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

[LI-er]

Can you display both cylinders on both computers (T1 & T2) and have redundancy rather than using the computers separately?

No but I have a backup to my primary tank computer in my BCD pocket.

 

[LI-er]

I run a 30cf pony and won't use it to extend a dive unless I need to drain it for viz

Why would you drain a full tank just to get a visual? Especially one that you might not even use for quite some time? Leave it filled, don't worry about an expired viz or hydro until it's time to refill it. Unless of course there will be an issue with an upcoming weeklong livaboard trip or whatever.

 

[grantmac]

While I personally don't have a problem with your plan it's not one I'd follow.
Do you rent your back gas tanks? What type of BCD do you have?

 

[SouthernSharktoothDiver]

Hey everybody! I really appreciate all the responses/advice. I meant to respond to all this sooner and more individually, but I got drafted into helping a buddy repair his dock, so I wasn't able to check in and respond as soon as I'd have liked. Picking up where I last responded, here's my thoughts/answers:

@happy-diver I may well decide my current plan is too much work and opt for a larger tank, but at the moment I'm young and strapped for cash, so using a pony as 2 in 1 redundancy and dive extension seems worthwhile to me.

@34109411 Sort of the same answer as I told happy-diver, I'm mostly just trying to get my money's worth out of the pony. I never intend to fully deplete any of my air sources. My main is an Al 80, and I plan on lowering it to ~600 psi (my normal ascent reserve under these conditions) and then switch to the pony.

@LI-er I've considered your approach before writing this thread; however, I prefer to keep my tanks above 500 psi, both as a precaution against moisture infiltration (which of course requires far less than 500 psi, but I like a buffer), as an emergency reserve, and because it prevents "hot fills." I think my local fill guy will allow me to fill the pony alongside my other two tanks at no additional cost, so it's just the cost of the vis/hydro at that point.

A general note to point out: I'm planning to do all of this with a max dive depth of 40 ft, and a usual end depth of 20 ft, so my reserve needs are likely less extensive than most. Obviously, if I go dive offshore to 60 or 100 feet, I'll keep a much larger reserve in my 80 and have no plans of using the 19 outside of emergencies.

@rjack321 My basic thought process is this: I don't really "need" a pony, since I'm within CESA depths. I want a pony, for extra safety since I'm not confident in my ability to CESA under these conditions, but I don't strictly need it. As such, I'm kind of left wanting to get my money's worth out of the extra gas I'm carrying, and the extra 15 minutes of bottom time does this for me, especially since I can do it without compromising my redundancy. Plus, I figure the habit will be good practice for switching between bottles so that, in the event of an emergency, it's all muscle memory. All of that being said, I don't actually want a larger tank that much, currently. I'm ok with my current bottom times. It's more a function of "I've got the air, may as well use it." Finally, I'm avoiding sidemount because I'm not trained in it, no one I know irl is trained in it, and I don't have the gear. I don't feel comfortable trying it, as much of a novice as I am, without someone who knows what they're doing to show me the ropes IRL

@KenE I don't plan to switch back unless the Pony should fail. I plan to finish the dive and ascend using the Pony, and leave the 80's remaining gas (~600-700 psi, my usual ascent gas) as an emergency backup and for use with my BCD inflator. In the event the 80 fails, I orally inflate my BCD and surface with the pony. If the pony fails, I switch back to the 80 and surface. If both fail....someone up there hates me and there's really nothing for it at that point. A final note, my stow point for the reg is on a chest D-ring, where I would see a reg freeflow. Since this is the most likely source of failure (current pushing the purge button) I can eliminate that risk.

@NothingClever I've said it above as well, but I want to respond to everyone individually cause you've taken the time to comment and I want to take the time to respond, but basically, I'm just trying to get some use out of a piece of gear that would otherwise never be used except in emergencies. Increasing bottom time is a happy side effect, redundancy is the goal.

@MidOH All of these are great options; none of them are within my budget. Someday....

@Wibble You are, of course, right that the bottle I'm using isn't the biggest, but 19 cu ft isn't exactly puny either. I've done the math. Going off double my SAC, I could ascend from 100 ft and complete a five minute safety stop while not exceeding 30ft/min and still have a solid margin for error. I've also calculated that, at my normal SAC rate and a depth of 40 ft (the deepest I go for what I'm doing in this instance), I should have 15 minutes of gas time on my pony. That seems like enough to me. Do you agree? Lastly, I've also seen at least one post about a diver grabbing the wrong reg, and when I read it I decided to automatically get in the habit of a "five minute check" where, no matter what else I've got going on, I check my SPG five minutes after splashing. If I grabbed the wrong one, I've still got plenty of air in the pony and can switch back without trouble.

Yep we had a diver drown here in "shallow" water because his pony tank was part of his gas plan and when he went to breath off it the volume wasn't there. Unbalanced tank gas plans are almost always pretty bad. For instance, breath pony first, still descending and its nearly empty (that's the plan right?)... Switch to the main tank and discover the 2nd stage diaphragm is forked. Does the pony have enough gas for you to exit safely? No the whole plan is you burned through the reserves tank first. So now you are near maximum depth on a tiny nearly empty tank.

My plan is designed to always have redundant gas supplies. I won't even touch the pony until near the end of my second dive (except to check its function and of course in emergencies), at which point I will still have ~700 psi in my 80, which should be more than enough reserve for 40 ft of water.

@MrChen That is my goal. Based on my stated plan, do you feel I'm not doing so sufficiently, and if so, why?

@scubadada I also use mechanical SPG's, so no worries about fancy AI computers in my case. Perhaps eventually, but not today.

@chrisjur What sort of failures should I be concerned about, given my plan? This is not a rhetorical question. I'm new (17 dives) and I don't know what I don't know. I think I've proofed this plan against anything less than a double reg failure, but if I'm wrong, I'd sure love to know it before I find out the hard way.

@justinthedeeps I've actually considered a 30 or a 40, but a 19 is what I found, and I figured it's a good thing to have. If it does turn out that the value of the extended time isn't worth it, I can always shop for a 40 and keep the 19 as a strict pony for when I don't want to use it for extensions (Like, say, a 60 foot dive, where my pony would be strictly for emergency use)

@grantmac I own all my tanks (2 Al 80's and an Al 19) and I dive a Cressi Aquawing BPW BCD with a 38 pound wing)

 

[rjack321]

This is pretty much par for SB

"Hey I have this great idea what you all think of it?"
"Yeah no not so great, there's a simpler better way"
"Well I have 10 dives now and I think its great so Imma do it anyway"
"Carry on"

 

[Wibble]

@Wibble You are, of course, right that the bottle I'm using isn't the biggest, but 19 cu ft isn't exactly puny either. I've done the math. Going off double my SAC, I could ascend from 100 ft and complete a five minute safety stop while not exceeding 30ft/min and still have a solid margin for error. I've also calculated that, at my normal SAC rate and a depth of 40 ft (the deepest I go for what I'm doing in this instance), I should have 15 minutes of gas time on my pony. That seems like enough to me. Do you agree? Lastly, I've also seen at least one post about a diver grabbing the wrong reg, and when I read it I decided to automatically get in the habit of a "five minute check" where, no matter what else I've got going on, I check my SPG five minutes after splashing. If I grabbed the wrong one, I've still got plenty of air in the pony and can switch back without trouble.


My plan is designed to always have redundant gas supplies. I won't even touch the pony until near the end of my second dive (except to check its function and of course in emergencies), at which point I will still have ~700 psi in my 80, which should be more than enough reserve for 40 ft of water.

The most important thing is that you have thought about it. You’ve done the calculations and you know the limitations.

 

[SouthernSharktoothDiver]

@rjack321 I acknowledge that, if bottom time were my priority, a bigger tank is simpler and better. But my priority is not simply to extend bottom time. I want redundancy at low cost and encumbrance, and if I can get extra bottom time, more the better. Thus far, the issue you've raised with my plan assumes I'm using the pony on descent, then switching to the main. This is not what I'm doing. This is the opposite of what I've said I'm doing. So, in light of what I'm actually doing, is there a valid safety risk to this, and if so, what is it?

@Wibble Good to hear. Just wanted to check that my thinking was right.

Just to clarify, my major reason for this post is that I've seen a lot of people saying "don't use your pony to extend your bottom time. That's not what ponies are for." And for a deep dive, I'd agree, but for what I'm doing, I don't see an issue with this plan, but I'm not blind to the notion that there might be one. I'm open to criticism or flaws in the plan. That's the reason for this post.

 

[NothingClever]

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

@Rob9876 nailed it above.

From your first post, it seems to me more gas to increase BT had greater emphasis than the redundancy.

If I were in strong current and low vis, I’d go for the most streamlined rig I can assemble. I wouldn’t want the extra drag to impact my ability to maneuver.

Also, are your primary and back up regulators in such poor shape that you need to bring two more along?

Anyways, do what you like but your method doesn’t seem to be passing a peer review.

 

[Wibble]

@Wibble Good to hear. Just wanted to check that my thinking was right.

Just to clarify, my major reason for this post is that I've seen a lot of people saying "don't use your pony to extend your bottom time. That's not what ponies are for." And for a deep dive, I'd agree, but for what I'm doing, I don't see an issue with this plan, but I'm not blind to the notion that there might be one. I'm open to criticism or flaws in the plan. That's the reason for this post.

Your 19cf cylinder is a quarter of an ali80, or using normal values that's 19 x 28 = 532 litres.
If you're at 10m/33ft/2ata and your SAC is 30 litres/min (double 15 litres/min typical SAC) then you'd be consuming 2 x 30 = 60 litres/min.

Your little pony would last 500 (litres max volume) / 60 (stressed consumption at 2ata) = 8 minutes

Sure, it's all rounding up to be prudent, but if you've got an issue your breathing rate will go through the roof.

Aside from the small amount of gas in the 19cf pony, there's plenty of other issues with pony bottles that make them a poor choice for open circuit diving (or CCR bailout for that matter):

• where are they mounted and can you get to the valve? The popular mount them on the side of a large cylinder beloved of older UK divers (15 litre 232 bar steel cylinder with a steel 3 litre 232 bar pony) often means the valve is not accessible by the diver unless it's inverted
• mounting as a stage cylinder clipped to your harness is good, but it gets in the way and swings around
• the regulator connected to the pony needs a readable pressure gauge. Many divers use a button gauge which they cannot see in the water
• it must be regularly practised otherwise you're just as likely to forget how to use it when the crap hits the fan
• what happens if there's a freeflow on the pony reg - can you quickly turn the valve off? A freeflow will empty a small pony in a few seconds!
• where's the pony's second stage stashed? Bungeed to the side of the pony means it is a pig to re-stash.
• your primary reg (longhose?) also has the shorthose bungeed to your neck (or if an octopus, stuffed in a pocket somewhere). Where is the third regulator from the pony stashed for immediate use?

PONY cylinders in the UK stand for "Piss Off Not Yours".


For balance (I appreciate you've got the pony and anything's better than nothing), if you were sidemounting, you'd have two regulators, one from each cylinder, one would be the long hose, the other the bungeed short hose under your neck. You breathe from alternate tanks for about 10 mins either side (30 bar / 500psi). Therefore you've always got enough gas even if a freeflow happens or, perish the thought, you need to donate to some other diver. And all the other benefits of sidemount too.

 

[happy-diver]

A lot of people on here, myself included, would treat a low-vis, high current dive quite seriously.