How low with spare air

[herbdb]

I agree, bigger is better. I like to use a 20 or 30 cu ft bottle depending on the dive.

But I do have a couple of 6cu ft bottles, with combination valve / first stages sold by H2Odessy that I use when I need to travel by air and know the dives will be shallow. I have experimented with them in our local cold water quarry. I can make a normal ascent with a 3 min safety stop from 60', but the reg is breathing wet and hard at that point. My son could not get a full safety stop with his 6 cu ft bottle.

I can't see a lot of utility from 3 cu ft. In hindsight, I wish I had gone with 12 cu ft for these travel bottles. They would still be small and light.

 

[rhwestfall]

VZ - nice!

---------- Post added November 20th, 2013 at 03:28 PM ----------

I agree, bigger is better. I like to use a 20 or 30 cu ft bottle depending on the dive.

But I do have a couple of 6cu ft bottles, with combination valve / first stages sold by H2Odessy that I use when I need to travel by air and know the dives will be shallow. I have experimented with them in our local cold water quarry. I can make a normal ascent with a 3 min safety stop from 60', but the reg is breathing wet and hard at that point. My son could not get a full safety stop with his 6 cu ft bottle.

I can't see a lot of utility from 3 cu ft. In hindsight, I wish I had gone with 12 cu ft for these travel bottles. They would still be small and light.

well, you might have a usable argon bottle on your hands....

 

[Hatul]

Steve Lewis' article notwithstanding, my rule for solo diving is 19 cu ft pony for deep open water dives and 13 cu ft for dives<60 ft. True if you have an OOA situation it would be better to have more gas, but it's a compromise and I don't want to lug a 40 cu ft tank on every dive.

Also, my calculations are less conservative. For example if I have a catastrophic failure such as a burst hose I'm not sticking around at depth for a minute, I can still breathe a bit from the main tank, and the first part of the ascent can be made faster than 30 ft/min.

 

[rhwestfall]

my diving here is square profile, and associated with a mooring line. I'm not, unless an absolute last resort, going to do an open water ascent from where I am if/when something happens. Swim back to the mooring, and up, or rig and run an up line. Those activities will be done in the "comfort" of a reasonable gas cusion. My 30 fits that need, though a 13/19 will be with me on <60' dives if there wasn't a deep dive that day. I've done a open water ascent from 115', following a panicked diver/buddy, and prefer to not do it again..... SA just doesn't cut it for me, nor do i want my buddy to feel it is an adequate tool for us.

 

[Steve_C]

Following up on victorzamora's comments.
1) is very important
2) I had a spare air and after a few dives I thought about how much it really gave me. Spare air now sits in the closest and has been replaced by a slung AL19. This is smaller than an AL40 and much less hassle. On the other hand it is 1/4 of an AL80 in terms of volume which will get me up with a safety stop from the depths I normally dive.

 

[dumpsterDiver]

The OP may find this article enlightening... USING ADDITIONAL REDUNDANCY: the maligned and misunderstood pony bottle | Doppler's Tech Diving Blog

The calculations are somewhat arbitrary and very subjective. For people that are diving to 130 feet, MAYBE a 30 or 40 cu-ft bottle is needed, but the fact remains that most sport diving is done at a depth considerably less than that, probably less than 100 feet.

The problem, as mentioned already, with one of these big heavy pony bottles, is that people just are not going to use them. A 40 cu-ft bottle is absolutely ridiculous, say for a female recreational diver who is small and relatively weak and will be dealing with rough water exits, climbing a ladder etc for a 50 or 90 ft dive. A bottle that big takes a lot of the fun out of the dive for many people, and they just won't take it with them.

I would rather see more people take small pony bottles on all dives (say over 50-60 ft) rather than very few divers take ridiculously big bottles.

If we adjust some of the assumptions, we can justify a much smaller bottle...

Don't assume that the diver is going to totally freak out on the bottom, you just gotta stick the reg in your mouth and go up. In other words, why assume it takes 60 seconds to leave the bottom?

Don't assume the breathing rate is going to go super high, I would be comfortable in assuming a SAC of 1,0 ft^3/min.

Don't assume that the ascent rate will be 30 feet per minutes for the start of the ascent, in reality if people are stressed they will probably go FASTER. In the very unlikely event of a total scuba failure, I am willing to revert to the "old" ascent rate of 60 fpm (at least for the first 30 or 50 feet).

Don't assume that the diver will do a safety stop (again in the extremely unlikely event of a total scuba failure).

The reality of the matter is that scuba units rarely fail completely. Generally the diver that needs redundancy has been stupid and inattentive and has breathed the tank too low (or it is getting hard to breath). If this is the case, then the diver can switch over to the pony at the first indication of a problem, begin the ascent and then when they are making headway, getting their breathing under control and are reaching depths that they are more comfortable with (say 50 or 30 feet), they can decide to switch back to the main tank and "milk" some air out of it. If they can do this, they will be saving the pony gas and can contemplate the practicality of the performance of a safety stop at 20 feet or so. You can gently sip a large tank for quite some time at 20 or 30 feet, even if it was close to "empty" on the bottom. This might also be practical if a tank is free-flowing... it can still be used for part of the ascent... maybe....

A few years ago, I ran out of air, while solo in 120 or 130 feet and I was completely out of breath when it occurred. My sac rate was probably 1.5 when the tank got hard to breath. I only had a 6 cu-ft pony... I made it to the surface, used a little of the main tank and did do a safety stop, but you can be damn sure I didn't screw around on the bottom, or ascend at 30 feet per minute for the first 60 feet.

 

[scubadada]

OK, I dive a 19 cu ft pony for all of my solo dives. Is a 3 cu ft Spare Air a replacemet for my pony, absolutely not, I have posted my gas requirements on numerous previous posts. So...whats better, running our of air within recreational limits and doing a CESA or having 3 cu ft of air. I don't think anyone would argue this point except to argue that it is wrong to run out of air, no disageement.

---------- Post added November 20th, 2013 at 07:01 PM ----------

The calculations are somewhat arbitrary and very subjective. For people that are diving to 130 feet, MAYBE a 30 or 40 cu-ft bottle is needed, but the fact remains that most sport diving is done at a depth considerably less than that, probably less than 100 feet.

The problem, as mentioned already, with one of these big heavy pony bottles, is that people just are not going to use them. A 40 cu-ft bottle is absolutely ridiculous, say for a female recreational diver who is small and relatively weak and will be dealing with rough water exits, climbing a ladder etc for a 50 or 90 ft dive. A bottle that big takes a lot of the fun out of the dive for many people, and they just won't take it with them.

I would rather see more people take small pony bottles on all dives (say over 50-60 ft) rather than very few divers take ridiculously big bottles.

If we adjust some of the assumptions, we can justify a much smaller bottle...

Don't assume that the diver is going to totally freak out on the bottom, you just gotta stick the reg in your mouth and go up. In other words, why assume it takes 60 seconds to leave the bottom?

Don't assume the breathing rate is going to go super high, I would be comfortable in assuming a SAC of 1,0 ft^3/min.

Don't assume that the ascent rate will be 30 feet per minutes for the start of the ascent, in reality if people are stressed they will probably go FASTER. In the very unlikely event of a total scuba failure, I am willing to revert to the "old" ascent rate of 60 fpm (at least for the first 30 or 50 feet).

Don't assume that the diver will do a safety stop (again in the extremely unlikely event of a total scuba failure).

The reality of the matter is that scuba units rarely fail completely. Generally the diver that needs redundancy has been stupid and inattentive and has breathed the tank too low (or it is getting hard to breath). If this is the case, then the diver can switch over to the pony at the first indication of a problem, begin the ascent and then when they are making headway, getting their breathing under control and are reaching depths that they are more comfortable with (say 50 or 30 feet), they can decide to switch back to the main tank and "milk" some air out of it. If they can do this, they will be saving the pony gas and can contemplate the practicality of the performance of a safety stop at 20 feet or so. You can gently sip a large tank for quite some time at 20 or 30 feet, even if it was close to "empty" on the bottom. This might also be practical if a tank is free-flowing... it can still be used for part of the ascent... maybe....

A few years ago, I ran out of air, while solo in 120 or 130 feet and I was completely out of breath when it occurred. My sac rate was probably 1.5 when the tank got hard to breath. I only had a 6 cu-ft pony... I made it to the surface, used a little of the main tank and did do a safety stop, but you can be damn sure I didn't screw around on the bottom, or ascend at 30 feet per minute for the first 60 feet.

Where's the Like icon for this post, I can't find it? Consider the post liked

 

[tomfcrist]

The calculations are somewhat arbitrary and very subjective. For people that are diving to 130 feet, MAYBE a 30 or 40 cu-ft bottle is needed, but the fact remains that most sport diving is done at a depth considerably less than that, probably less than 100 feet.

True enough, however consider that if a diver is buying a pony, why not buy one that will serve more than 1 purpose. A 30 or 40 can be used as redundant gas all the way to 130 as a recreational diver, and also serve as d deco bottle should the diver decide to go tech in the future.

The problem, as mentioned already, with one of these big heavy pony bottles, is that people just are not going to use them. A 40 cu-ft bottle is absolutely ridiculous, say for a female recreational diver who is small and relatively weak and will be dealing with rough water exits, climbing a ladder etc for a 50 or 90 ft dive. A bottle that big takes a lot of the fun out of the dive for many people, and they just won't take it with them.

There is nothing big nor heavy about an al 30 or 40. An AL 19 is almost just as cumbersome and quite honestly an AL 6 isnt good for anything but shallow dives. Yes you can make it up from 100' on a 6, but I'd rather have enough redundant gas to take my sweet time coming up.

I would rather see more people take small pony bottles on all dives (say over 50-60 ft) rather than very few divers take ridiculously big bottles.

I agree completely. I have an AL19 that i use regularly, although i see no problem with dragging my 40 around either.

 

[dumpsterDiver]

True enough, however consider that if a diver is buying a pony, why not buy one that will serve more than 1 purpose. A 30 or 40 can be used as redundant gas all the way to 130 as a recreational diver, and also serve as d deco bottle should the diver decide to go tech in the future.




There is nothing big nor heavy about an al 30 or 40. An AL 19 is almost just as cumbersome and quite honestly an AL 6 isnt good for anything but shallow dives. Yes you can make it up from 100' on a 6, but I'd rather have enough redundant gas to take my sweet time coming up.




I agree completely. I have an AL19 that i use regularly, although i see no problem with dragging my 40 around either.

A 40 cu-ft tank IS heavy. In fact, it weighs MORE than half as much as a standard 80 cu-ft Aluminum tank. Some divers don't even want to use a standard 80 aluminum tank because it is too big, too heavy and cumbersome. Many people will use those tiny little steel 80's because they are smaller and lighter... You will be selling ice makers to Eskimos before you sell them a 40 cu-ft bottle,

Most ALL the people will NEVER get into tech diving and will NEVER need a stage bottle. And if they ever do progress to tech diving, they can sell the bottle and buy a new bigger one for a total lost of $75... NOTHING compared to the cost of Tech diving....

 

[tomfcrist]

A 40 cu-ft tank IS heavy. In fact, it weighs MORE than half as much as a standard 80 cu-ft Aluminum tank. Some divers don't even want to use a standard 80 aluminum tank because it is too big, too heavy and cumbersome. Many people will use those tiny little steel 80's because they are smaller and lighter... You will be selling ice makers to Eskimos before you sell them a 40 cu-ft bottle,

Most ALL the people will NEVER get into tech diving and will NEVER need a stage bottle. And if they ever do progress to tech diving, they can sell the bottle and buy a new bigger one for a total lost of $75... NOTHING compared to the cost of Tech diving....

You are honestly going to try to make the arguement that an al 40 is heavy? or cumbersome? Im not buying it. If the added weight of a 40 is too heavy for someone, than they need to go to the gym bro.

I think the reason most divers stray away from AL 80's has less to do with overall weight and more to do with piss poor bouyancy properties.

All that being said, I have no issues with divers using 19's......i use them too, but after 100' i dont consider them sufficient. MY opinion of course. You are entitled to yours as well.