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[rescuediver009]

By your definition, any scuba reg ever made would be overbalanced. I cannot think of any scuba reg ever made that did not incorporate a mechanisim for compensating for increased ambient pressure. This is in fact a defining trait of all scuba regulators and is what sets them aprart from other types of pneumatic regulators.

That would strongly suggest that your definition of overbalancing is not what is commonly meant by "overbalancing".

Sorry I meant about piston regs. What about a sherwood first stage or an SP4. Only the flow through pistons are overbalanced if they have the flow ports on the side of the reg.
As far as diaphragms go, I forgot to mention that they require the little flow port that leads behind the hp seat. Is this a little clearer? If not please so enlighten me.

#5 on the diagram...

 

[redacted]

Sorry I meant about piston regs. What about a sherwood first stage or an SP4. Only the flow through pistons are overbalanced if they have the flow ports on the side of the reg.
As far as diaphragms go, I forgot to mention that they require the little flow port that leads behind the hp seat. Is this a little clearer? If not please so enlighten me.

#5 on the diagram...

#1 is the equivilant of the ambient chamber on a piston 1st.

#5 is the balance chamber which counters the downstream force. With a "balanced" piston, there is no downstream force when once it locks up.

 

[DA Aquamaster]

Balanced diaphragm first stages use a balance chamber behind the seat carrier to compensate for the pressure of the air coming from the tank that would otherwise tend to hold the seat against the orifice. They use a small bleed hole through the center of the soft seat that provides IP air to the balance chamber. The diameter of the balance chamber is the same diameter as the orifice so that the pressure acting on each side of the seat carrier is the same.

If a diaphragm first stage is not balanced, the IP will increase as tank pressure falls as there is less air pressure from the tank attempting to hold the valve closed.

 

[Zaphod]

Eh, what? Flow ports on the side of the reg? Do you mean the ones leading to the ambient chamber? I think you kinda have to have them in a piston and it doesn't mean it's overbalanced.

I don't really think the definition of overbalanced is really all that mysterious, though. It simply means that the IP increases slightly more than 1 atm every 33ft. There are numerous ways of doing it and I'm sure every one is patented. I understand that the overbalance is about 20% in the Apeks/Aqualung design.

I remember seeing an article or TSB from Sherwood a while back that described their first stages as being slightly overbalanced (before the marketing term was invented). It had something to do with the design of their springs (the little coned washers) having some sort of non-linear spring coefficient.

The little port that goes behind the seat (to the balance chamber) only provides balancing. Just remember, that the old Aqualung designs (yes, even those with "load transmitters" and those that were open to the environment) were not, in fact, overbalanced. It has more to do with geometry than what is or isn't there.

Sorry I meant about piston regs. What about a sherwood first stage or an SP4. Only the flow through pistons are overbalanced if they have the flow ports on the side of the reg.
As far as diaphragms go, I forgot to mention that they require the little flow port that leads behind the hp seat. Is this a little clearer? If not please so enlighten me.

#5 on the diagram...

 

[Zaphod]

Technically, second stages can't (or shouldn't) be balanced. Even the "balanced" seconds are truly underbalanced. There's slightly more downstream pressure on the seat so if you get a runaway IP, it will still release. It's a geometry thing again. The area on the seat upon which the IP acts is a little larger than the area on the rear of the "shuttle valve" in the balance chamber.

Yes all theflowthough piston designs are overbalanced thanks to the holes that allow water pressure to increase the IP

There is no such thing as an overbalanced second stage. Just balanced. The ones you mentioned are balanced similarly to the entire Apeks line.

As far as what uberspeed is asking. That is where I think you would find the difference between balanced and non balanced second stages.

 

[DA Aquamaster]

I don't really think the definition of overbalanced is really all that mysterious, though. It simply means that the IP increases slightly more than 1 atm every 33ft. There are numerous ways of doing it and I'm sure every one is patented. I understand that the overbalance is about 20% in the Apeks/Aqualung design.

I fully agree with everything else you said and I am familiar with this definition of overbalancing.

I suppose this could be done relatively easily if, for example, the ambient pressure side of the piston head (in a piston reg obviously) were slightly larger than the compression chamber/IP side of the piston head. It would require a slightly stepped design, a matching step with adeqate working range in the body or cap of the reg and an extra piston head o-ring and would neccesarily increase the complexity of the design. But I don't see the point - it is not worth the trade offs.

A 20% increase in IP would be about a 12 psi difference from the surface to132 ft. That 12 psi increase in IP over ambient would require the second stage to be detuned so that it does not freeflow at depth (a very bad place for that to occur). It has been pointed out a couple times previously in this thread that even a balanced second stage still must be biased with a smaller than ideal balance chamber to ensure it still functions like a downstream second stage to vent excess pressure in the event the IP becomes excessive.

There is no doubt some slight benefit in increased flow rate due to the 12 psi increase in IP that would help offset the effects of increased viscosity of the air at depth. And the second stage would in fact breathe easier at depth due to the increased IP.

But on the downside, there are better ways to deal with viscosity such as larger and better designed air passages that will as a side effect improve flow rate in all situations. And the "improvement" you would see at depth in terms of improved cracking effort is instead just the removal of the detuning you end up with the lower IP's found at shallower depths.

It is in my opinion a far better approach to have a very stable IP (relative to ambient pressure) that is not affected by either depth or falling tank pressure. If the IP remains constant relative to ambient pressure (eg. 140 psi greater than ambient at 0 ft and 140 psi greater than ambient at 132 ft) that will enable precise tuning of the second stage for max performance and ease of breathing at all depths.

 

[uberspeed]

From what I can gather a balanced second stage is not that great an advantage. That being said, sounds like an unbalanced second stage is more susceptible to a badly tuned first stage, is my understanding off? Is it worth the $100 or so premium? I guess that's what I'm trying to get to...

 

[Zaphod]

The freeflow aspect is an interesting one and I agree. That's exactly why a downstream second stage is not recommended with an overbalanced first stage. The Aqualung rep drilled that into me at the last seminar. I'm sure you won't find an official line on this, though. This is especially true for octos like the LPO because (a) it breaths like a beast anyway, (b) it's therefore tuned to the fine edge so the customer doesn't get pissed off, and (c) it's then fairly easy for a 12 psi increase to cause a slight freeflow at depth. Downstream seconds that are generally easier to breath from (perhaps with a larger diaphragm) seem to be a little more tolerant to the issue along with the sorta-balanced-but-not-quite seconds.

If I remember correctly, the overbalancing in the Apeks/Aqualung design has something to do with the small decrease in the diameter of the "hydrostatic transmitter" plastic doohicky as it passes between the two diaphrams. The crossectional surface area of this stem seems to decrease by about 20%. Perhaps somebody with a more mechanical background can confirm (I got a C in my ME classes). My guess is with the surface area changing, but the force remaining constant across the stem, the pressure in PSI increases by 20% near the internal diaphragm relative to the external diaphragm. A look at the older Aqualung designs that had a dry sealed system and were certainly not "overbalanced" will show that their equivalent part did not have this step.

I don't know how it's done (or if it's done) with a piston.

I fully agree with everything else you said and I am familiar with this definition of overbalancing.

I suppose this could be done relatively easily if, for example, the ambient pressure side of the piston head (in a piston reg obviously) were slightly larger than the compression chamber/IP side of the piston head. It would require a slightly stepped design, a matching step with adeqate working range in the body or cap of the reg and an extra piston head o-ring and would neccesarily increase the complexity of the design. But I don't see the point - it is not worth the trade offs.

A 20% increase in IP would be about a 12 psi difference from the surface to132 ft. That 12 psi increase in IP over ambient would require the second stage to be detuned so that it does not freeflow at depth (a very bad place for that to occur). It has been pointed out a couple times previously in this thread that even a balanced second stage still must be biased with a smaller than ideal balance chamber to ensure it still functions like a downstream second stage to vent excess pressure in the event the IP becomes excessive.

There is no doubt some slight benefit in increased flow rate due to the 12 psi increase in IP that would help offset the effects of increased viscosity of the air at depth. And the second stage would in fact breathe easier at depth due to the increased IP.

But on the downside, there are better ways to deal with viscosity such as larger and better designed air passages that will as a side effect improve flow rate in all situations. And the "improvement" you would see at depth in terms of improved cracking effort is instead just the removal of the detuning you end up with the lower IP's found at shallower depths.

It is in my opinion a far better approach to have a very stable IP (relative to ambient pressure) that is not affected by either depth or falling tank pressure. If the IP remains constant relative to ambient pressure (eg. 140 psi greater than ambient at 0 ft and 140 psi greater than ambient at 132 ft) that will enable precise tuning of the second stage for max performance and ease of breathing at all depths.

 

[rescuediver009]

The unbalanced diaphragm that DA explained is shown in a poseidon regulator. I agree that the over-balancing is achieved with the flow port in behind the HP seat giving it the more than ambient increase in IP.
Zaphod, I would think that the only reason that it was suggested that the sherwoods were overbalanced, is because they have that constant bleed that is always venting (even if it is very little). I think that the conical washers just allow the moving orifice a greater range of movement giving ti the perofrmance lacking in the Brut.