Couple questions on a pony bottle for bail out

[mrdre]

A 24 minute dive at 130 feet is not a no-deco recreational dive. In fact it requires almost an hour of decompression. And it would still require only 2 minutes to ascend to 20 feet if he had to blow off his deco stops from 70 to 20 feet.

I guess I just don't get your point.

That's what I got. We can try something more conservative - 120 feet. My point was that a diver with 120 cft single tank staying within recreational depth may hit deco requiring 40 cft to bail out. I'm just trying to think when 40 cft pony would be useful. I understand it's not "no-deco recreational dive", but so what?

 

[Greg_Vic_Diver]

My bottom SAC is about 1.0. Deco SAC is about 0.5 and working SAC is about 2.0 for very short bursts. Double those since average depth in the example is 33 feet. I'm not sure what "time to deal with issue at depth" means since you would just deploy the pony second stage and immediately start to ascend. Ascent to first stop is also the only stop. There is no second stop when we are talking about no deco recreational diving.

Or are you guys confusing a pony bottle with a deco cylinder?

You said a AL40 would allow you to safely (as a bailout) get to the surface after a 1.2 pp02 dive to 30m for 50 mins. That's equivalent to an OC 30% nitrox dive. After seeing the schedule I see why....

Let's back up a bit and say it was a recreational dive, one to 30m (100ft) for 30 mins using 32%. What amount of gas say cu ft (pony bottle) would you need if you had air (normoxic nitrox) in your pony, wanted to do your safety stops, required 1 min at depth to deal with your issue, signal your buddy and ascend using a SAFE ascent rate?

 

[kwinter]

That's what I got. We can try something more conservative - 120 feet. My point was that a diver with 120 cft single tank staying within recreational depth may hit deco requiring 40 cft to bail out. I'm just trying to think when 40 cft pony would be useful. I understand it's not "no-deco recreational dive", but so what?

We are talking not only about recreational depths, but also no-decompression diving. So it doesn't matter what size primary tank the guy has as long as he is diving within NDL. There is just no dive I can come up with that would come close to needing 40 cf of air for a bailout scenario.

You said a AL40 would allow you to safely (as a bailout) get to the surface after a 1.2 pp02 dive to 30m for 50 mins. That's equivalent to an OC 30% nitrox dive. After seeing the schedule I see why....

Let's back up a bit and say it was a recreational dive, one to 30m (100ft) for 30 mins using 32%. What amount of gas say cu ft (pony bottle) would you need if you had air (normoxic nitrox) in your pony, wanted to do your safety stops, required 1 min at depth to deal with your issue, signal your buddy and ascend using a SAFE ascent rate?

That's still outside the parameters since (with my configuration) a 30m dive for 30 min on 32% would require 9 minutes of deco even staying on the nitrox. With a switch to air after the 30 minute BT, the deco time goes up to 15 minutes yet the ascent still only requires 18 cf of air. BTW, my configuration is with a 30 fpm ascent rate. Gas use would be even less at 60 fpm.



MultiDeco 4.04 by Ross Hemingway,
VPM code by Erik C. Baker.


Decompression model: VPM - B/E


DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0m
Conservatism = + 4


Dec to 30m (1) Nitrox 32 18m/min descent.
Level 30m 28:20 (30) Nitrox 32 1.27 ppO2, 24m ead
Asc to 9m (32) Air -9m/min ascent.
Stop at 9m 0:40 (33) Air 0.40 ppO2, 9m ead
Stop at 6m 4:00 (37) Air 0.34 ppO2, 6m ead
Stop at 3m 10:00 (47) Air 0.27 ppO2, 3m ead
Surface (47) Air -9m/min ascent.


Off gassing starts at 17.8m


OTU's this dive: 43
CNS Total: 15.9%


17.6 cu ft Air
81.8 cu ft Nitrox 32
99.4 cu ft TOTAL




DIVE PLAN COMPLETE

 

[Croomrider]

kwinter,

What conservatism are you using in your calculations. According to the NOAA 32% Nitrox tables 30 minutes to 100 feet is listed as the NDL. Your calculations show that time as needing deco. I am not saying that I wouldn't rather play it safer if planning the dive, but we are talking about an emergency and most likely a good safety stop. However, I agree that 40cf is not needed.

Mike

 

[kwinter]

kwinter,

What conservatism are you using in your calculations. According to the NOAA 32% Nitrox tables 30 minutes to 100 feet is listed as the NDL. Your calculations show that time as needing deco. I am not saying that I wouldn't rather play it safer if planning the dive, but we are talking about an emergency and most likely a good safety stop. However, I agree that 40cf is not needed.

Mike

Like it shows, that was run using VPM-B/E with conservativism of +4. Similar results using Buhlman B with GF of 30/80, giving 7 minutes of deco instead of 9 with VPM. Interesting that air bailout using 30/80 only adds 1 minute of deco compared to an additional 6 minutes using VPM. According to the nearest table I could grab, NDL on 32% is 30 minutes at 98 feet, not 100. At 109 feet it is only 25 minutes. So a little deco at 30 minutes, 100 ft, 32% seems right. Whether it is 1 minute or 10 doesn't matter. We seem to agree that there is no NDL dive that could require 40 cf of air for ascent.

 

[jadairiii]

My bottom SAC is about 1.0. Deco SAC is about 0.5 and working SAC is about 2.0 for very short bursts. ...

That explains a lot. You may want to consider a gym membership.....or golf.

 

[kwinter]

That explains a lot. You may want to consider a gym membership.....or golf.

Maybe you should understand. My actual bottom and deco rates are significantly lower. But for conservative planning purposes to build in reserve, I can use those when doing the computations manually. And the point was that 40 cf was way too much. Using a lower SAC rate would reduce the necessary gas even further.

And there's nothing wrong with golf. My shoulders just don't allow it anymore.

 

[mrdre]

We are talking not only about recreational depths, but also no-decompression diving. So it doesn't matter what size primary tank the guy has as long as he is diving within NDL. There is just no dive I can come up with that would come close to needing 40 cf of air for a bailout scenario.

True, there seem to be no NDL dive which would require 40cf. 19 cf is more than enough. But (rereading Mick's post...) he didn't mention NDL dives, he didn't mention recreational dives either. He just mentioned 1 tank dive and 40 cft for "deeper" dive. He could mean 150-180 feet dives. I assumed that he was going over to deco. If Mick would shed some light....

 

[jadairiii]

Maybe you should understand. ..... And the point was that 40 cf was way too much. Using a lower SAC rate would reduce the necessary gas even further. ...

Your numbers, not mine. But a 40 is not over kill if you have a working SAC of 2. At 100', with that SAC rate you will burn more that 1/2 of that 40 just getting to the surface not counting solving the problem or doing any kind of safety stops/deco. And remember every dive is a decompression dive, regardless of what they told you at PADI. If you have to fix a problem on the bottom and would like to do some safety stops, that 40 is gone before you get there.

So I think I do understand. Which also lends itself to why a pony bottle is not the best option. This opinion has nothing to do with DIR/GUE/ABC/NBC/CBS but the reality of what most people's (honest) SAC really is and the fact that in an emergency even a 40 cuft bottle has questionable value at 100', unless you are just going to "blow and go" and pray you don't have a PFO.

 

[grf88]

40 cubic feet not enough to bail out from a NDL dive to 100 ft.!!! Why is it that some people have to take their diatribe against pony bottles to the point of absurdity?