If you read what I posted earlier, the market for 30 cf tanks is higher than 19 cf tanks, once he tries to sell it.
19 or 30 cubic feet?
- Thread starter rye_a
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If you read what I posted earlier, the market for 30 cf tanks is higher than 19 cf tanks, once he tries to sell it.
Interesting discussion as a pony is my next item on the list. I'm leaning towards a 19cf simply because I believe it is enough air to survive an OOA from recreational depths. Would 30cf or 40cf be better and safer? Sure, but a 19cf slung to me will be a lot more valuable than a 40cf sitting in my garage or car. I think I'll be more likely to actually bring the smaller 19cf on all dives, especially when flying or where I have a long walk through soft sand for a beach dive.
Let's be realistic. The odds of a sudden complete malfunction of air supply at 130ft (we are recreationally diving) is very low. Unless you are in an overhead or have a mandatory stop, the 19cf should be adequate to get you to the surface. Safety stops are a good practice but not mandatory. So I'll use the 19cf to get to my safety stop, then hang at 15 feet for the stop or as long as the pony lasts. Could there be circumstances where 19cf is not enough. For sure, but the boat could sink or you could get struck by lightning or eaten by a shark, too. Having the 19cf should knock the already low odds down enough where now the best thing you could do to improve safety is to not have that second doughnut for breakfast.
Let's be realistic. The odds of a sudden complete malfunction of air supply at 130ft (we are recreationally diving) is very low. Unless you are in an overhead or have a mandatory stop, the 19cf should be adequate to get you to the surface. Safety stops are a good practice but not mandatory. So I'll use the 19cf to get to my safety stop, then hang at 15 feet for the stop or as long as the pony lasts. Could there be circumstances where 19cf is not enough. For sure, but the boat could sink or you could get struck by lightning or eaten by a shark, too. Having the 19cf should knock the already low odds down enough where now the best thing you could do to improve safety is to not have that second doughnut for breakfast.
jeraldjcook
Contributor
The 19cf easily fits in my carry-on. A major factor in my consideration since it's mostly a vacation item.
I am glad people are at least not discussing a 6cft.......
I do want to stress that the original idea behind spare airs was for downed pilots to be able to have air while swimming like hell to the surface...again its a rescue device that is available to get someone to the surface. We can fix blow lungs, we can fix DCS, we can't fix DEAD....not yet.
I'm sorry that I don't have much to add to the discussion re pony bottle sizes but I do want to add that 8 l/minute is FAR too low a SAC rate for calculations.
Over a period of about 50 dives my SAC rate has crept down* to hover around the 15 l/min (~ 0.52 cft/min) mark on average. That is 13 l/min on a good day and 17 l/min on a bad. This is in cold, dark water btw. I think that you will find that this is about the average SAC rate amongst most moderately experienced divers.
I'm not saying that you won't find people with lower SAC rates (because you will) but you will NOT find anyone with one at 8 l/min ... never mind find an average SAC of 8 l/min.
Why this isn't predicted by your predictions based on tidal volumes and breathing frequency I don't know but I do now that if you have two measurements for a quantity, one derived empirically and one derived based on theory, you go with the empirical data.
That said, I love that you are taking the time to properly explore the question with calculations and good educated guesses rather than stating "x cft is too much/too little/your mother is fat" like some prophet handing down holy words.
*Disclaimer: this was calculated using my Vytec computer. I know that it is also an estimate and not accurate but I seriously doubt that it is off by as much as 7 litres.
Over a period of about 50 dives my SAC rate has crept down* to hover around the 15 l/min (~ 0.52 cft/min) mark on average. That is 13 l/min on a good day and 17 l/min on a bad. This is in cold, dark water btw. I think that you will find that this is about the average SAC rate amongst most moderately experienced divers.
I'm not saying that you won't find people with lower SAC rates (because you will) but you will NOT find anyone with one at 8 l/min ... never mind find an average SAC of 8 l/min.
Why this isn't predicted by your predictions based on tidal volumes and breathing frequency I don't know but I do now that if you have two measurements for a quantity, one derived empirically and one derived based on theory, you go with the empirical data.
That said, I love that you are taking the time to properly explore the question with calculations and good educated guesses rather than stating "x cft is too much/too little/your mother is fat" like some prophet handing down holy words.
*Disclaimer: this was calculated using my Vytec computer. I know that it is also an estimate and not accurate but I seriously doubt that it is off by as much as 7 litres.
jeraldjcook
Contributor
No ... it wasn't. 8 l/min was assumed to be the average SAC rate in the calculations until it was pointed out how inaccurate this was.
After that Stimpy then redid the calculations again, this time assuming a SAC rate of 20 l/min to be more representative of reality (which was admittedly done before my post).
The post which you quoted is the second lot of calculations with the new, more realistic, value for the breathing rate.
After that Stimpy then redid the calculations again, this time assuming a SAC rate of 20 l/min to be more representative of reality (which was admittedly done before my post).
The post which you quoted is the second lot of calculations with the new, more realistic, value for the breathing rate.
jeraldjcook
Contributor
I have just read through this whole thread because I too am interested in a pony bottle. I got the 6 ft*3 Spare Air and I still think it has a purpose. Not for deco accents from 150 ft, maybe not even from 60 ft*3, but I got it to get to my insta-buddy more than to get to the surface.
I do most of my diving with insta-buddies. Some are very skilled and wonderful. Others are divers in the same ocean. That said, I wanted something I could easily pack, and easily fill at my destination. Given that you have to remove the valves, most shops, if they know, will require a new VIP. Since I can fill the little bottle off my main tank (I usually do this the first dive, which is usually a buoyancy test for the group, and we don't go deep or long, the missing 6 ft*3 from my 80 (or 77) is not a problem. But, if I had to get a new VIP that can get expensive, and time consuming. I'm not even sure how I would do that on boat dives where they bring just the number of tanks you bought on the boat. No compressor. So it would be a special trip to fill (or so went my reasoning).
Ok, now I am thinking I may be doing more solo diving (often with another diver in the same ocean) so I want a bail out bottle. If I decide to go tec, I'll get a deco bottle. I'm very much leaning towards the 19 ft*3, but since there is so much discussion on what one really needs, I think I'll borrow one and try it out from 80-90 feet. Granted, I'll probably be more relaxed than in a real OOA situation, but if I don't make it to the surface in a leisurely accent with a 5 min safety stop, as some believe, then I know I won't in a true OOA situation. If I get there with 1/2 a tank, it will probably work. I'll just have to make some judgments, but I think by diving it, I will make smarter ones. They won't be based upon resale value, or upon potential use for something I'm not buying it for now. It will be based on what I need to do the instant job, with some reserve.
And, I'm really hoping that the 19 ft*3 will work, since I believe I can build and trans-fill that from an 80 ft*3, and still have enough left in the 80 to do an easy dive (and bypass the extra VIP). I haven't run the calculations yet. I do know that I'd end up with less than 2000 psi in both with a 40 ft*3 tank trans-filled. In addition, I'm not sure I'd pack (due to weight and size constraints) a 30 or 40 ft*3 pony on my recreational dives. Especially if the airlines follow suit and limit you to one checked bag (and on some airlines, that bag is limited to 30 pounds).
Is my reasoning flawed?
I do most of my diving with insta-buddies. Some are very skilled and wonderful. Others are divers in the same ocean. That said, I wanted something I could easily pack, and easily fill at my destination. Given that you have to remove the valves, most shops, if they know, will require a new VIP. Since I can fill the little bottle off my main tank (I usually do this the first dive, which is usually a buoyancy test for the group, and we don't go deep or long, the missing 6 ft*3 from my 80 (or 77) is not a problem. But, if I had to get a new VIP that can get expensive, and time consuming. I'm not even sure how I would do that on boat dives where they bring just the number of tanks you bought on the boat. No compressor. So it would be a special trip to fill (or so went my reasoning).
Ok, now I am thinking I may be doing more solo diving (often with another diver in the same ocean) so I want a bail out bottle. If I decide to go tec, I'll get a deco bottle. I'm very much leaning towards the 19 ft*3, but since there is so much discussion on what one really needs, I think I'll borrow one and try it out from 80-90 feet. Granted, I'll probably be more relaxed than in a real OOA situation, but if I don't make it to the surface in a leisurely accent with a 5 min safety stop, as some believe, then I know I won't in a true OOA situation. If I get there with 1/2 a tank, it will probably work. I'll just have to make some judgments, but I think by diving it, I will make smarter ones. They won't be based upon resale value, or upon potential use for something I'm not buying it for now. It will be based on what I need to do the instant job, with some reserve.
And, I'm really hoping that the 19 ft*3 will work, since I believe I can build and trans-fill that from an 80 ft*3, and still have enough left in the 80 to do an easy dive (and bypass the extra VIP). I haven't run the calculations yet. I do know that I'd end up with less than 2000 psi in both with a 40 ft*3 tank trans-filled. In addition, I'm not sure I'd pack (due to weight and size constraints) a 30 or 40 ft*3 pony on my recreational dives. Especially if the airlines follow suit and limit you to one checked bag (and on some airlines, that bag is limited to 30 pounds).
Is my reasoning flawed?
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