On using a small tank to increase bottom time and provide redundancy

[SouthernSharktoothDiver]

Hey all!
So, some context: I do most of my diving in 20-40 ft of high current, low vis water. I have a "buddy" on these dives, but realistically, if we're not using a tether (which we do if things seem risky today, but usually not as it's an entanglement risk) we're unable to readily come to each other's assistance. We remain with depth for a CESA, but frankly I'm not sufficiently confident in my ability to do one of those under these conditions to wager my life on it. For this reason, I have acquired a 19 cu ft aluminum scuba cylinder, which I intend to kit out for use as a redundant air supply.

Now, in order to get my "money's worth" out of the bottle, I'm considering using it as a part of my gas plan to extend my dive time (hence why I've avoided using the term, "Pony bottle" since that refers to a cylinder which is not part of the gas plan). I have planned it out that I would do the first dive of the day as normal, using the 19 ft bottle only in the event of an emergency. Then, for the second dive, I would drain my main bottle until I would normally ascend, then switch over to the small cylinder to finish out the dive. Based on my SAC rate and the amount I'd want to keep in reserve, I think this would buy me about 15 minutes of dive time at my usual depth.

Under this gas plan, I'd always have a fully redundant air supply, but I'd be using both as part of my intended dive. I have seen posts on here which say this is a Very Bad Idea (Tm) and I would like to shop the idea around a find out why. Please note that I'm not requesting advice on how to mount/sling my pony/stage bottle, and while I'm aware many divers believe side mount or independent twins to be preferable, I feel a 19ft pony to be sufficient for my depth, and I don't really want to spend the money on side mount or twins, especially since I don't see a need for such a bulky setup under these conditions. I say all this to try and avoid spawning another thread on pony bottle logistics, which has kind of been beaten to death on this forum.

Anyway, I look forward to getting advice/opinions, thanks in advance!

 

[Rob9876]

Just to clarify, when you say "drain my main bottle until I would normally ascend" are you saying you would switch to the 19cf when your main tank is at 700psi (or 500psi)? That is to say -- if after switching your 19cf failed for some reason, you would still have a reasonable amount left in your main tank to switch back to and safely ascend?

If so, then that seems o.k. to me. If you're not leaving that much in your main tank, then you're really only fully redundant on the first dive, not the end of the second, if you do that.

 

[SouthernSharktoothDiver]

Just to clarify, when you say "drain my main bottle until I would normally ascend" are you saying you would switch to the 19cf when your main tank is at 700psi (or 500psi)? That is to say -- if after switching your 19cf failed for some reason, you would still have a reasonable amount left in your main tank to switch back to and safely ascend?

If so, then that seems o.k. to me. If you're not leaving that much in your main tank, then you're really only fully redundant on the first dive, not the end of the second, if you do that.

Yes, this is exactly what I have in mind. My dives are typically shallow enough to not need a safety stop (since I head for the shoreline when I’m ready to turn around, I usually end up in 20 ft of water or less before ascending), and the currents make one impractical in any case, so ascending only requires the gas to breath for less than a minute at shallow depth, and to fully inflate my BC once on the surface (which could be done orally, but why not use the power inflator?)

I usually ascend with 600-700 psi and end at 500-600, so I would keep a 700 psi reserve on the main cylinder before switch to the pony.

 

[happy-diver]

Soon enough you may decide, that the best way to extend a dive is to get out and start another dive
but until then, a good exercise would be, to get on your little tank before you hit reserve on your big
tank and when you hit reserve on your little tank, get back on your big tank then complete your dive

So after tiring of having to fill the little tank and swapping things around, you may buy a bigger tank
and have more air and a pony to ease your mind and you can get out of the water like everyone else

 

[34109411]

Hey all!
So, some context: I do most of my diving in 20-40 ft of high current, low vis water. I have a "buddy" on these dives, but realistically, if we're not using a tether (which we do if things seem risky today, but usually not as it's an entanglement risk) we're unable to readily come to each other's assistance. We remain with depth for a CESA, but frankly I'm not sufficiently confident in my ability to do one of those under these conditions to wager my life on it. For this reason, I have acquired a 19 cu ft aluminum scuba cylinder, which I intend to kit out for use as a redundant air supply.

Now, in order to get my "money's worth" out of the bottle, I'm considering using it as a part of my gas plan to extend my dive time (hence why I've avoided using the term, "Pony bottle" since that refers to a cylinder which is not part of the gas plan). I have planned it out that I would do the first dive of the day as normal, using the 19 ft bottle only in the event of an emergency. Then, for the second dive, I would drain my main bottle until I would normally ascend, then switch over to the small cylinder to finish out the dive. Based on my SAC rate and the amount I'd want to keep in reserve, I think this would buy me about 15 minutes of dive time at my usual depth.

Under this gas plan, I'd always have a fully redundant air supply, but I'd be using both as part of my intended dive. I have seen posts on here which say this is a Very Bad Idea (Tm) and I would like to shop the idea around a find out why. Please note that I'm not requesting advice on how to mount/sling my pony/stage bottle, and while I'm aware many divers believe side mount or independent twins to be preferable, I feel a 19ft pony to be sufficient for my depth, and I don't really want to spend the money on side mount or twins, especially since I don't see a need for such a bulky setup under these conditions. I say all this to try and avoid spawning another thread on pony bottle logistics, which has kind of been beaten to death on this forum.

Anyway, I look forward to getting advice/opinions, thanks in advance!

We have more or less the same dive profile beside that i am solo diving only. It seem you have good understanding of some concept. First what size in CF is your main tank ? If you worry about that part to "extend your dive" you could get a 100 or 120 steel tank you will have like 3500 psi og gas (more or less). Not sure it's a good idea to use that 19cf to extend your dive. You should use your main tank and let's say at 1000 psi you go up. I will use the 19cf for redundant air source but not to extend your dive.

Usually what your refer too is a stage bottle thay usually imply some deco obligation. But let's not get too complicated. Not sure why you will "drain" your bottle in any given scenarios.

I really thing the best option for you is to get a larger tank to do a short story.

Cheers

 

[LI-er]

Here's how I do it when I dive solo, which is most of my dives.

In addition to whatever cylinder is provided by the Dive Op, I carry either a 19cf or 30 cf back mounted pony bottle, which has a transmitter to feed an AI computer on my left wrist (I don't take the 30 cf bottle on plane trips). On my right is my primary wrist computer which gets the signal from my main tank. So I always know the pony bottle is full or conversely if there's a leak or free flow. The pony bottle regulator is on a rubber hose that hangs right below my chin. I do not carry an octopus second stage. In the event of an OOA emergency with another diver I would hand them (or they would grab) my primary and I would switch to my pony regulator.

On almost every dive I draw my main tank down much lower than typically recommended levels before ascending, although the amount will vary with depth and complexity of the dive to some degree. I won't be in a wreck at 120' at 500 psi but on an 80' reef dive I probably would be. I rarely actually draw a breath from the pony but having it as a reserve allows me to use more gas in the main tank on each and every dive. My main tank almost always has less than 250 psi at the end of the dive.

There have been a handful of (interesting) dives where I do actually draw from the pony to extend the dive even more. In which case I'll draw the main tank down to recommended levels (700-1000 psi), switch to the pony bottle, breathe it down to about 150-200 psi, then go back to main tank and ascend.

I recently bought an airfill whip so I can fill the pony if I do happen to use it, and it avoids the inconvenience of having to get it filled at a ship prior to a boat dive, and I also don't have to deal with tank inspections for it.

 

[34109411]

Here's how I do it when I dive solo, which is most of my dives.

In addition to whatever cylinder is provided by the Dive Op, I carry either a 19cf or 30 cf back mounted pony bottle, which has a transmitter to feed an AI computer on my left wrist (I don't take the 30 cf bottle on plane trips). On my right is my primary wrist computer which gets the signal from my main tank. So I always know the pony bottle is full or conversely if there's a leak or free flow. The pony bottle regulator is on a rubber hose that hangs right below my chin. I do not carry an octopus second stage. In the event of an OOA emergency with another diver I would hand them (or they would grab) my primary and I would switch to my pony regulator.

On almost every dive I draw my main tank down much lower than typically recommended levels before ascending, although the amount will vary with depth and complexity of the dive to some degree. I won't be in a wreck at 120' at 500 psi but on an 80' reef dive I probably would be. I rarely actually draw a breath from the pony but having it as a reserve allows me to use more gas in the main tank on each and every dive. My main tank almost always has less than 250 psi at the end of the dive.

There have been a handful of (interesting) dives where I do actually draw from the pony to extend the dive even more. In which case I'll draw the main tank down to recommended levels (700-1000 psi), switch to the pony bottle, breathe it down to about 150-200 psi, then go back to main tank and ascend.

I recently bought an airfill whip so I can fill the pony if I do happen to use it, and it avoids the inconvenience of having to get it filled at a ship prior to a boat dive, and I also don't have to deal with tank inspections for it.

LOl i did get a transfer whip also to fill out my

Here's how I do it when I dive solo, which is most of my dives.

In addition to whatever cylinder is provided by the Dive Op, I carry either a 19cf or 30 cf back mounted pony bottle, which has a transmitter to feed an AI computer on my left wrist (I don't take the 30 cf bottle on plane trips). On my right is my primary wrist computer which gets the signal from my main tank. So I always know the pony bottle is full or conversely if there's a leak or free flow. The pony bottle regulator is on a rubber hose that hangs right below my chin. I do not carry an octopus second stage. In the event of an OOA emergency with another diver I would hand them (or they would grab) my primary and I would switch to my pony regulator.

On almost every dive I draw my main tank down much lower than typically recommended levels before ascending, although the amount will vary with depth and complexity of the dive to some degree. I won't be in a wreck at 120' at 500 psi but on an 80' reef dive I probably would be. I rarely actually draw a breath from the pony but having it as a reserve allows me to use more gas in the main tank on each and every dive. My main tank almost always has less than 250 psi at the end of the dive.

There have been a handful of (interesting) dives where I do actually draw from the pony to extend the dive even more. In which case I'll draw the main tank down to recommended levels (700-1000 psi), switch to the pony bottle, breathe it down to about 150-200 psi, then go back to main tank and ascend.

I recently bought an airfill whip so I can fill the pony if I do happen to use it, and it avoids the inconvenience of having to get it filled at a ship prior to a boat dive, and I also don't have to deal with tank inspections for it.

Wow i am surprise a low you can go on air !!) Honestly at 1500 i turn over, 1200 ascent and i make sure i have 1000 left. I also use a transfill whip to fill out my pony it's handy. I am also always thinking to ditch that octo but not sure yet about it. By PADI and SDI standards they recommend to keep it i think.

Be safe cheers

 

[rjack321]

Just get a larger tank or properly sidemount 2

 

[KenE]

My concern for this scenario is when the failure occurs on the primary tank after switching to alternate source. If the primary tank free flows you may not notice it in high current. Then when you go to switch back to ascend you find you have no gas in the tank. I would likely opt to drain the first tank before switching, yes I would lose redundancy but I would know how much gas I had to ascend with the reg in my mouth. I run a 30cf pony and won't use it to extend a dive unless I need to drain it for viz, and only if I have a buddy who was informed of the plan. I have been tempted because I suck gas, but ultimately found it was not worth the risk.

 

[NothingClever]

Bigger tank, mate.