Balanced or Unbalanced for Pony

[LJinFLA]

I really hope this doesn't turn into debate about the use of a pony. I have already decided they make sense to me for deeper dives, regardless of who my buddy is, so please let's not derail this. I do believe that the buddy is the first and best alternate, but...

My question is this: I am looking at buying a new or used reg for my pony setup and thought that would be an easy decision. Now I am getting advice from divers and dive shops that seem to be inconflict over whether I should go Balanced or unbalanced for this particular application. There seems to be a difference of opinion. I use balanced for my primary regs. My primary is an Apeks ATX200. My octo is an ATX40. Both Balanced.

For my pony, cost is somewhat of an issue but not totally. The unbalanced appear cheaper. Is there a difference that is significant for pony usage. Are there advantages/disadvantages to one over the other in this application?

LJ

 

[Web Monkey]

There's no reason to buy an unbalanced regulator, except that it's a little cheaper. In fact, you'll be better off with a used, balanced reg that you have rebuilt than a new unbalanced reg.

The unbalanced regs breathe harder as you go deeper, and hard breathing is exactly what you don't want if you just had a failure on your primary system.

Terry

My question is this: I am looking at buying a new or used reg for my pony setup and thought that would be an easy decision. Now I am getting advice from divers and dive shops that seem to be inconflict over whether I should go Balanced or unbalanced for this particular application. There seems to be a difference of opinion. I use balanced for my primary regs. My primary is an Apeks ATX200. My octo is an ATX40. Both Balanced.

 

[redacted]

It's hard to get simpler and more reliable than a good unbalanced piston. The performance decrement due to the unbalanced design occurs as tank pressure drops and is most noticable at depth. Low tank pressure and depth should not be what a "bail out" bottle encounters. Smaller and lighter are nice too.

 

[DA Aquamaster]

You need to distinguish between a balanced/unbalanced first stage and a balanced/unbalanced second stage.

First stage:

An unbalanced first stage will not breathe harder as you go deeper, but rather will get slightly harder to breath as tank pressure falls to around the 500 psi range and noticeably harder to breathe below that. This can be a very valuable clue that you are getting low on air, particularly with a pony bottle where you go from full to empty fairly fast.

An unbalanced first stage is unvariably going to be a piston first stage as I am unaware of any unbalanced diaphragm seconds stages still in production. The unbalanced "inline" or "flow by" piston design is as simple as it gets with one moving part and no dynamic o-rings exposed to high pressure air, so high reliability and low maintenence are both traits you can expect.

On the other hand a balanced first stage may be either a piston or diaphragm design. Balanced first stages will deliver very stable intermediate pressure and generally but not always) have higher flow rates than unbalanced designs. The down sides are higher cost, higher parts count and in some respects increased maintence/or more potential for something to go wrong.

Second stage:

An unbalanced second stage usually uses a time tested down stream poppet design that uses a lever, spring and seat containe din an aspirator body so they are very simple and reliable designs. They are also easy to service and most tend to use generic sized seats that can be found anywhere and can in a pinch be substituted between brands. Most of these seats can also be flipped over and reused, something that is quick and easy to do in the field on most models with only a little training. These are very nice features to have if the reg develops a slight freeflow during your trip.

The downside is that the orifices in unbalanced second stages are sometimes smaller and they sometimes offer slightly lower flow rates. Inhalation effort is also much more dependent on intermediate pressure. So when paired with an unbalanced first stage, you will experience an increase in inhalation effort as the tank pressure falls to around 500 psi. This can be viewed as a good thing or a bad thing depending on the application.

Balanced second stages:

Balanced second stages normally use a modification of the downstream second stage design with a flow through poppet design incorporating a balance chamber to partially equalize air pressure on each side of the poppet. The balance is only partial to ensure that there is still some downstream effect so that excessive intermediate pressure can vent through the second stage.

A larger orifice can be used, so flow rates can be higher. Also the spring pressure used in the second stage is lighter so seats do not tend to encounter problems with an excessive seating groove developing quite as fast. On the other hand, poppets normally have the seats permanently attached and are very specialized and very non interchangeable between brands, so if you need to change the seat/poppet due to a slight freeflow and are someplace with no dealer and no parts, it will not be repairable until you get home.

Balanced second stages are affected to a much lower degree by changes in intermediate pressure, so there will be less rise in inhalation effort if paired with an unbalanced first stage.

Other issues:

A better question to ask with regard to a second stage is perhaps "adjustable" or "non adjustable." A diver adjustable inhalation effort can be nice in that it can be set quite high to prevent unintentional air loss during the dive, and then can be adjusted for minimum breathing effort when in use.

Almost as nice are flow vane adjustments to help prevent a massive freeflow if the second stage purge button gets bumped during the dive.

The purge itself is another consideration as you want a design that is not prone to getting easily bumped.

Size and weight are considerations and unbalanced inline piston designs tend to be the smallest and lightest first stage designs. A nice feature when the gear bag starts to get heavy.

I think there is a real advantage to having an unbalanced first stage on a pony reg, as it will warn you when tank pressure is low - very important on a pony with no SPG - as you will have several breathes with progressively higher inhalation efforts to warn you of a near empty tank. This will enable you to ascend to 15' on the pony and then make a normal safety stop and remain there until the pony gets low and then provide you with ample warning and time to make a normal 30fpm ascent to the surface.

In contrast a balanced first stage will give you virtually no warning when the pony is nearly empty as inhalation resistance will only increase once tank pressure falls below the intermediate pressure. This usually only occurs on the next to last breath or last breath with a small volume tank like a pony. This discourages saftey stops on the OOA ascent with a pony as it can leave you suddenly sitting with no air at 15 ft and force you to make a emergency swimming ascent from 15 ft. Not that big of deal, but still far less desireable than a normal ascent.

A DIN connection is also a good thing to consider as it eliminates one of the knobs sticking out from the valve/first stage combination. They tend to be both more secure and less prone to snagging fishing line etc. if you sling the pony like a stage bottle, the lack of a yoke knob eliminates the potential to turn the yoke knob rather than the tank valve in a stressful situation.

Personally my favorite pony/deco reg is a DIN equipped Scubapro Mk 3 Balanced Adjustable. The unbalanced first stage gives ample warning of low tank pressure even with the balanced second stage and the adjustable inhalation effort allows the second stage inhalation effort to be set heavy during the dive to prevent unintentional gas loss and adjusted for easy breathing when the reg is being used.

A current production equivlalent would be a Mk2 G250 or Mk 2 S600, which would have to be special ordered as SP does not package them that way normally.

A Scubapro Mk 2 R190 or Mk 2 R390 would also be good but less expensive (and in stock) choices for a pony reg as would a Aqualung Calypso. None of them have adjustabel second stages (other than flow vane adjustments) but all are very reliable unbalanced piston designs with very reliable conventional downstream second stages. All of them also have relatively light weight and compact first stages relative to other balanced diaphragm and balanced piston designs.

 

[LJinFLA]

DA Thanks for the incredible lesson on the differences. Since I will be using this mostly at depth I must agree with the Monkey that the last thing I think I would want when my primary fails at depth is a reg that I will overbreath while rushing on adrenilin. That was my original concern with unbalanced.

I do totally get your point about the warning provided by unbalanced since ponies are almost instantly empty when empty. However, I plan to sling this pony and have a SPG on a short hose so I should be aware of where I am on gas supply. My plan for the pony is first to never use it exept in an emergency (other than testing and practice), second if I have an emergency at depth and go to the pony I am going straight to the surface with no safety stop (unless I absolutley have to make a safety or deco stop because I exceeded NDL, and even then only if air allows) and third, keep my eyes on the SPG the whole trip up.

I also plan on filling the pony with Nitrox to get maximum off-gassing on the trip up since it will likely be a no-stop trip. Thus I need a reg that is EAN40 compatible right out of the box.

I can't help but notice that even with your advice about the benefits of warning with an unbalanced first stage, you still dive and recommend balanced. The Mk2, and Calypso are both Balanced according their sites.

LJ

 

[AtomicWalrus]

Consider getting an OMS deco reg for your pony bottle. I've been using one with a pony for about 6 months, and it's been great. The reg is unbalanced, but it's O2 clean and quite inexpensive. If you're diving on EAN40, you won't be deploying your pony at more than about 80 ft anyway, and I can say from experience that the OMS reg breathes just fine there (and my primary is an Atomic M1.)

DA Thanks for the incredible lesson on the differences. Since I will be using this mostly at depth I must agree with the Monkey that the last thing I think I would want when my primary fails at depth is a reg that I will overbreath while rushing on adrenilin. That was my original concern with unbalanced.

I do totally get your point about the warning provided by unbalanced since ponies are almost instantly empty when empty. However, I plan to sling this pony and have a SPG on a short hose so I should be aware of where I am on gas supply. My plan for the pony is first to never use it exept in an emergency (other than testing and practice), second if I have an emergency at depth and go to the pony I am going straight to the surface with no safety stop (unless I absolutley have to make a safety or deco stop because I exceeded NDL, and even then only if air allows) and third, keep my eyes on the SPG the whole trip up.

I also plan on filling the pony with Nitrox to get maximum off-gassing on the trip up since it will likely be a no-stop trip. Thus I need a reg that is EAN40 compatible right out of the box.

I can't help but notice that even with your advice about the benefits of warning with an unbalanced first stage, you still dive and recommend balanced. The Mk2, and Calypso are both Balanced according their sites.

LJ

 

[halocline]

DA Thanks for the incredible lesson on the differences. Since I will be using this mostly at depth I must agree with the Monkey that the last thing I think I would want when my primary fails at depth is a reg that I will overbreath while rushing on adrenilin. That was my original concern with unbalanced.

I do totally get your point about the warning provided by unbalanced since ponies are almost instantly empty when empty. However, I plan to sling this pony and have a SPG on a short hose so I should be aware of where I am on gas supply. My plan for the pony is first to never use it exept in an emergency (other than testing and practice), second if I have an emergency at depth and go to the pony I am going straight to the surface with no safety stop (unless I absolutley have to make a safety or deco stop because I exceeded NDL, and even then only if air allows) and third, keep my eyes on the SPG the whole trip up.

I also plan on filling the pony with Nitrox to get maximum off-gassing on the trip up since it will likely be a no-stop trip. Thus I need a reg that is EAN40 compatible right out of the box.

I can't help but notice that even with your advice about the benefits of warning with an unbalanced first stage, you still dive and recommend balanced. The Mk2, and Calypso are both Balanced according their sites.

LJ

The mk2 is not balanced, neither is the R190 second stage. However, I can tell you from experience that the increase in inhalation effort at low tank pressure is pretty slight at 500lbs; I did a little test before buying this reg in which I stayed on a training platform with a buddy (full tank, ready to share air if needed), breathing the mk2/R190 with 500lbs in my tank; just stayed there breathing normally, trying to guage the increase in effort as tank pressure dropped. It really becomes noticable at more like 250lbs; up until then you'll probably hardly notice it. And, despite what others might say, it's not like you can't get enough air; it's just that you'll begin to notice the breathing effort a little more. In my test, I went right down to 100lbs before I surfaced, and at no time did I feel starved for air. (In case there are readers out there concerned for my safety in doing this, I mentioned that I did this test with a buddy who knew what was going on and the whole thing took place on the training platform at about 25 ft depth.)

What DA recommended was an unbalanced piston 1st stage and the balanced adjustable 2nd stage, although I think he mentioned the R190 as a perfectly acceptable 2nd stage for this. You can get a new mk2/R190 for about $200, I think, and it has the advantage of its reputation for being very reliable even with occasional use and service.

 

[nategasser]

I also plan on filling the pony with Nitrox to get maximum off-gassing on the trip up since it will likely be a no-stop trip. Thus I need a reg that is EAN40 compatible right out of the box.

Don't you always want air in a pony?

How lousy to be at 120 feet on EAN28, run out of gas, and have a pony full of EAN36!

Nate.

 

[LJinFLA]

Thanks for the correction on the MK2 Matt. The site was pretty unclear.

Also Nate and Walrus, I did not mean to suggest that I was going to put EAN40 in the pony. I just meant to say that the reg needed to be EAN 40 ready since that is what most manaufacturers of modern regs build their regs to if they are listed as nitrox compatible(unless of course they are designed specially to be up to pure oxygen rated). I had thought that I might put EAN30 - 32 in it. The few deep dives that I have done below 100 ft I have used air in my primary tank. i guess my thinking was that if I had to go to the pony I would not be staying at those depth long enough to worry about Tox or exceeding pp limits. But your point is well taken. What if I am OOA because I am entangled and trying to free myself. I would not want to go onto too rich a mixture while I do that at depth.

Does anyone have any other thoughts about using Nitrox in the pony? What would be the ideal mixture if I should use Nitrox. Or, should I use Air or some lower mixture of Nitrox like EAN28 to get the benefits of off-gasing on the way up?

LJ

 

[DA Aquamaster]

I had thought that I might put EAN30 - 32 in it. The few deep dives that I have done below 100 ft I have used air in my primary tank. i guess my thinking was that if I had to go to the pony I would not be staying at those depth long enough to worry about Tox or exceeding pp limits. But your point is well taken. What if I am OOA because I am entangled and trying to free myself. I would not want to go onto too rich a mixture while I do that at depth.

Does anyone have any other thoughts about using Nitrox in the pony? What would be the ideal mixture if I should use Nitrox. Or, should I use Air or some lower mixture of Nitrox like EAN28 to get the benefits of off-gasing on the way up?

LJ

You have various levels of possible controversy that will result depending on the answer given.

No controversy:

Use air if it's a pony and then you don't have to sweat the PPO2's

Very little controversy:

32% makes sense as the PO2 is only 1.58 at 130 ft. and while that is more than the 1.4 recommended for the working portion of the dive, it would be fine for an ascent and under the acceped PPO2 limit of 1.6 for acent and deco. Not much controversy there even if you took a couple of minutes at the bottom to get untangled. Running out of gas due to an entanglement on the bottom is a much larger concern if you are using a small pony. (Larry's advice: get at least a 19 and a 30 is better plus can double as a deco bottle later. A 40 is even better in that regard)

Some controversy:

36% produces a PPO2 of 1.78 at 130 ft, and while it is heresy and will doubtlessly result in me being pummelled by hordes of divers who take their nitrox training as the absolute gosphel according to PADI, SSI, etc, you would be extremely unlikely to tox on 36 percent on ascent from 130 ft to and past 110 ft/1.6 PPO2. If you pushed the ascent rate a bit to 60 fpm, you are only going to be over the 1.6 PPO2 for 20 seconds plus the time you take to switch to the pony and initiate the ascent.

And in practice, given that you are probably switching from 21% at that depth, the effective PPO2 in your lungs will probably not rise over 1.6 until you have taken a few breaths or in other words before you reach 110 ft anyway. Even in the event it does, the US Navy allowed much more extreme O2 exposures for short periods of time and a minute or two at 1.78 is in all likely hood not going to kill you. No guarentee on that though, so there will be those who advise not even considering it. On the other hand, if you buy a large pony, sling it and start using 36% on ascent and at the safety stops, you add some cushion against DCS on every dive which mediates the elavated risk you would encounter on the relatively few dives and relatively small percentage of those dives that you actually do below 110 ft.

Extremely controversial (and very unsafe):

40% would result in a PPO2 of 1.97. Again, while an extreme PPO2 of approx 2 for a minute or two would maybe not result in oxtox, you have to ascend to 100 ft to get under a PPO2 of 1.6 and to 80 ft to get under a PPO2 of 1.4 so any delay on the bottom could get very problematic and could be potentially fatal. It would be one of those things that fall under the clause of being willing to breathe whatever you have when you have run out of everything else. The deco benfits are better at the safety stops, but definitely not worth the risk.

More training is always better than even the best advice on Scubaboard:

So my suggestion would also be to take Deco Procedures and Advanced Nitrox courses and gain a better understanding of high PPO2's and the potential benfits for decompression, safety stops, etc on hotter nitrox mixes and then make your own call on what to use. Until then, 32% would be a good maximum limit for the pony for dives to a max of 130 ft.

That said, and going back to "no controversy", it only makes sense to use Nitrox in the pony if you actually plan to use it for accellerated deco/or safety stops. If you are stictly using it for a pony to be used only in emergency situations, then go with air and you won't have to worry about the PPO2, at least at sane depths.

Personally, I have one deco bottle that gets used this time of year when I am not inclined to always use doubles due to long walks and steep shorelines that are a neccesary evil with the local shore diving. Since I am not doing dives outside the normal NDL's with a single tank, it functions as a pony/redundant air source and has air in it (with an MOD label of 187 ft).