Balanced vs unbalanced 2nd's

[Packhorse]

I have never dived deep on a unbalanced 2nd stage but I read that they are harder to breath from at depth.
This I do not understand. They are still depth compensated, They only difference is they do not compensate for changes in IP.

So is all this talk about unbalanced 2nd stages breathing harder at depth just hype or am I missing something? (assuming a balanced 1st)

 

[hg frogman]

Yes, that's more than 50% hype, but not completely.

First, "balanced seconds" are not really (fully) balanced, because otherwise the second stage would never open in case of an overpressure, and the hose would explode. "Balanced" seconds are actually "downstream override" seconds, they use a small balance chamber to reduce (but not to annihilate, as if they were fully balanced) the force needed from the spring to close the valve. It allows for a slightly lighter poppet, and because the spring is softer it also reduces the wear on the seat when the 2nd stage is not used.

The softer spring means a smaller cracking pressure but not that much smaller. "Balanced" seconds are then (marginally) better breathers than "unbalanced" ones, but that comes at a price : more complexity (especially for servicing them), more price, and more sensitivity to salt, sand, and so on.

For air flow the "balanced" seconds are often a bit better than unbalanced ones (like 1600 liters per minute for a Scubapro G250V opposed to 1200 liters/mn for a Scubapro R190, if I'm not wrong) but many unbalanced seconds can be good enough at depth provided they're nicely tuned. This bigger flow doesn't always come so much from balancing the second stage, simply manufacturers tend to offer higher-end casings for balanced seconds.

An advantage of balanced seconds is, they often have a cracking pressure knob that you can turn to prevent a free-flow in case of a strong current, or if your reg is getting detuned (this knob increases the pressure of the spring by making it shorter). But this also adds complexity, more o-rings, and a potential for failure.

Another point is that "balanced" seconds are much less sensitive to IP variations. This can be a drawback (your second doesn't tell you immediately of an IP creep in your first stage) or an advantage (some first stages, eg the Aqualung Legend, are "overbalanced", ie their IP increases at depth "to make breathing easier", and so they require "balanced" seconds, also for the octopus).

In fact the "balanced" vs "unbalanced" question is much more important for the first stage. No doubt that only the balanced first stages provide enough flow at depth in most circumstances. For second stages this is more of a "marginal improvements" vs "simplicity" choice, IMO.

Finally there is one exception in the world of unbalanced second stages, namely the Mares Abyss which is unbalanced yet hat an enormous flow thanks to a Mares patented feature named "bypass" (or "vortex assisted design"). This reg has been down to 313 meters (more than 1000 feet) underwater with Mark Ellyatt, so you (and I) should be able to breathe from it at depth.

 

[halocline]

I have never dived deep on a unbalanced 2nd stage but I read that they are harder to breath from at depth.
This I do not understand. They are still depth compensated, They only difference is they do not compensate for changes in IP.

So is all this talk about unbalanced 2nd stages breathing harder at depth just hype or am I missing something? (assuming a balanced 1st)

It's basically hype. There's no difference between unbalanced and balanced 2nd stages in terms of depth compensation. What frogman said about most balanced 2nds being higher flow, by overall design and not because they're balanced, is true.

If you want to experiment, take 2 scubapro 109s, convert one to balanced/adjustable, and take them both on a deep dive. You'll find it difficult to tell the difference between them at any depth. With a stable IP from the 1st stage, you can theoretically set the cracking pressure on each exactly the same, although I have found in practice there seems to be a slight difference. This could just as easily be differences in my individual regs (I own a couple of unbalanced ones and a couple of balanced ones). Awap says he can't tell the difference between his unbalanced 109s and his balanced ones, and he's a pretty sharp guy.

 

[captain]

Just unneeded complication.

 

[redacted]

I suspect there is some truth in the claim. Unbalanced 2nds tend to be lower end models using the classic downstream design which tends to limit performance unless the manufacturer employs special design features like Mares does. But one thing that I believe is true is that unbalanced 2nds have to use a heavier machanical spring than balanced 2nds. (Yes, the G250 and the R190 do use the same spring but under quite different initial tension.) While you might be able to tune both to the same cracking pressure, it is still going to take more force for additional increments of seat movement with the heavier spring than with a lighter spring. But much if not all of this inherent difference can be overcome with well designed venturi effects. The Scubapro 109/156 is probably the cleanest comparison as it isolates the balanced/unbalanced variable from other design features. Freshly tuned with a fairly new LP seat, it is hard to detect much difference in the water. But I do find myself tweaking the unbalanced versions more often than the balanced versions, I sure due to some seat set. I think I can see a little difference on a magnehelic but that is hard to tell.

I am surprised that the flow rate of a G250 is so different from an R190. They both use the same orifice and operate at the same IP. I suspect that some of the difference noted above may be from the 1st stage as the R190 is often associated with a Mk2 and the G250 with a higher performance balanced 1st.

 

[hg frogman]

Awap,

I may be wrong about those flows. That's what I got from some Scubapro documentation and I didn't check it thoroughly. I agree it's surprising. Maybe it comes from the shape of the poppet and barrel (nothing to do with the balancing anyway). Or maybe it comes from using two different first stages as you suggest.

Cheers.

 

[Packhorse]

Thanks for the answers guys. Pretty much confirming what I thought.

This has not been my experience. I have converted several 109s to 156's and required new G250 springs on all of them. From memory the 109 spring is thicker and stronger.

 

[redacted]

Thanks for the answers guys. Pretty much confirming what I thought.



This has not been my experience. I have converted several 109s to 156's and required new G250 springs on all of them. From memory the 109 spring is thicker and stronger.

I'm sorry. I meant 190 nor 109. I can't tell you how many times I have screwed that up. Yes, the 109 uses a clearly heavier spring. I corrected my post also. Thanks

 

[LeadTurn_SD]

I have never dived deep on a unbalanced 2nd stage but I read that they are harder to breath from at depth.
This I do not understand. They are still depth compensated, They only difference is they do not compensate for changes in IP.

So is all this talk about unbalanced 2nd stages breathing harder at depth just hype or am I missing something? (assuming a balanced 1st)

Interesting question. My experience is it is just hype. I'm sort of a "throw back" I guess, but all things being equal I prefer "simple" over more "complex" designs. But there are some current balanced adjustable 2nd's out there I'd love to try....

I've taken unbalanced classic downstream 2nd's quite deep. If properly tuned they've worked just fine. Better than fine actually

If you have a balanced 1st stage, there should be no change in IP to be concerned with.

I think the main "advantages" a balanced adjustable 2nd stage brings to the table are the ability to tune the regulator a little closer to freeflow, and more important, the ability to DE-tune the regulator in case it does freeflow a little , and the ability to use a lighter spring. The ability to detune (strong current, etc) I think would be the biggest selling point for me.

I currently dive both balanced adjustable (Zeagle Flathead) and unbalanced classic downstream (Mares Abyss) regs.... well the Mares is slightly different from a regular classic downstream due to it's design that optimizes venturi effect... Anyway, despite the Zeagle Flathead being a fantastic reg, I seem to prefer the way the Mares breaths, and usually use the Zeagle on a stage.

Best wishes.

 

[Luis H]

As mentioned above, a pneumatically balanced second stage is not 100% balanced it should have enough bias to bleed any over pressure.

The so called non-balanced on the other hand is should be tuned to be mechanically balanced during service. That is to say that the pressure force and spring force are supposed to be tuned to balance each other.

Here is something I wrote before. A lot of this has already been covered above, but I like bullet form.

In short here are pros and cons:

Un-balanced second stage:
Pro
• Simpler, one less O-ring
• Seat can be flipped over or replaced with generic seat from most any manufacturer (at least in the Scubapro 109)
• If properly tuned breath great with a balanced first stage.

Con:
• The spring has to be strong enough to hold air pressure and when not in use it pushes on the seat poppet putting wear on seat.
• It doesn’t stay in tune as long due to spring pressure.
• Breathing performance is affected by IP variations


Balanced:
Pro:
• Not affected as much by IP changes. Initial cracking effort is the same as in an unbalanced, but it should breathe a bit better during the full breathing cycle (most people do not notice the difference with properly tuned regulators).
• The spring is much lighter so it doesn’t push as hard on the seat and therefore holds a good working tune for a lot longer period of time (probably many years versus a few years between service intervals).

Cons:
• It is a bit more complex with one extra O-ring sealed chamber.
• The seat is not generic and needs to be purchased from the manufacturer.


Over all the difference is subtle, but I do like the balanced since I have too many regulators to service as often as they should and balanced seem to hold the tune a bit longer. I also like the mechanism and do not find it at all complex.

If you use a balanced second it is not a bad idea to have an unbalanced octopus… it is not essential since the regulators are not perfectly balanced (they will bleed any over pressure), but I am concerned what will let go first.

Diver adjustable second stages are also often associated with balanced, but they are not an integral part of it. I also like adjustable second stages because it does give the diver more options.


This is probably one of those threads that may be good as a sticky since the subject comes up often.