Max depth for unbalanced regs???

[keyshunter]

It is good to see that some of “the usual suspects” showed up with some good explanations. For a moment this thread looked like a runaway train.

The explanation from Mattboy is very good (with the minor correction from Awap).

The only thing I would like to add is that personally, when speaking about second stages, I like to use the term “pneumatically balanced” second stages.

In a properly adjusted conventional (down stream demand valve) second stage all the forces should very closed to balanced. The spring force is basically balancing the pneumatic force caused by the intermediate pressure pushing on the soft seat. This is often referred to as mechanically balanced.

Luis,
Oceanic advertises their GT3 and Delta 4 second stages to be mechanically balanced, and self adjusting. What's that about? I have never looked inside one, but looking at the schematic, it appears to be a simple downstream with a tube which goes over the lever lock nut for the manual external adjustment. Is this correct?

 

[Luis H]

But what about all this talk about overbalance 1st stage vs balanced 1st stage? Is this a bunch of bull? Something about diving deeper with a over balance will breathe better than just balance. Then I hear that even if you have an over balanced 1st stage the 2nd stage being neumatically balanced will kill the funtion of the over balance anyway...

Anyone have any insight?

Thanks,

I thought you saw the explanation of “overbalanced” first stage in the other thread, but I guess my explanations are not always clear. I think Mattboy also wrote about the overbalanced first stages in another recent thread.

 

[halocline]

But what about all this talk about overbalance 1st stage vs balanced 1st stage? Is this a bunch of bull? Something about diving deeper with a over balance will breathe better than just balance. Then I hear that even if you have an over balanced 1st stage the 2nd stage being neumatically balanced will kill the funtion of the over balance anyway...

Anyone have any insight?

Thanks,

I hear different things about what "overbalanced" is supposed to mean. In SP's case, it apparently is a slight flare of the piston stem so that IP stays (theoretically) exactly the same throughout the full supply range. I think the MK25 does this, and I would suspect the Atomic regs do something similar. DA would know alot more about it.

With the aqualung regs (and maybe apeks) the idea is that IP increases more than the increase in ambient pressure with depth. So a much more accurate term for this would be "over-depth-compensated" in that it has nothing to do with balancing IP and supply pressure. But, of course "over-depth-compensating" doesn't roll off the tongue like "over balanced" and when would any self respecting marketing guy let accuracy get in the way of a good scheme.

Interestingly, if it was really "overbalanced" meaning IP would rise as the tank emptied, what you'd have is a regular obsolete unbalanced diaphragm. That would be a nice trick to sell as "overbalanced."

The problem with the increase in IP above ambient at increasing depth is that with a balanced 2nd, there is essentially no effect as the higher IP just works harder to keep the valve closed, and with an unbalanced downstream 2nd, you'd have to tune it so it breathed badly at shallow depths or it would freeflow at depth.

But, lets assume for a minute that they really do this, and the IP rises more than ambient. How?

 

[Luis H]

But, lets assume for a minute that they really do this, and the IP rises more than ambient. How?

That is easily accomplished with the newer environmental kit on diaphragm first stages. They use a rigid pressure transferring rod from the outer diaphragm to the inner HP diaphragm. All you need is that the effective area of the outer diaphragm be a bit larger than the area of inner high pressure diaphragm.

IMHO, as I mentioned before, this so called “overbalanced” was just an unintended side effect of the environmental kit…and marketing just ran with it.

 

[halocline]

Thanks Luis, I should have thought of that.

 

[Luis H]

Luis,
Oceanic advertises their GT3 and Delta 4 second stages to be mechanically balanced, and self adjusting. What's that about? I have never looked inside one, but looking at the schematic, it appears to be a simple downstream with a tube which goes over the lever lock nut for the manual external adjustment. Is this correct?

I am not really familiar with the Oceanic regulators, but I have also looked at their diagrams. I agree that they looked like a basic down stream demand valve. Therefore, as I mentioned IMO, they can be adjusted to be mechanically balanced.

The self adjusting part I am not sure what they mean by that, but I will try one potential explanation:
Since the IP pressure is pressure compensated, the poppet in the second stage is automatically self adjusting to the increase in IP. This happens because the back side of the poppet is exposed to ambient pressure.

This is just a guess; there may be something actually meaningful behind that statement. But heck…maybe I should have been in marketing instead of engineering…I think I could BS with the best of them.

 

[Nemrod]

But did we ever get an example of a currently produced unbalanced first stage regulator?

N

 

[Stu S.]

The unbalanced Oceanic Alpha 7 meets the Navy Spec at 198 feet. At 140 feet it seemed just fine to me. The Oceanic rep on this board says it is good down to 200, and then you need the Delta model.

I used my (single hose) Healthways Scuba Star regulator at 145 feet in the Bahamas in 1979 without difficulty. I believe it is unbalanced. I did many cold dives and some below 120 feet with the Scuba Star with no problems.

If these things were risky, I would have died multiple times over the past 31 years.

 

[Couv]

But did we ever get an example of a currently produced unbalanced first stage regulator?

N

Yes, Scuba Pro MK 2, Aqualung Calypso, Mares R2, Sherwood Brut and similar "flow by" piston first stage regulators.

No diaphragm regs built today (that I know of) are unbalanced.

 

[fisherdvm]

I believe that all the sherwood bruts made today and in the past are unbalanced first and unbalanced second stage. As far as I know, vacationers regularly take these rentals to 100 ft plus and not complaining about it, right?