What’s the deal... SP 108 HP verses 156BA

[tbone1004]

@halocline called Hooke's law.
Spring Force=Spring Constant*delta length

 

[Open Ocean Diver]

I understand how they work I’ve read the bible. (Still learning mostly from this great site) I know an air balanced downstream valve uses a lighter spring to significantly reduce the work of breathing... so it is said. They are partially balanced because it still needs to work as a safety valve in the invent of IP over pressure.

Since I can’t get in the water I was just playing with my toys and trying to see If I could measure the benefit of air balancing, with these to regulators. So far I can’t, admittedly I’ve only been in 15 foot pool with the 108 for some subjective testing.

I took a dowel and pushed on the seat through the orifice on both, I didn’t really find much difference in force to open the valve. However what I plan on doing when I store long term is to back out the orifice.

I have a 50 year old never used single adjustment downstream regulator that still seals tight and works great. Go figure...

 

[Angelo Farina]

The cracking pressure does not depend on the balanced/unbalanced poppet, it is just the "extra"pressure you apply to the spring beyond the equilibrium point, which is the point were the seat is still closed, but the forces acting on it are summing to zero.
In an unbalanced reg, these forces are just two:
- the spring, which pushes the poppet against the orifice.
- the air exerting the IP against the seat, pushing the poppet away from the orifice.
Some extra spring load is always required, for avoiding that the reg starts free flowing easily.
In a balanced second stage, at the equilibrium point, there is a third force: the air which entered the balance chamber and pushes the poppet against the orifice.
As the balance chamber has a diameter slightly smaller than the orifice, this force is always smaller than the second one, hence the spring needs to exert the missing force.
This means that the second stage is not really balanced, but it still seals even if the IP increases, and does not become too hard to breath if the IP decreases.
This is the real benefit of a balanced second stage.
However, if the IP is truly constant (as using a properly serviced MK5), then this advantage vanishes. You cannot measure any difference in cracking pressure if the two regs are tuned for the same value...

 

[halocline]

I understand how they work I’ve read the bible. (Still learning mostly from this great site) I know an air balanced downstream valve uses a lighter spring to significantly reduce the work of breathing... so it is said. They are partially balanced because it still needs to work as a safety valve in the invent of IP over pressure.

Balancing a 2nd stage does not on its own reduce work of breathing. You still have the basic opposing forces of pressurized air coming from the first stage trying to open the valve, and either a mechanical spring (unbalanced) or combination of air pressure and mechanical spring (balanced) on the opposite side, keeping the valve closed. That side must always have a slight advantage or the reg will be free-flowing. The difference, as Angelo said, is that in a balanced design, any fluctuation in IP from the 1st stage is partially matched by pressure in the balance chamber. This is really important with unbalanced (or poorly balanced) 1st stages. But if you have a 1st stage that provides stable IP (within a few psi) throughout the entire supply range, there is no practical difference between two 2nd stages that are identical other than a balanced vs unbalanced poppet. The classic example of this would be a 109 with the original poppet vs one that has been updated to the s-wing poppet. It's almost impossible to tell them apart if they're connected to a MK25 (for example) and tuned the same. There is a slight theoretical difference, in that once the 2nd stage is cracked open and flowing, IP always drops; it has to by design. So in a balanced poppet, that means that the air pressure in the balance chamber will also drop, meaning that there is less force trying to close the valve. With a good 1st stage, the IP drop is really slight; more under severe demand, like gulping for air at great depth. But in normal use it's pretty slight and it's very difficult to tell the difference. There are great 2nd stages that are not air balanced.

You are right that you can't have a 100% balanced valve, meaning that the balance chamber always must be slightly smaller in diameter than the opening in the orifice. That way in the event of a major IP spike the 2nd stage will act as an over-pressure release valve and you won't blow out a hose.

 

[Open Ocean Diver]

I’m still grappling with the idea according to the book air balancing reduces the work of breathing. Because of the upstream force allows you to reduce the spring force to close the valve against the down stream force, however the lever still has to open the valve which is pushing on the lighter Spring plus the upstream force to open the valve. Equilibrium yes valve is closed all balanced, is the difference when the valve actually opens and loses some of the balancing effect the work of breathing is easier versus when it first cracks open? Hope i said that right.

 

[Couv]

Apparently the .215 spring found in the C370 is the same as a .216 but "coated", with what I don't know but apparently it's more corrosion resistant or it could just be magic SP fairy dust.

I'd be interested to know what this guy's springs are coated with.
Scubapro G250/G200B/G150/109 Balanced Second Stage Regulator Repair Parts Lot | eBay

 

[buddhasummer]

I'd be interested to know what this guy's springs are coated with.
Scubapro G250/G200B/G150/109 Balanced Second Stage Regulator Repair Parts Lot | eBay

At that price I suspect gold.

 

[Open Ocean Diver]

I'd be interested to know what this guy's springs are coated with.
Scubapro G250/G200B/G150/109 Balanced Second Stage Regulator Repair Parts Lot | eBay

 

[halocline]

I’m still grappling with the idea according to the book air balancing reduces the work of breathing. Because of the upstream force allows you to reduce the spring force to close the valve against the down stream force, however the lever still has to open the valve which is pushing on the lighter Spring plus the upstream force to open the valve. Equilibrium yes valve is closed all balanced, is the difference when the valve actually opens and loses some of the balancing effect the work of breathing is easier versus when it first cracks open? Hope i said that right.

I think you mostly have it figured out. Keep in mind that the only difference between the unbalanced and balanced poppet is that in the balanced poppet, IP provides the majority of the opposing force. As the valve opens, IP drops, therefore the opposing force drops. But it's not much. You could quantify it if you could accurately measure the area of the end of the stem of the poppet in the balance chamber, and the area of the seat within the orifice. Lets say for argument that the poppet stem has 50% of the area. That means that IP is providing half the opposing force. (It's probably more, but lets keep it simple) Assume the area of the seat is .1 square inches. That would mean the poppet stem is .05. Let's say IP is 100PSI and drops 5PSI during inhalation. That means there is 10 lbs of downstream force on the seat when closed and 9.5 when opened. For opposing force, we'll ignore cracking effort for arguments sake, and say that the mechanical spring is providing 5 lbs of force and the balance chamber 5 lbs. When the valve opens, the mechanical spring is still providing 5 lbs (not really, because it's being compressed, but we'll also ignore that for the moment) but the balance chamber is now providing 4.75 instead of 5 because IP has dropped 5%, so it's force must also drop 5%. So instead of 10 lbs opposing force, you now have 9.75 lbs. Not a lot of difference.

In reality, there is cracking effort that needs to be factored in, and the change in opposing force of the mechanical spring which gets compressed. That's beyond me! I would probably assume that the larger unbalanced mechanical spring would have a different increase (probably smaller) than the lighter balance spring simply because the unbalanced spring is much larger, and therefore being compressed a smaller fraction of it's length by the action of the lever. I could be wrong about that one. Maybe Luis will chime in and correct me as he has done many times in the past.

 

[Open Ocean Diver]

I think you mostly have it figured out. Keep in mind that the only difference between the unbalanced and balanced poppet is that in the balanced poppet, IP provides the majority of the opposing force. As the valve opens, IP drops, therefore the opposing force drops. But it's not much. You could quantify it if you could accurately measure the area of the end of the stem of the poppet in the balance chamber, and the area of the seat within the orifice. Lets say for argument that the poppet stem has 50% of the area. That means that IP is providing half the opposing force. (It's probably more, but lets keep it simple) Assume the area of the seat is .1 square inches. That would mean the poppet stem is .05. Let's say IP is 100PSI and drops 5PSI during inhalation. That means there is 10 lbs of downstream force on the seat when closed and 9.5 when opened. For opposing force, we'll ignore cracking effort for arguments sake, and say that the mechanical spring is providing 5 lbs of force and the balance chamber 5 lbs. When the valve opens, the mechanical spring is still providing 5 lbs (not really, because it's being compressed, but we'll also ignore that for the moment) but the balance chamber is now providing 4.75 instead of 5 because IP has dropped 5%, so it's force must also drop 5%. So instead of 10 lbs opposing force, you now have 9.75 lbs. Not a lot of difference.

In reality, there is cracking effort that needs to be factored in, and the change in opposing force of the mechanical spring which gets compressed. That's beyond me! I would probably assume that the larger unbalanced mechanical spring would have a different increase (probably smaller) than the lighter balance spring simply because the unbalanced spring is much larger, and therefore being compressed a smaller fraction of it's length by the action of the lever. I could be wrong about that one. Maybe Luis will chime in and correct me as he has done many times in the past.

I think it just clicked in thank you that’s a great explanation something that should be added to the book. So to summarize when the valve first cracks IP drops upstream force is less and the valve becomes easier to open with the lighter spring? Lightbulb moment.