes up to 0.5 if I am somewhat active. [as measured by computer so 0.4 is not really resting]

What are you doing while in trim?

Dove with an instabuddy last weekend who was probably well over 1.0. He had buoyancy and trim but his legs were moving almost all the time in a bicycle motion. Mine only move when I want to move. Then it is slow but wide kicks. (Unless close to the bottom and then I adjust to avoid silting)

 

[uncfnp]

If I remember correctly, divers that do this start on the reserve, breathe to a set level then switch to back gas?

 

[CptTightPants21]

Also take into consideration the conditions of the dive? Is it a quarry or open ocean? is there current, etc?

Using the above SAC rate, you would need 200psi for a 3 min safety stop at 15ft, another 100psi to get from 100 to 15 ft. If this was an open ocean dive I would do what you did and turn around at 1000psi. If this is a quarry, I might push it a little until 700psi and look to end the dive with around 300-400psi left in the tank. Everyone has different comfort levels, but these are other things to take into consideration.

 

[nimoh]

it takes practice to deploy and stow a stage reg without changing depth, and using a stage bottle also makes gas planning more complicated....and all to get an extra 3 minutes of bottom time?

I think your best course of action is to dive shallower, enjoy the extended bottom time and save the deeper diving for when you SAC rate is better...and it will get better.

If I remember correctly, divers that do this start on the reserve, breathe to a set level then switch to back gas?

yes, the idea is to keep your reserve in your back gas which is accessible by two second stages in case you get into a gas sharing situation with a buddy. Also, in an emergency, empty stage bottles can be dropped to improve streamlining.

In cave diving, divers will commonly drop the stage on the line after breathing half of it, and then pick it up and breathe it on the way out. However, they still have reserve in their back gas.

 

[uncfnp]

....and all to get an extra 3 minutes of bottom time?

And that brings up the suggestion for getting Nitrox certified if he is going to do these dives.

 

[Wookie]

Some operators (and I am one of them) consider use of a pony (Pony - emergency redundant air supply) to constitute an out of air emergency and act accordingly. If the out of air emergency is a result of some equipment failure, no harm. If it is because you haven't planned an adequate gas supply for the dive you are doing (you ran out of air), well, we get to sit down and discuss your failure to properly plan a dive.

Planning to rely on an emergency backup like a pony is poor planning IMHO. It's like carrying an SMB because you plan to get lost because you never learned to navigate. I am a great believer in carrying a pony. I am not too keen on using one.

 

[uncfnp]

So Wookie. If a diver on your boat decided to sling a 40, start the dive by breathing it down to a set level, switch to back gas and finish the dive, all to a predetermined dive plan, you would have an issue with this?

 

[luscioman]

Most divers will go with the larger tank like a 100 or 120. However if your dive op doesn't have an issue with it you should be fine with brining the extra 40. Do you have an extra Reg for it? If not, factor in that cost compared to buying a larger tank. I would also suggest getting the book breathology by Stig which is a world record holder.


https://www.breatheology.com/