Steel HP 23 vs Al 19 redundant air source

[halocline]

I'm debating between these two choices for a backmount pony mounted on the right side of the main tank upside down with a left mount valve and 40" hose run under the right arm. Main gas is HP100. Diving 90' or less in all cases....

I'm leaning to the steel HP 23.

New divers like yourself ask specific questions like this from time to time, and the responses are often much as they were in this thread, for good reason. You are debating between two options that are probably both bad decisions. I would politely suggest that you consider exactly why you want the redundant bottle, what actual purpose it serves, and what real-life problem it solves, based on the diving scenarios you are in. I would definitely discuss it with experienced technical divers that use redundant gas for specific reasons, and have many years of experience with the whole issue of redundant gas.

Here's my take: I never use redundant gas in recreational, open water, buddy diving, because I never allow myself to be in a situation where I don't have immediate access to either the surface and/or my buddy. If neither of those things are true, what you have is a de facto overhead situation, and to safely dive in an overhead situation requires far more than carrying a pony bottle. I did not have a full appreciation for this until I learned to dive in a 'real' overhead. (I'm primarily a cave diver) There is a reason that technical divers (good ones, anyhow) have trained in a whole variety of ways to deal with problems underwater. The extra gas is only a small part of it.

However, if you are truly determined to dive with two tanks on your back, forget the back mounted pony and learn to dive with doubles. It's a FAR better solution to having more gas than you need and provides real redundancy. If you absolutely insist that what you need is a pony, pick either one (the 19 or 23cft), it doesn't make a damn bit of difference, but sling it so that you have some semblance of control over it. Most people use AL bottles for bailout because they tend to change your buoyancy less than steel bottles. A 30 or 40cft bottle is probably a better investment in that it has real use as a deco bottle down the road if you go that route. But ponies do have their uses; mine is for filling tires and checking regulators.

And since you are asking for people with more experience than yourself to freely give advice, I suggest you try harder to be gracious in their responses, even if it is not what you want to hear. You're not paying for any of this!

 

[Lostdiver71]

I would go with the steel for the 20% extra gas. I personally use a 19 and sling it instead of back mount for a few reasons. The first is even with a weight on the opposite side I didn't like the trim, I also like the ability to unclip it and give the bottle to someone else and there is not always enough room in the tank holders on the boat without leaving one empty. Also changing over your BC to another tank, with the pony attached, is a real pain and if you mount it upside down there is a risk of damaging the regulator if you sit down in the wrong place. Make sure to check it on a regular basis to make sure that there are no problems and rinse the steel and dry it well, it will last you more than your lifetime. Mount it how you are most comfortable, just make sure you get familiar with it in a controlled environment, I personally always test new stuff in shallow water with no current. It really stinks to jump off the charter boat and find out that new piece of equipment that you set up is not working properly.

 

[Barnaby'sDad]

It’s already been stated, but I’ll also throw out there that if you’re going to be diving solo down to ~90’...I’d want something larger than a 19cf or 23cf. If you run into a situation where you need it, you’re going to blow through gas quickly.

That being said...of your two choices...I’d go for the AL19. It’ll likely be easier to unload whenever the times comes. Ex. No longer want it, want to get a larger tank, etc.

Regardless of what you pick...sling it. That and as people have said...the “larger” little tanks are easy to sling. Ex. The 40. I did a few solo ~100’ dives with mine. It didn’t distract me from what I was doing (teeth hunting).

 

[2airishuman]

I'm debating between these two choices for a backmount pony mounted on the right side of the main tank upside down with a left mount valve and 40" hose run under the right arm. Main gas is HP100. Diving 90' or less in all cases.

The steel 23 would provide a bit more air than the 19, but be slightly more negatively buoyant at -2.64 full and -0.66 empty. I would most likely counter this with 2lb trim weight on the left shoulder strap to counter potential rolling to the right.

The aluminum 19 is a bit shorter height wise (2" shorter) and more buoyant at -1.7 full and -0.3 empty. Not significant, but something to consider.

Land weight is irrelevant, as I'll be loading these from my truck straight to the boat.

I'm leaning to the steel HP 23.

1) You will have trouble getting HP fills in many resort destinations which will limit you to around 19 or 20 cf anyway
2) If you decide to fill it from an AL80 as many people do you will once again limit the contents to the same as you would have in the AL19
3) You will typically need a 2# trim weight with the AL19 to counter the negative buoyancy when full. The FX23 you would need another pound
4) Steel in this situation is added cost for, at best, little or no benefit
5) Usual choice for a backmounted pony is an AL40

Other posters upthread have indicated the risks inherent in a back-mounted pony which are significant

 

[2airishuman]

If someone drowns because they drained their pony bottle, then they should have been prepared to switch over to their main tank, which would have been on from the start. If you are out of air, you switch tanks. It's why you have two tanks. No need to drown at all.

There have been fatalities including one among a group of divers I know locally.

One scenario involves forgetting to open the valve on the main cylinder after testing everything and turning the air off before the dive. This in and of itself is fairly common. Then the diver breathes the pony reg by accident, gauge shows 3000 before the dive, just enough air to inflate the BC, 15 minutes later the pony is empty and no usable regs and SPG shows 0. Diver tries to switch to the pony not realizing he's already on it, breaths from an empty tank, panics

Other scenario diver loses regs at some point during the dive, sweeps, grabs the pony reg, doesn't realize it's the pony reg, breathes down the pony, tries to switch to the pony, panics.

Those have actually happened resulting in fatalities.

There is also a problem that the pony reg may freeflow without you noticing and unless you have an SPG for it you will never be able to tell something happened.

I use an AL19 and have experimented with steels and sling the pony rather than backmounting it as it is a far safer configuration

 

[divezonescuba]

Just a quick question, if it is such a hazard why is it common in the PSD realm to have a pony on your back mount? I would agree that these are a bit small as far as capacity.

I don’t think this has been answered.

Many PSD divers that use tank mounted redundant air cylinders typically have a releasable mount. They have to be able to release it themselves and provide it to another PSD if necessary.

Some of the members of the Texas State Guard dive team which we have equipped are using AL30s mounted with the Trident aluminum pony bottle mount. With this system, you pull a pin and the AL30 is released.

 

[2airishuman]

own dedicated regulator, which is the same as my primary, but different color and different color hose.

Color is difficult to discern at depth

Color is difficult to discern in poor visibility

Color is difficult to discern when someone has kicked you in the face with a fin and knocked your mask off and knocked the reg out of your mouth

 

[2airishuman]

Btw...

say you are out of gas at 30m...

1 minute to get your act together, a perfect 3 minute ascent, and 3 minutes at 5m

that is 4bar minutes + 7.5 (3 * 2.5 bar average) + 4.5 = 16 bar minutes so air used is 16 * your RMV. For a relaxed and practiced person that might be 20 but for a person breathing like a train it might be 40.

you start with about 600l so 16*20
Is ok but 16*40 is not. So any ascent that is not quite sorted is a risk and you will have to skip some of the safety stop.

Other fun includes free flows jumping in, you can easily lose 50 bar like that, and general lost gas through turning it on, testing etc. Also take care that the reg is charged when in the water and again at depth so as to avoid flooding the regulator. Check it still works at depth every so often. Otherwise you might flood it, not notice and only check at the surface until finding out it isn’t in a good state when you really need it.

I believe the purpose of a pony is to permit a direct ascent to the surface. One minute is an eternity to spend fiddling around on the bottom -- why would anyone do that? What are they doing? What possible thing would you want to do between the time you put the reg in your mouth and start heading for the surface? These calculations -- elevated RMV plus long time at depth plus slow ascent plus safety stop plus surface reserve -- aren't real world. They result in an unnecessarily large and heavy rig.

If it's that high risk of a dive it's probably best undertaken with a twinset, a buddy, or both. Or maybe not undertaken at all

 

[Barnaby'sDad]

Color is difficult to discern at depth

Color is difficult to discern in poor visibility

Color is difficult to discern when someone has kicked you in the face with a fin and knocked your mask off and knocked the reg out of your mouth

Exactly. You should literally know what each regulator is/what they go to (which cylinder) with your eyes closed...by feel/location.

I believe the purpose of a pony is to permit a direct ascent to the surface. One minute is an eternity to spend fiddling around on the bottom -- why would anyone do that? What are they doing? What possible thing would you want to do between the time you put the reg in your mouth and start heading for the surface? These calculations -- elevated RMV plus long time at depth plus slow ascent plus safety stop plus surface reserve -- aren't real world. They result in an unnecessarily large and heavy rig.

We could literally fill volumes talking about the stupid **** that people do while under stress.

Edit: Being specific...it’s VERY common for people in stressful and/or distracting situations (ex. 90’ down and something goes wrong) to fixate on one thing and ignore everything else going on around them. Ex. The pilots on a commercial flight years ago ran the aircraft into the ground and killed everyone onboard because everyone on the flight deck was fixated on a non-emergency fault indicator bulb.

 

[2airishuman]

We could literally fill volumes talking about the stupid **** that people do while under stress.

I am skeptical that one minute of gas will help. The question really is this: What problem(s) are we trying to solve with the pony? If the answer is that we need a the most reliable, redundant gas supply possible to deal with a wide range of possible problems e.g. entanglement, navigational confusion, crocodiles on the surface, 5 minutes of meditation to overcome panic, etc., then the solution is a twinset.

A pony is better understood as a backup air supply to protect against an equipment failure on the primary supply, period.