Pony vs. Doubles -- Philosophical Difference?

[hobodiver]

The one single tank failure that is not serviceable underwater is a blown o-ring on the tank neck. The diver is forced off the back gas and on to the pony when the gas is gone (usually very quickly).

In this case, would you be able to use the isolation manifold fast enough to have sufficient gas in the other tank to make it back?

It's hard to argue that 160 cf of air isn't going to provide a better safety margin than 80+30 (or whatever), particularly in a fault tolerant configuration. But what about say a 2 tank boat charter? Do you bring 2 sets of doubles to provide the same margin of safety on both dives?

 

[micklock]

An interesting combination. While the 40 makes sense when paired with the 80 in numbers, the depth at which the 80 is usable to doesn't really neccessitate the use of a pony, unless one is solo diving. While I have absolutely no problem with solo diving, I think this setup would be cleaner as just a simple set of double 80's.

There is a fine margin between the depth where a pony is not needed and doubles makes more sense than a pony. Planning would indicate this range to be about 60 to 100 fsw. For that little window, I'd just have a single handy and set of doubles handy. I'd dive the doubles if diving solo deeper than 60. Shallower than that, and it's a CESA in lieu of a buddy.

One thing about using the 80 and the 40 is it takes about 13lbs off my back compaired to two 80s. I used this set up at whitestar, 40' to 76' at the bottom of the rock crusher. When I went to gilboa I used two 80s. Today I used two 40s in the small lake I live on, 30' to 40' and the water was 48f. I just got used to diving with doubles, I can't remember the last time I used a single tank. My regs and weights are set up for doubles and most of my dives are solo. I feel better having a redundant tank instead of doing a CESA, if I dove in 100' vis I might feel different. Vis has been getting better in my lake in the last month it was almost 5' today

 

[mempilot]

In this case, would you be able to use the isolation manifold fast enough to have sufficient gas in the other tank to make it back?

In a single tank configuration, a blown o-ring at the tank renders that tank unusable, h-valve or not. This is where the pony comes into play, and hopefully the choice of size was appropriate.

With doubles, yes, the isolation valve can be closed and the gas in the good tank saved. Depending on gas planning, one should be able to turn the dive and surface with this failure on a set of doubles.

It's hard to argue that 160 cf of air isn't going to provide a better safety margin than 80+30 (or whatever), particularly in a fault tolerant configuration. But what about say a 2 tank boat charter? Do you bring 2 sets of doubles to provide the same margin of safety on both dives?

Doubles get a single tick more than the single/pony in the fault tolerance category. Prior to isolation manifolds, divers used twinsets but they weren't connected. They swapped back and forth between regulators. This graduated to the iso manifold after divers were found dead with an empty tank plumbed to the mouth, and the other tank still had gas in it. This 'independent' doubles is kind of like the large single, large pony format. The resemblence is erie if the pony is part of the gas planning. Other than for emergency use, it should be left out of bottom time requirements.

The two tank boat dive is something I used to do all the time on the set of doubles. I would plan the first dive with a turn at 3/4 on the spg. Then I use another 1/4 getting back and to the surface. That left 50% for the second dive. You just do the math based on 50% for the second dive. Let's say you used a set of double 80's. Maximum gas spent on dive one would be 80 cf. The reserve from that dive didn't get used, so now you have 80 cf for the second dive. You would turn dive two at about 53 cf and surface with 26 cf in reserve. You can plug your SAC and depth into these figures to get how much time these dives could have been. The compare the times against the RDP to see if you would be time limited or gas limited. For recreational dives, a set of AL 80 doubles works just as good as two seperate AL80's. I only look to have a big reserve on dives deeper than 60 feet, so if the second dive was shallower than that, I'd come up with around 250psi in the twin set (equiv of 500psi in a single 80). If the second dive was deeper than 60, then I plan on being at the surface with the 26 cf.

 

[mempilot]

One thing about using the 80 and the 40 is it takes about 13lbs off my back compaired to two 80s. I used this set up at whitestar, 40' to 76' at the bottom of the rock crusher. When I went to gilboa I used two 80s. Today I used two 40s in the small lake I live on, 30' to 40' and the water was 48f. I just got used to diving with doubles, I can't remember the last time I used a single tank. My regs and weights are set up for doubles and most of my dives are solo. I feel better having a redundant tank instead of doing a CESA, if I dove in 100' vis I might feel different. Vis has been getting better in my lake in the last month it was almost 5' today

I find no fault in your choice for those dives. I'd only do a CESA from 60 feet. Your configuration makes sense for your dives. I'd probably sling an AL40 and put an h-valve on the backgas, but that is just me. Put's another regulator into the budget (the downside). I used to dive like this many years ago. When I needed twins for deeper dives, I just started using them for the 60 to 100 range as well. Just easier to keep less configurations.

One suggestion after looking at your picture with the setup. You may want to consider putting a opposite hand valve on your pony. The way it is now, you'd probably have a heck of a time turning the valve on if you forgot at the surface, or turning it off to stop a massive freeflow or failure. The other option is to invert the tank and reach down your left side to the valve like we do on the rebreathers.

The rebreather has been a tremendous improvement in weight and gas. I can dive for 5 hours on 12 cf of trimix, 12 cf of O2, and 5lbs of sorb. It costs me about about $15 to do any depth, where as on OC, it would cost me in the neighborhood of $100 just for the backgas, and another $25 for the decompression mix. I shaved just shy of 100 lbs off my total kit weight as well.

There are a whole new set of issues to discuss around fault tolerance there though. It seems like you have choosen a suitable tool for the job.

 

[DA Aquamaster]

In this case, would you be able to use the isolation manifold fast enough to have sufficient gas in the other tank to make it back?

It's hard to argue that 160 cf of air isn't going to provide a better safety margin than 80+30 (or whatever), particularly in a fault tolerant configuration. But what about say a 2 tank boat charter? Do you bring 2 sets of doubles to provide the same margin of safety on both dives?

I'd add to that list a burst disc failure - whihc is more likely than an extruded or failed neck o-ring. If you replace the o-ring at each VIP and ensure the valve is a bit past hand tight, you will never experience a tank neck o-ring failure.

I do agree with you in another respect. If you can't get to the isolator quickly, you could have a problem in that all of your gas is at risk. A blown burst disc will empty a full 80 cu ft tank in something like 80-90 seconds, so figure twice that time for doubles and then you have to halve it again as you may be in the middle of the dive with only 80 cu ft total.

When I used to do lots of ice diving and very cold water diving with a dry suit, heavy under wear and dry gloves with heavy liners, I would use independent doubles (or manifolded doubles with the isolator closed) to guard against that remote possibility.

Gas management is simple 1/3rd of the first tank, 2/3rds of the second tank and then back to the first tank for the second third.

 

[mempilot]

I'd add to that list a burst disc failure - whihc is more likely than an extruded or failed neck o-ring.

I would like to make a comment here. There is no history of a burst disc (essentially an OPRV) failing below the surface. The bust disc tends to let loose during an overfill or during storage in elevated temperatures. Once the tank is in the water, cooling and breathing lower the pressure at a rate that bust disc failure is almost unlikely.

An extruded neck o-ring on the other hand can fail for a few different reasons. One, it may be old and vulnerable. However, if it was recently replaced, it may have been contaminated prior to installation. This contamination can cause breakdown and failure. Also, like you mentioned, extrusion, or not seated properly, can cause failure as well, but usually rears it's head during filling or shorthly there after like the burst disc.

The other less looked at failure of the o-ring is improper material exposed to high concentrations of O2. This can and does cause premature failure of o-rings, and I've seen it happen quite a bit.

So, the burst disc isn't much of a potential failure point once the tank is dived, but the o-ring is.

If you replace the o-ring at each VIP and ensure the valve is a bit past hand tight, you will never experience a tank neck o-ring failure.

Absolutely wrong. Sorry, but degredation of the material happens due to contamination and high O2. I've seen o-rings blow out on people gear, my own included. And it wasn't from blatent improper handling.



I am a proponent of doubles for another simple reason. 1st stages can and do fail. I've seen 1st stages start porting gas overboard due to o-ring failure in the DIN barrel. This can not be resolved underwater. Shutting down the post is the only option. With a manifold, all the gas in that tank is still available via the secondary regulator. A single tank with an h-valve allows the same procedure. Without the h-valve, or manifold on doubles, immediate surfacing is required unless a pony is available. This is why I dived doubles with a manifold, or single with an h-valve when diving below 60 feet. The pony became a neccessary component when diving below 60 with a single tank due to the threat of o-ring failure on the neck, or in the 1 in 10,000,000 chance a burst disc let loose at less than service pressure . Since the single with pony was somewhat conveluted due to the size of pony truly required, the kit was easily streamlined by using doubles. Doubles became the setup of choice for dives deeper than 60 feet, regardless of RDP. Then, then only reason to sling was for decompression.

 

[micklock]

One reason I switched to independents from a manifold was I worried about having all my back gas at risk if I wasn't able to shut a valve off quickly, I had a remote cable and practiced with it but still was confident enough. Another reason was I had a third matching tank I could take with me and swap out on the second dive. I can't reach my valves very good, but with independents is it as important to shut down a tank with a free flow or other failure? I would just switch to the other tank and end the dive.

 

[mempilot]

One reason I switched to independents from a manifold was I worried about having all my back gas at risk if I wasn't able to shut a valve off quickly, I had a remote cable and practiced with it but still was confident enough.

A lot of being able to reach the valves is proper setup of the bp and harness. This is a whole other converstation, so I won't get into it here, other than it takes some practice and fine tuning of the plate and webbing. Also, stretching the tendons and muscles in the arms and shoulders is important, as is a proper fitting exposure suit (especially drysuit).

Another reason was I had a third matching tank I could take with me and swap out on the second dive. I can't reach my valves very good, but with independents is it as important to shut down a tank with a free flow or other failure? I would just switch to the other tank and end the dive.

The ability to swap tanks is somewhat valid if you want to change mixes for the the second dive; otherwise, a single set of doubles works for both recreational dives as explained in an earlier post.

With independents, shutting down a valve with a freeflowing reg isn't as important, because you are commited to not relying on that gas in that event anyway. It can be a huge distraction though, but that is it. The important thing is proper gas planning, because you have to be ensured that your pony is of ample volume to get you from the furthest point in the dive back to the surface. This could be as simple as from the max operating depth to the surface with an appropriate stop, or from the turn inside an overhead environment, out, and then up with an apporpriate stop. Most people I see diving with Spare Airs or small ponies on 120 fsw dives are in for a real awaking when they find that their rate of breathing is twice what they planned for and they breathe down a SpareAir in a couple of minutes or end up doing a CESA when the pony runs out on the safety stop.

It looks like your choice of pony is very adequate, and you are in the minority of people I've seen with such setup. Usually, I see something like a 6 or 12 cf tank strapped to an AL80 on a sub 100ft dive, which is crazy and naive.

 

[micklock]

Thanks for the good info mempilot, I read that you should breath off the free flowing reg until it is empty and then switch to your other tank. I have three hp80s and two 40s, I drilled a hole in the backplate about 3" below the top hole and installed a grommet in my trek wing to match the hole so the tanks sit low for proper trim, so reaching the valves is a little harder. If I'm diving deeper I use two 80s.

 

[mempilot]

Thanks for the good info mempilot, I read that you should breath off the free flowing reg until it is empty and then switch to your other tank. I have three hp80s and two 40s, I drilled a hole in the backplate about 3" below the top hole and installed a grommet in my trek wing to match the hole so the tanks sit low for proper trim, so reaching the valves is a little harder. If I'm diving deeper I use two 80s.

You are right. You should breath the leaking gas down, as to not try would be a waste of good gas. I suggest you practice this skill before you rely on it though.

Sounds like you've got it squared away.