Balanced or unbalanced?

[redacted]

I'm familiar with the engineers (and sales) description of "air balancing". The same effect is accomplished with a single mechanical spring. I guess it could be called "mechanical" balancing. In both designs, air pressure is opposed by an arrangement of springs. Except for price, it is not particularly important that one design uses two springs and the other, simpler, design a single spring.

Not quite the same thing. The mechanical spring has to be overcome thoughout the inhale portion of the breathing cycle while the air sping force is reduced as soon as gas begins to flow. The difference may be barely perceptible by the user and the venturi effect may totally overcome any difference. But they are not quite the same.

 

[DA Aquamaster]

Lets look at the difference at a practical level.

1. Pescador is correct that if the IP is stable there is little or no advantage to a balanced second stage and if properly adjusted for minimum cracking effort, all other things being equal a balanced and unbalanced second stage will breathe equally well. The best example of this would be the SP R109 and R156 where the difference is the "balanced" versus unbalanced poppet.

2. However if you put both of them on an unbalanced second stage (for example a Mk 3), you will notice an increase in inhalation effort on the R109 long before you notice or measure any increase on the R156. The drop in IP that occurs in the unbalanced piston first stage as tank pressure drops reduces the downstream force acting against the fixed spring pressure in the R109's unbalanced second stage. In the R156 the air balanced forces also decrease as the Ip drops resulting in less increase in cracking effort.

3. Finally, it is important for the second stage to still act as a downstream design in the event of a leaking HP seat in the first stage so that the extra pressure can be vented. This means the area of the balance chamber is slightly smaller than the area of the seat inside the orifice, so the dopwnstream forces maintain a slight advantage and the system is by design less than perfectly balanced. The end result is that with a large drop in IP, inhalation effort will increase slightly, but not to the same degree as with the unbalanced second stage.

So much of it is a matter of degree and of semantics, but there is a degree of air balancing occuring in the system beyond what would be considered a pneumatic spring.

The Scubapro D series regs used a center balanced design, but it was again designed with a slight downstream bias in the interest of over pressure relief.

 

[Luis H]

This is an interesting thread and I tend to agree with most of what has been posted.

I have seen some technical terms define differently in different industries and some definitions even change with time. Since I am an engineer and not an English major (heck English is not even my first language), I can't comment if there is one truly correct definition for what is a balanced first of second stage, but I do have my opinions.

To me the definition of a balanced system is a system that is in equilibrium and more specific to the discussion is that the forces are or will obtain an in equilibrium. From this general definition I then tend adapt the more specific definitions adopted by different industries.

In the case of the Scuba industry I tend to think that the definition of a balanced first stage is one that obtains equilibrium without being affected by the actual tank (source pressure).

The geometry of the Sherwood regulators first stages may look similar to some non- balanced regulators, but they do accomplish a constant IP independent of tank source pressure (a friend tested several Sherwood first stages and the IP was totally unaffected by tank pressure). The summation of the two sets of springs (the coil spring and the Belleville washer's spring) and the two pneumatic devices (the primary piston and the piston behind the floating volcano orifice), add up to balance all the forces and the outcome is a constant IP, independent of tank pressure.

A primary advantage of any balanced first stage is that the gas flow orifice on the primary closing valve can be larger, since the area of the opening times the tank pressure will not affect its operation. The larger pneumatic force of the larger area is balanced by an equal and opposite pneumatic force.


When referring to second stages I also tend to use the term pneumatically balanced in a similar fashion as most of the industry (specifically Scubapro).

I have equated the balancing chamber as similar to a pneumatic spring, but I said similar, not exactly a pneumatic spring. In general I tend to think of an actual pneumatic spring as one with a fixed amount of gas and therefore provide a linear (not to be confused with straight line) spring constant. Again this is just a definition that may vary in different industries, but in this situation the balancing chamber provides more of a proportional control force than a fixed spring constant. The force varies with gas pressure not position (spring forces/ spring constants are normally defined as a function of position change or compression/ extension).


One important correction I would like to make is that in a pneumatically balanced second stage when the demand valve opens the air source to the balancing chamber is not automatically cut off. The pressure drop discontinuity in a volcano orifice with soft seat type of pneumatic valve occurs around the perimeter of the volcano orifice, not the face of the orifice. Actually, depending on flow velocity, it occurs just outside the perimeter of the orifice. This is a very important concept to understand. The pressure all along the IP volume is relatively constant including the pressure in the balancing chamber. There is a very small pressure drop due to flow, but it is almost insignificant compared to the pressure discontinuity that occurs around the perimeter of the orifice where the soft seat is barely separating from it when it opens.

To fully understand how a pressure reducing gas valve works it almost requires a course in compressible gas flow and supersonic flow. Yes, the flow in that miniscule region where there is a large pressure discontinuity becomes supersonic and there is a static shock wave (a.k.a. a pressure discontinuity).

The conclusion is that the pressure discontinuity occurs around the perimeter (or just outside) of the volcano orifice (unless you retract the soft seat so far back that it is far from the flow path, the discontinuity will always occur outside the location of the smallest opening). Therefore, the gas source into the balancing chamber is not cut off, but instead is modulated by the average gas pressure inside the volcano orifice.


BTW, the term volcano orifice is not really an industry standard term, but I like to use since IMHO it is very descriptive to the geometry of most valve orifice (it does not apply to a Poseidon Cyklon 300 first stage).

 

[Thalassamania]

I guess that's why I've always loved the Cyklon 300.

My real favorites have been pilot actuated second stages like the Omega II (I hope Omega III rumors are for real). Your explanation opens my eyes as to why they can perform so much better - thanks.

 

[Luis H]

I guess that's why I've always loved the Cyklon 300.

My real favorites have been pilot actuated second stages like the Omega II (I hope Omega III rumors are for real). Your explanation opens my eyes as to why they can perform so much better - thanks.

The geometry description doesn't apply, but the first stage valve works in a similar fashion. Instead of a hard volcano orifice and soft seat, it is just has a hard cone seat and soft square edge orifice.

I tend to like the more common volcano orifice and soft flat seat. It is easier to reproduce or re-surface the seat if the original manufacturer quits supporting its older vintage regulators (and it works great).

 

[Thalassamania]

Sure, but with my old Cyclon 300s I can flip the first stage seat or use a piece of pencil eraser.

 

[halocline]

Just one question about the SR1 for Pescador; I believe that it is a flow through piston very much in the style of the MK25 and Atomic regs, not a floating orifice flow by design like the earlier Sherwoods. Am I incorrect in this? I don't see how Sherwood could be touting this reg as high flow or "technologically advanced" if it were in fact a continuation of the older Sherwood/MK2 flow by design.

Regarding the drop in pressure in the balance chamber of a downstream 2nd stage like the B/A, I have wondered how much pressure drop does actually occur when the valve is opened. Let's say that IP measured somewhere between the two stages drops 10 PSI during inhalation. Luis, are you saying that if it were possible to measure IP in the balance chamber, it would not also drop 10 PSI, but some smaller amount? I understand that the airflow around the edge of the orifice is fairly complex and that the flow rate is influenced by the geometry of the seat/orifice interface. I guess that's why SP went to the round piston edge in the MK10+ and MK20, as well as to the conical seat. This would increase the aerodynamic capacity (I don't know of a correct term for that) and allow greater airflow given the very tiny opening between the orifice (piston edge and the HP seat.

 

[Luis H]

Regarding the drop in pressure in the balance chamber of a downstream 2nd stage like the B/A, I have wondered how much pressure drop does actually occur when the valve is opened. Let's say that IP measured somewhere between the two stages drops 10 PSI during inhalation. Luis, are you saying that if it were possible to measure IP in the balance chamber, it would not also drop 10 PSI, but some smaller amount? I understand that the airflow around the edge of the orifice is fairly complex and that the flow rate is influenced by the geometry of the seat/orifice interface. I guess that's why SP went to the round piston edge in the MK10+ and MK20, as well as to the conical seat. This would increase the aerodynamic capacity (I don't know of a correct term for that) and allow greater airflow given the very tiny opening between the orifice (piston edge and the HP seat.

The intermediate pressure drop you see during the breathing inhale cycle is due to the first stage not keeping up with the immediate air flow demand. My observation is that the pressure dip can vary from as little as about 5 psi for the higher performance first stage to more than 15 or even 20 psi (some regulator may even be higher or lower).

I have measured this pressured drop simultaneously in several different points of a regulator and the pressure drop is the same as expected. My instrumentation is not high precision so there are limits to my measurements, but I don't expect any major pressure drops due to flow within the IP volume.

I have never measure the pressure inside balancing chamber in a balanced second stage, but it would be basically the same pressure as behind the orifice at all times. The balanced chamber is a dead end with the flow only in and out of it and a very small volume. Therefore, there would be only minimal flow pressure loss since there would be minimal flow (mostly static air inside the chamber). The pressure on both sides of the poppet would be always basically the same. The only pressure force difference would be due to the designed area difference.

As I mentioned above the large pressure drop happens just outside the perimeter of the orifice. The pressure inside the orifice is fairly uniform and about the same as anywhere in the LP chamber, including hoses, LP area in the first and second stages, etc. There are some friction and other minor pressure loses during flow, but they are small compared to the actual pressure drop in the first stage valve or the second stage valve.



BTW, the pressure drop is just out side the orifice in an outward flowing orifice. It would be just inside the orifice in an inward flowing orifice (for example in a flow through piston or a balanced diaphragm similar to the Conshelf).

 

[Tortuga68]

Q: Is 'unbalanced' the same as 'overbalanced'?

 

[halocline]

"Overbalanced" as used by aqualung and others to describe their regs as easier breathing at depth has nothing to do with air balancing or otherwise compensating a stage for variance in supply pressure. It's used by those manufacturers to describe a design in which the IP is supposed to increase at depth above and beyond the change in ambient pressure. "Over-depth-compensating" would be a more accurate description, but doesn't have the same marketing ring, does it?

I'm kind of glad you brought it up, because it gives me a chance to rant against another example of BS marketing ploys. The idea is that if the reg has a higher IP at depth, it will deliver more air and be 'easier breathing' at depth. Well, in the first place, as has been discussed in this thread, most higher performance 2nd stages are designed to compensate for changes in IP through diverting downstream air pressure and using a portion of it to provide some upstream counter force. What do you think happens when these balanced 2nd stages see higher IP downstream force from the "overbalanced" 1st stage? They simply provide more upstream counter force.

Then there's the issue of higher IP supposedly providing more flow. The problem here is that any high performance 1st stage already provides WAY more flow than any 2nd stage, or in most cases pair of 2nd stages, can handle under full purge. The MK25, for example, has enough flow potential to theoretically empty an AL80 in under 15 seconds.