I am thinking about redundancy and such and have considered getting a pony bottle. ... Thoughts?
The problem that you are equipping yourself for is a regulator failure towards the end of your dive. Your first response should be to obtain assistance from your buddy. Odds are that you will be quite a bit shallower than 80 FSW and thus a free ascent should not be that big a deal, in fact I don't really see a free ascent from 80 feet as a big deal for a well trained diver, but I realize that approach has faded from view along with what I consider to be quality training. Anyway ... more gas is always better as long as you don't burden yourself with more gear than you can handle. The difference between a the smaller bottles and a 30 or even 40 are so small that I really do see any sense in getting anything but a 40. But will a 13 get you to the surface? Absolutely. Even at SAC of 1.0, you'd need a bit less than 6 cubic feet for your ascent, leaving plenty of air for a "safety stop" if you're so inclined.
Hi OldNSalty,
Just more food for thought: You may want to consider an Al30 rather than an Al19. The reason is that the physical size and weight difference between the two tanks is negligible, the 30 only costs slightly more (check prices online to confirm this for yourself), and it holds almost double the air.
Best wishes.
In this case bigger is better.
well I would suggest an al40, but you have to have your harness setup to sling it off your (usually left) side. but any size would be better than nothing. Its important to have enough air/ nitrox in your bottle to be able to bring you up and allow you to do your stops. if slung you have to be able to take it off your harness to be able to hand it back up into the boat, so practice is a good idea. I believe that all divers should have a pony stage setup of some kind when diving deeper than 60ft. I know its expensive to do so.
A 40 is great. Slung is better, IMHO.
I don't dive without a redundant system, and usually now that means doubles but thats another thread.
I own an aluminum 19 which I backmount for any dives in "good" conditions and shallower than say 60ft (depending on conditions)
I own an aluminum 40 which I sling for any dives in "poorer" conditions or deeper than 60ish feet (depending on conditions)
T
If you have a 40 why would you ever dive with a 19?
A 13 cu ft will get you to the surface from 120'. No, you will not get a safety stop with that, it is an emergency, I'm out of breathing gas ascent to safety, period. Safety stops are for normal dive ascents to allow a little cushion to your off-gassing.
If anyone tells you a 13 cu ft cyl is too small, they are speaking from mis-information, or adding more to the issue than "I need to go to the surface".
At a SAC rate of 1.0, 12 cubic feet will get you to the surface at 30 fpm from 120 feet, so you are quite correct. There's not enough gas there for a 3 min. "safety stop" (which as we have discussed in other treads is totally unnecessary with a 30 fpm ascent rate).
I'd like to get to the surface safely and without getting bent. That consitutes a slow, controlled ascent. For a dive that is deeper than 20m, it includes time at the bottom to resolve a problem, deeper stops and a shallow safety stop. 13 cu ft just doesn't cut the mustard.
I can't do imperial calculations, but here it is in metric:
Assuming a stressed breathing rate of 30 litres per minute at a depth of 40m. It is likely to take me a minute to get things under control. In that minute, I will use;
30 * 1 * 5 = 150 litres of gas (5.29719 ft³)
Average depth on the ascent is 20m, or 3ATM. Assuming a safe ascent rate of 10m per minute it will take 4 minutes to reach the surface. In that ascent, I will use:
30 * 4 * 3 = 360 litres of gas (12.7132 ft³)
Like I say, for me a safe ascent from 40m includes at least a one minute stop at half my maximum depth. I don't like the idea of my faster tissues bubbling away. So I can add another 90 litres of gas. (3.17831 ft³)
Finally, a 3 minute stop at 5m. Let's assume I've got my breathing under control at this point and back down to 20 lpm. For my safety stop, I will use:
20 * 3 * 1.5 = 90 litres of gas (3.17831 ft³)
So my total gas use is 150 + 360 + 90 + 90 = 690 litres of gas (24.3671 ft³)
Using Google to convert from litres to cu ft - that's 24 cu ft, about twice what a 13 cu ft gives you.
Assuming that you feel comfortable enough to make a direct ascent to the surface, without resolving any issues at depth, whilst maintaining a safe ascent rate of 10 m/min, you will use 360 litres - Google gives me 12.71 cu ft. There is absolutely zero margin for error with a 13 cu ft tank.
I have measured my consumption rate in various circumstances, and it varies from around 13 litres per minute (0.459090 ft³) to as high as 50 litres per minute (1.76573 ft³). If it were on the higher end of that, with a 13 cu ft it's not getting me to the surface. If I have a 30 cu ft cylinder, I have the option to not include my deeper stop or my safety stop.
The difference of carrying a 13, a 19 or a 30 cu ft tank on a dive is barely noticeable. Larger tanks give you options to work through your ascent safely. Just because it's an emergency, that doesn't mean you have to compromise safety.
YOU may be willing to do that, but to advise someone that they should is really not ethical for a diving professional.
I hope this "misinformation" is of use.
The reality is that if you want to carry enough gas to hoover whilst solving a problem, making a slow ascent to a deep stop, making a slow ascent to a "safety stop" and then surfacing ... wear doubles and dive to overhead environment Bingo Air rules, that's your only safe bet. If you want to carry enough gas to get you back to the surface in an easy controlled fashion from 80 feet (even 120) a 13 will do, but I can see no reason to not get a larger flask.
Nope, no use to me. You are using a linear caculation to work a problem that works on a darivation of the change of volume with the change of pressure over time.
Second, I qualified my opinion that it is an "emergency gas source" only to provide egress to the surface.
Safety stops are for people ignoreant of the medical and testing basis of decompression theory. So for an emergency ascent a safety stop is irrellevant. You are just adding conservitive redundatcy to the original posters question and adding bulk to the system that the diver has to lift on the surface to get to and exit the water. How easy it is to sling a 200 cu ft cylinder underwater is also not as important.
For the OP, a pony is just a better way to, as the dreaded PADI agency puts it, CESA to the surface.
And yes I am comfotable enough underwater to make a direct ascent to the surface, at 30 feet per minuite, Oh wait!, 9.144 meters per minuite, safely using a 13 cubic foot (0.368119 cubic meter) cylinder if my main supply of breathing gas fails at approximately 30.480 meters depth.
What is there to sort out at depth if you have no breathing gas? Are you planning a contengency to do a couple of S-drils on the way up?
Sorry, but if you gas consumption varies by 25% than you are not a very confident, comfotable diver and therfore you should not be diving deeper than about 60 feet (18 meters). To do the kind of diving you are refering to a diver must be calm, and thoughtful underwater. Getting excited or stressed easily is not a good trait for an advanced / technical diver.
I have to agree, it's really not that exciting to change regulators and go to the surface.
Emphasis on the word "theory". Any dive to 40m has the potential to cause a DCS incident. Given the relatively short NDLs at 40m (what ever "no decompression" is), you're essentially saying it is acceptable to bounce dive to 40m and come straight to the surface. The risk of a Type II incident is much higher under those circumstances.
I don't know how this has entered the discussion, but what is wrong with a single "bounce dive" to 40m? Even at 60 fpm both ways without a deep stop or a "safety stop?" I've made hundreds of such dives deploying and recovering instruments ... just down and back. But I don't do them without a significant surface interval. Face it, that's just a comfortable free dive for a lot of folks.
I'd agree that a 30 cu ft is significantly different in size from a 13 cu ft, but a 19 cu ft is not. It also has a marginal swing in buoyancy. Again, the difference from a 30 cu ft to a 19 cu ft is barely noticeable.
I agree, and the difference to go to a 40 is no big deal.
"dreaded PADI agency"....? Leave your prejudices at home.
The point here is PADI's pushing of the "safety stop" the disingenuousness that that engenders.
When the RDP was developed the standard ascent rate (U.S. Navy) was 60 fpm, so the RDP was designed for that same rate.
The model PADI uses predicts that the decompression required to keep the risk of DCS to a negligible level consists of a diver trying their best to ascend at a constant rate of 60ft./minute. The idea of the divers trying their best to stop at 15', then trying their best to rise to the surface under control, well ... that was added by the lawyers not by anyone with a background in decompression.
But, at least, a three minute, ten to fifteen foot stop will not hurt you, at least from a decompression or practice standpoint.
The reality is that "Safety Stops" are antique artifacts. In this day of 30 FPM, computer controlled ascents "safety stops" should be irrelevant. I say that as one of those who was in on originating them. The idea of safety stops came out of some research that showed that recreational divers were, in point of fact, making their ascents between 100 and 120 feet per minute, back when a standard ascent was 60 fpm. The ascent problem was noted by the National Underwater Accident Data Center, it was investigated and confirmed by Glen Egstrom at UCLA, the safety stop was suggested by Andy Pilmanis of the USC Catalina Lab Chamber back in the 1970s. It was first adopted by NAUI after the AAUS Decompression Computer Workshop where Bruce Bassett (as I recall) suggested perhaps a stop between 20 and 10 feet for two to three minutes would be every bit as effective as actually getting divers to slow down to 60 fpm. In addition it would also cover most errors of one depth too deep or one time too long.
Then along came dive computers with ascent meters and such and the reason for the stop was forgotten, just the procedure was remembered.
The reality is that with a 30 FPM ascent you're not going to have detectable bubbles anyway. If you are using 60 FPM based tables (and remember that the original U.S. Navy tables were almost based on 120 FPM to satisfy the needs of Doug Fane's UDT folks) Spenser did show that a 60 fpm ascent following 50 min at 2.8 ata would exhibit bubbles in some cases. But ... I believe that the PADI tables were doppler tested and a 60 fpm ascent within the limits of those tables did not show bubbles, thus demonstrating that making such a stop a waste of time and a singularly foolish criterion for emergency ascent planning.
I'm not disputing that a pony is a preferable option to a CESA. I'm merely saying that it is unethical for you to say that a 13 cu ft is enough to get the OP to the surface - when you have no idea what their gas consuption rate is. It simply may not be big enough.
Let's take an example. I teach the use of pony bottles on a course, and recently I had a student (name changed), Dave.
Dave's average SAC was 30 lpm. On a stress test his SAC hit 60 lpm. Let's assume that he read your post that said "13 cu ft is enough to get you to the surface from 40m". I wonder how fast his ascent rate would be to get him there.....?
Average depth = 20m, 3ATM... so approx 180 lpm average. A 13 cu ft tank holds roughly 360 litres. That gives him 2 minutes to ascend, at a rate of 20m/min.
I find a 60 fpm ascent rate perfectly acceptable as part of an emergency plan. In fact, you're likely better off to do 120 fpm to 60 fsw then 60 fpm to 15 FSW, hold there till you run out of gas, and then free ascend.
Increasing the size of the tank to 19 cu ft, or a total of around 540 liters. That gives him 3 minutes to ascend, at a rate of 13m/min. Still too fast for my liking, but much better than 20m/min.
And the OP's SAC rate is the same as yours.......?
To a point, more is better.
Plenty, in some circumstances. Orientation, for example. Or maybe you can't ascend directly to the surface - one of my favourite wreck dives here is in the middle of a shipping lane on route to a major port. I'm not sure I'd want to make a direct ascent to the surface with out the option to stop at some stage and check for boat traffic.
This is outside the scope of the OP and the discussion.
Your arrogance is amazing.
This is outside the scope of the TOS.
Have ever thought that maybe gas consupmtion varies with things other than stress? Something as simple as swimming against a current, for example?
This is outside the scope of the the discussion unless the current is running down.
Also, don't you find it odd that you are saying that a diver needs to be "calm and thoughtful", yet you are saying that they should carry the smallest possible air source that gives them no option but to make a direct ascent to the surface, with no option to stop and think? You're simply adding a stressor that doesn't need to be there.
No one ever said that, the point was that 13 was sufficient, that's all.
Do you also advocate not having an SPG on the pony as well?
This is outside the scope of the OP and the discussion.
Just because a 13 cu ft is the right option for you, doesn't mean it's the right option for everyone - give people the tools to work out what is right for them, rather than forcing your opinions on them.
No force is being applied other than the force of logic.
40 cuft pony man Luxfer is the best trims out nice and you an use as a deco bottle if you ever decide to become a Tech diver.
Yup.
