Pneumatically Balanced 2nd Really True???

[DA Aquamaster]

Exactly, the amount of air flowing through the poppet is irrelevent. You need just enough flow to equalize the pressure on both sides of the poppet and the volume of the balance chamber is small so little volume is needed even if the Ip drops 20 psi or so.

Aqualung refers to the poppet as a "shuttle valve" and it is not a bad way to conceptualize it. The poppet shuttles back and forth opening and closing the valve.

In order to provde some donwstream bias and to ensure the valve is stable and will naturally return to a closed position, there is a little more pressure holding it closed than is poushing on the upstream side. The downstream bias is produced by the area of the oirifice exposed to pressure from the first stage being slightly larger than the area of the poppet expoosed to an equal pressure in the balance chamber. So, even without the lever being activated if the IP gets too high (usually around 170-190 psi) the poppet will be forced open to vent excess pressure from a leaking HP seat before a low pressure hose could rupture (rated pressure at least 240 psi plus usually a 150% safety margin - roughly a 400 psi burst pressure). The spring in the sytem on the other hand provides stability by ensuiring the valve rests in the closed position.

 

[FFMDiver]

I'm thinking the spring would provide around 90% of the opposing force and the shuttle, poppett (whatever) is providing another 10% to balance the opposing air from the 1st stage...something much smaller than the spring...but I bet its main role (besides the free flow you mention) is to act a s a cracking buffer that allows the initial burst of air to have more of a shock absorber than the big spring can provide thus significantly enhancing cracking performance...and increasing the performance of the breathability of the reg?

Exactly, the amount of air flowing through the poppet is irrelevent. You need just enough flow to equalize the pressure on both sides of the poppet and the volume of the balance chamber is small so little volume is needed even if the Ip drops 20 psi or so.

Aqualung refers to the poppet as a "shuttle valve" and it is not a bad way to conceptualize it. The poppet shuttles back and forth opening and closing the valve.

In order to provde some donwstream bias and to ensure the valve is stable and will naturally return to a closed position, there is a little more pressure holding it closed than is poushing on the upstream side. The downstream bias is produced by the area of the oirifice exposed to pressure from the first stage being slightly larger than the area of the poppet expoosed to an equal pressure in the balance chamber. So, even without the lever being activated if the IP gets too high (usually around 170-190 psi) the poppet will be forced open to vent excess pressure from a leaking HP seat before a low pressure hose could rupture (rated pressure at least 240 psi plus usually a 150% safety margin - roughly a 400 psi burst pressure). The spring in the sytem on the other hand provides stability by ensuiring the valve rests in the closed position.

 

[redacted]

With ths Scubapro balanced barrel poppet, the diameter of thge orifice is .188 inches and I measure the diameter of the balance chamber as about 1/8 incn or .125 inches. The proportion of the force provided provided by the pneumatic balancing should be proportional to the surface area which is proportional to the square of the radiuses. Ignoring the little hole, I think that works out to the balance chamber pushing upstream with about 50% of the force pushing downstream on the LP seat. So the spring has to provide the other 50%, maybe a little more.

I was under the impression bases on some discussions on this board and tests by Phill Ellis (dive Sports) that the Apeks design may provide even a greater proportion of the force from the balancing effect; to the point where the provision of an effective emergency release of excessivly high IP may be compromised in some cases.

I would think that balancing has relatively little effect on cracking presure (with my 109 and 156 cracking pressure seem to be about the same) but should have an effect on the work related to the rest of the inhale portion of the WOB cycle. Balance or unbalanced, the force maintaining the seal should be the same - whatever it takes for stability.

 

[FFMDiver]

Pi R squared (area of a circle)???

should be proportional to the surface area which is proportional to the square of the radiuses.

 

[redacted]

Pi R squared (area of a circle)???

The Pi(e)s cancel. Unless they are chocolate cream.

 

[FFMDiver]

Well, 50/50 - good. I learned a lot, thanks folks..

 

[Luis H]

With ths Scubapro balanced barrel poppet, the diameter of thge orifice is .188 inches and I measure the diameter of the balance chamber as about 1/8 incn or .125 inches. The proportion of the force provided provided by the pneumatic balancing should be proportional to the surface area which is proportional to the square of the radiuses. Ignoring the little hole, I think that works out to the balance chamber pushing upstream with about 50% of the force pushing downstream on the LP seat. So the spring has to provide the other 50%, maybe a little more.

I was under the impression bases on some discussions on this board and tests by Phill Ellis (dive Sports) that the Apeks design may provide even a greater proportion of the force from the balancing effect; to the point where the provision of an effective emergency release of excessivly high IP may be compromised in some cases.

I would think that balancing has relatively little effect on cracking presure (with my 109 and 156 cracking pressure seem to be about the same) but should have an effect on the work related to the rest of the inhale portion of the WOB cycle. Balance or unbalanced, the force maintaining the seal should be the same - whatever it takes for stability.

You beat me to it, but here is what I wrote earlier.
I just took the dimensions with a dial caliper.

Well, I don’t know about the Apeks, but here are some numbers for the Scubapro Balanced adjustable.

Volcano orifice ID = 0.193 inches, therefore Area = 0.0292 sq in.
Using an IP of 140 psi the pressure force is F = 4.09 Lb

The balancing chamber ID = 0.158 inches, therefore the Area = 0.0196 sq in.
Again using an IP of 140 psi the balancing pressure force Fb = 2.74 Lb.

The ratio of Fb to F is 2.74 Lb / 4.09 Lb = 67.1 %

Therefore, based on the parts that I measure, the majority of the force comes from the pneumatic balancing chamber allowing a much smaller mechanical spring.

In some ways the air pressure in the balancing chamber can be though of as a self adjusting pneumatic spring. It is not exactly a pneumatic spring, but for the purpose of this discussion it is a reasonable description.

There are performance advantages on the balancing pneumatic chamber design and in addition the use of a lighter spring tend to save the soft seat.

One advantage of the self adjusting pneumatic spring is that changes in IP don’t have as much effect in performance as with a mechanical fixed spring.


Edit:

The difference between your 0.188 in and my 0.193 in doesn't seem that significant, but from 0.125 inches to 0.158 inches seems strange. Have they changed the balancing chamber size?
I am using G250 service kits.

 

[redacted]

Edit:

The difference between your 0.188 in and my 0.193 in doesn't seem that significant, but from 0.125 inches to 0.158 inches seems strange. Have they changed the balancing chamber size?
I am using G250 service kits.

I beat you to it because I used a simple ruler. I'm sure your measurement is more accurate.

 

[DA Aquamaster]

Awap is correct that under stable IP conditions the unbalanced 109 and balanced 156 designs can produce inhalation (cracking) efforts that are identical. A even better comparison is the balanced G200B and the unbalanced G200 as the two regulators are otherwise identical and neither is adjustable. In this case the difference in performance begins to show only when there is a change in intermediate pressure. This creates a situation where the change in pressure has much less effect on the balanced poppet, but a much greater and more noticeable effect on the unbalanced poppet due to the fixed spring pressure comprising all the opposing force and in essence becoming increasingly excessive as the IP (and the related downstream force acting against the poppet and spring) drops. In the balanced R156 and unbalanced R109 the same effect occurs - but the adjustment knob allows the spring pressure to be adjusted to reduce the spring pressure mechanically and enables the diver to manually recreate the balanced situation as the IP drops during the dive as would occur with an unbalanced first stage.

 

[Luis H]

Awap is correct that under stable IP conditions the unbalanced 109 and balanced 156 designs can produce inhalation (cracking) efforts that are identical. A even better comparison is the balanced G200B and the unbalanced G200 as the two regulators are otherwise identical and neither is adjustable. In this case the difference in performance begins to show only when there is a change in intermediate pressure. This creates a situation where the change in pressure has much less effect on the balanced poppet, but a much greater and more noticeable effect on the unbalanced poppet due to the fixed spring pressure comprising all the opposing force and in essence becoming increasingly excessive as the IP (and the related downstream force acting against the poppet and spring) drops. In the balanced R156 and unbalanced R109 the same effect occurs - but the adjustment knob allows the spring pressure to be adjusted to reduce the spring pressure mechanically and enables the diver to manually recreate the balanced situation as the IP drops during the dive as would occur with an unbalanced first stage.

A balanced second stage will also perform better if the first stage IP swings any noticeable amount during the inhalation cycle.

Granted most respectable 1st stages would probably only drop about 5 to 10 psi during a normal breath cycle (the MK-25 may be less, I don't know), but even a small amount of pressure drop will create a small force imbalance with a conventional down stream second stage.

During heavier breathing this transient IP drop can be much more noticeable with some first stages.

A pneumatically balanced 2nd stage will tend to compensate for that.