to upgrade an 109 to BA

[Angelo Farina]

What you are describing is caused by the drop in IP during inhalation. In a balanced 2nd stage, IP is providing the lion's share of the upstream force against the poppet opening. When this drops during inhalation, so does the upstream force, allowing the valve to stay open with less opposing force.

Effectively I am using old MK5 first stages. The pressure gauge shows a significant drop of the IP when pressing the purge button.
However, when breathing normally, the IP gauge only shows a minimal pressure drop, from 140 to 130 PSI.
Do you think that this pressure drop is enough for causing the balanced second to work better than the unbalanced?

 

[AfterDark]

This:
Scubapro parts. S style poppet assembly with spring, LP seat, & balance chamber | eBay

Three years ago I converted 3 of my 4 109s to 156s I don't notice a difference between any of the 4. They are all connected to their own MK5s except the 109 it is my secondary, it evens gets paired with my 2 stage double hose when I'm buddy diving with it.

 

[Open Ocean Diver]

What you are describing is caused by the drop in IP during inhalation. In a balanced 2nd stage, IP is providing the lion's share of the upstream force against the poppet opening. When this drops during inhalation, so does the upstream force, allowing the valve to stay open with less opposing force.

Do you think the balance chamber has any effect once the valve is opened?

 

[lexvil]

Do you think the balance chamber has any effect once the valve is opened?

I think of it more alone the line as a closing enhancer/assist to make up for the weaker spring.

 

[halocline]

Do you think the balance chamber has any effect once the valve is opened?

Sure, it's pushing back against the poppet just like when the valve is closed, but with slightly less force because IP is lower.

 

[halocline]

Effectively I am using old MK5 first stages. The pressure gauge shows a significant drop of the IP when pressing the purge button.
However, when breathing normally, the IP gauge only shows a minimal pressure drop, from 140 to 130 PSI.
Do you think that this pressure drop is enough for causing the balanced second to work better than the unbalanced?

Yes, any pressure drop in the balance chamber is going to affect the way the poppet acts. How much, and how much in comparison to other forces....I don't know.

 

[Open Ocean Diver]

Sure, it's pushing back against the poppet just like when the valve is closed, but with slightly less force because IP is lower.

I would of thought once the valve cracked open the air would take the path of least resistance and go around the the little hole in the poppet.

 

[lexvil]

I would of thought once the valve cracked open the air would take the path of least resistance and go around the the little hole in the poppet.

By ending your breath you switch everything around.

 

[rsingler]

Do you think the balance chamber has any effect once the valve is opened?

Angelo Farina's pertinent comment

It's a dynamic flow issue. And yes, balanced makes a big difference.

 

[halocline]

I would of thought once the valve cracked open the air would take the path of least resistance and go around the the little hole in the poppet.

Discounting any venturi forces acting on air inside the poppet and balance chamber, the pressure in the balance chamber should drop exactly as much as IP does. Air doesn't need to travel into the hole in the poppet when the valve opens, it's already in there. But you bring up an interesting idea. Does the movement of air across the seat into the air barrel create a low pressure area which draws more air out of the poppet and balance chamber, resulting in a pressure drop in excess of the IP drop? I bet Luis would know that one. I don't. It seems reasonable that there could be some venturi effect of air moving laterally across the seat, but I really don't know.