balanced second stage that important?

[redacted]

In the old days, with unbalanced first stages (many of mine currently are of that vintage), you varied the interstage pressure to fine-tune the regulator. This was fine, until the octopus was put onto the system.

Now, I understand that some will say that the balanced second stage allows the manufacturer to gain in orifice size, etc. I don't think that is correct, but there is an outside chance that there is some things done with this design that couldn't be done otherwise. But I think the main reason is the "detuning" feature for the octopus, rather than breathing performance.

SeaRat

In the old days, you had single adjustment classic downstream 2nds. The orifice was fixed and all you could do was set the lever height. I can see where you could get some fine tuning with such a setup by adjusting the IP to reduce cracking pressure.

But most modern classic downstream regs (maybe all now) and many of those oldtimers had adjustable orifices so once you got lever height right, you can do additional tuning (or detuning) with the adjustable orifice. And yes, I do understand that each adjustment effects the other somewhat, but detuning a properly tuned 2 adjustment classic downstream for use as an octo is usually just a matter of another 30 degree turn on the adjustable orifice.

Balancing (pneumatic) of a 2nd stage allows a lighter spring to be used to counter the downstream force of the IP which theoretically results in less force required to get gas flowing when you inhale. I'm not sure if anyone goes with a larger orifice opening in their balanced 2nd. All my balanced and unbalanced Scubapro regs use the same orifice.

 

[pescador775]

Good discussion, thanks, all. We need to consider that the puny forces posed by the regulator IP on the second stage poppet may not justify the use of a balance chamber for "easy breathing". I believe there are other reasons to employ balancing, however. I'll be back when I have some time.

It's Saturday, and here are some thoughts following on a comment by another esteemed member:

Balancing (pneumatic) of a 2nd stage allows a lighter spring to be used to counter the downstream force of the IP which theoretically results in less force required to get gas flowing when you inhale. I'm not sure if anyone goes with a larger orifice opening in their balanced 2nd. All my balanced and unbalanced Scubapro regs use the same orifice.

As you imply, it has nothing to do with the orifice. The second stage orifices are/were large enough and always have been; the only important flow restriction is in the hose itself. Many people seem to confuse the mechanics of balanced first and second stages. The lighter springs in the diaphragm reg's balanced first stage are associated with softer seat materials since the poppet is no longer being abused by unbalanced forces and thus can be made compliant. This may also be true with the balanced 2nd stages. However, after that, the design goals diverge. The use of a lighter spring in the balanced second stage vs the unbalanced have little to do with breathing resistance. This trend in use of lighter springs is to reduce tension on the second stage poppet while the regulator is in a static, unpressurized state. The primary purpose is to protect the soft seat from wear and tear. When the regulator is pressurized, the balance chamber acts as a pneumatic spring increasing the tension on the poppet and its seat to maximum, in other words, the same as required for an unbalanced regulator. As a bonus-- possibly unexpected by the engineers-- when the regulator is in a dynamic state, pressure drops briefly during inhalation. This also reduces pneumatic spring tension ("balancing") by a small amount and could account for the reports of "easier breathing". It is also depth compensated which should help smooth out response. However, this perception is not borne out by simulator testing when comparing balanced vs unbalanced. That does not mean that the balanced second stage is not everything that some people claim. It means that the unbalanced 2nd stg regulator is a lot better than some think.

 

[pescador775]

Squid man, I recommend that you consult Scubadiving.com. One thing I have taken from this site is that unbalanced 2nds like the R380 have high performance while maintaining a relatively simple design. It was mentioned in my previous post that the "balanced" valve is capable of reducing spring tension at the moment of inhalation. This is due to a normal drop in air pressure in the first stage and due to a venturi effect which occurs at the poppet seat. This seat, which is drilled (hollow) senses the drop in pressure and transmits this change to the balance chamber which is actually a pneumatic spring. The reduction in pneumatic spring force makes breathing "easier". Despite it design and despite what it is called, the "unbalanced" 2nd stage also is subject to balancing forces. This is known as "depth compensation". As the diver descends, the IP increases due to the mechanicals of the first stage. As ambient, water pressure increases, the second stage diaphragm is depressed which also allows compensating air pressure to flow into the regulator box where it increases the force on the back side of the poppet seat. That is why your regulator does not free flow as IP increases due to increasing depth and increasing water pressure. The higher ambient pressure which builds inside the regulator box exerts a compensating force which keeps the poppet closed until the diver inhales. When the diver inhales the picture becomes more complex. As soon as the poppet pulls away from the orifice, the force on the orifice side of the seat drops. This opening force of IP pressure is reduced and the opposing, closing force due to ambient pressure on the poppet remains. Simultaneously, the rapidly flowing air causes a "suction" (venturi) which is attempting to pull the poppet toward the orifice by cancelling out the balanced state and potentially making breathing "harder". (This can cause regulator "chatter" unless the spring rates and IP's are in synchrony). This response to flow is the main difference with the balance valve which actively compensates for this effect by reducing pneumatic spring pressure, one of the two closing forces on the poppet/seat designed into a balanced system, the other being a mechanical spring. To compensate for potentially harder breathing, the unbalanced 2nd stage is also designed to create a flow pattern which streams through the box and creates an intentional, second venturi draft which pulls the diaphragm down relieving much of the downward force on the poppet/seat of the "unbalanced" valve. The result is that the simpler, unbalanced valve also is capable of high performance air delivery.