1st Stage Balancing -Am I correct?

[kikifromcayman]

This is what i understand to be true!PLease let me know if you guys agree/ disagree, and why???? -thanks!
Unbalanced Piston 1st Stage:

-Is NOT affected by changes in depth ie: it will breathe the same at the surface as it will at 100ft’ (This is because as Depth increases , IP increases, and this compensates for the Increasing Density of the air)

-IS affected by changes in tank pressure! ( Force = Pressure x Surface Area at either end of the piston)Gets Harder to breathe as Tank pressure drops

Balanced Piston First Stage:

- Is NOT affected by Depth (Exact same reason as Unbalanced Piston)

-IS NOT affected by Changes in Tank Pressure ( 1 End of the piston has its surface area reduced to a Minimal amount therefore no force is applied to it)

There is no such thing as an Over Balanced Piston.

Unbalanced Diaphragm 1st Stage (Poseidon):

- Is NOT affected by changes in Depth (Same reason)
- IS affected by changes in tank pressure ( IP Rises and Gets easier to breathe as tank pressure drops) This Rise in IP is more Dramatic than in an ‘Aqualung’ Overbalanced Diaphragm.

Balanced Diaphragm First Stage:

-is NOT affected by Changes in DEPTH (same reason)
-Is NOT affected by changes in Tank Pressure ( Balancing chamber at end of poppet + equal surface areas)

Overbalanced Diaphragm ‘Aqualung’ 1st stage

-IS affected by changes in depth ( IP Rises quicker than normal due to a 2nd larger diaphragm)
- Is Not affected by Changes in Tank Pressure, (same reason as Balanced diaphragm)

 

[redacted]

Basically right but a few finer points to consider. All first stages are balanced for ambient pressure as you have said. But unbalanced pistons tend to employ smaller orifice sizes to minimize the the effect of changing tank pressures. So, due to restricted gas flow, they may tend to breath a little harder as depth increases. Also, older "balanced" pistons were not perfectly balanced so there usually is a small change in IP as tank pressure changes. I think we are talking something less than 5 psi as opposed to the 10 to 15 psi change in the unbalanced piston. Newer balanced pistons have modified the shape of the piston shaft to minimize or eliminate this small change. Some may refer to that design as "overbalanced".

 

[pdoege]

Your statements appear to be correct.

Various regulators are better/worse at balancing and coping with tank pressure than others but the physics is the same.

Peter

 

[kikifromcayman]

Thanks for the replies awap and pdoege, that makes me feel more sure of things! Would you guys also agree with this statement:

A balanced second stage will be easier to breathe from....Period, it is not affected by depth or tankpressure.Even on the surface you will notice a balanced second stage is easier to breathe from (cause its pnematically/Mechanically )balanced

An unbalanced second stage is always harder to breathe from due to a thicker spring, it is not affected by depth BUT as tank pressure falls Inhalation effort increases!??????????

By the way what is the most common way of 'Mechanically' balancing a second stage??(I only really know about pnematically balancing them) Are the vast majority pnematically balanced??Pros and Cons of each???

 

[redacted]

No, I would not. I dive older Scubapro metal adjustable regs. Some are balanced and some are not. The balanced 2nd should and (properly tuned) probably does breath better, at least on a machine, but I usually have a hard time telling the difference.

I believe the concept of "mechanically balanced" generally applies to a classic downstream design with an added spring which is user adjustable. End result is much like my old Scubapro adjustable barrel poppet (unbalanced) in terms of how it functions.

 

[kikifromcayman]

so are you saying that in theory my statement should be correct but in reality you dont notice it??

 

[Charlie99]

Just my 2 cents worth -----

I was initially confused about unbalanced vs. balanced simply because "balanced" means different things in a 1st stage vs. a 2nd stage, and "overbalanced 1st stage" is referring to something completely different than balanced.

Balanced in reference to a first stage means that it has compensation such that IP is relatively constant with changing tank pressure. Both balanced and unbalanced 1st stages compensate for depth and keep IP at more or less a constant pressure above ambient pressure.

Balanced in reference to a 2nd stage means that is has compensation with respect to IP. This means that the cracking pressure will change little as IP changes.


Another point of confusion is that some manufacturers refer to a 1st stage that increases IP pressure faster than ambient pressure increases as being "overbalanced". In this case, the "overbalanced" does not refer to compensation for tank pressure, but instead means overcompensation for depth, and when deep the IP pressure above ambient increases.

 

[redacted]

so are you saying that in theory my statement should be correct but in reality you dont notice it??

Yes, I tend to have a hard time recognizing the performance differences. I'm sure some of this is because some performance differences will only become apparent under ceertain diving conditions.

 

[pescador775]

"Balancing" and "compensation" are different and are not interchangable. Balance refers to a device used in the first stage of diaphragm regulators called a balance valve. This device is a chamber which isolates the poppet stem from variable tank pressures. Balance in the context of piston regulators refers to a flow through piston which is protected by its inherent design from being influenced by variable supply pressure.

Compensation refers to the regulator's ability to vary its IP when exposed to variable ambient pressures. As the water pressure increases, a corresponding force is exerted on the regulators' internal components which are exposed to the wet, and the regulators mechanicals force a rise in IP. Normally, this pressure rise follows closely the rise in ambient absolute pressure. There are exceptions.

A few years ago, manufacturers started tinkering with compensation. They figured that a bias to the compensation mechanism such as to direct a non linear rise in IP would be beneficial. Thus, in a regulator with a first stage of this type the IP rises faster than ambient. Generally, this rise approximates a 20% slope which accumulates to about 30 psi "extra" pressure at 200 feet depth.

The manufacturers call this "overbalanced" which is a misnomer. There is no connection between the mechanics of compensation and the first stage balance valve. The non linear pressure rise can be called positive compensation or over compensation but not overbalancing which is nonsense, basically commercial propaganda (advertising).

Positive compensation is achieved by installing an extra diaphragm larger than the primary diaphragm, the one which acts in concert with the main spring. The larger diaphragm is in contact with the wet but the primary diaphragm is dry. Due to its larger size, the exterior diaphragm can exert a larger force which must be countered by the smaller diaphragm. The primary diaphragm reacts by commanding a higher IP than that which corresponds to the actual surrounding water pressure. Only by doing this and increasing the line pressure can the diaphragm shut off the flow of air.

I believe there are other designs aimed at producing positive compensation. I took a look at a SP MK 18 because the particular drawing showed a single diaphragm design advertised as "overbalanced". There is a mysterious disc located on the inside of the diaphragm. It is possible that this disc is there to produce a bias to the diaphragm's flexibility. If so, it would mean that there would be less resistance to flexing inward than outward requiring a higher IP to restore the diaphragm to neutral. I don't know, just a guess.

 

[pescador775]

so are you saying that in theory my statement should be correct but in reality you dont notice it??

No, the statements are incorrect. Unbalanced diaphragms are becoming rare but one of the highest performing regs was the unbalanced Poseidon Cyclone. Unbalanced pistons such as the SP MK 2 are still around and have very high performance. However, at tank pressures below 300 psi the cracking effort may increase. This is a safety factor effectively warning of low tank pressure. The cracking effort of unbalanced diaphragm designs actually drops at low tank pressure. These regs are generally easier to inhale at low tank press.