differential pressure difference between balanced and unbalanced

[diverui85]

There's hundreds posts about balanced and unbalanced regulators online with detailed explanations of the differences except a clear physics explanation of why having the HP inlet and the LP exit on two opposite chambers instead of being in the same chamber makes that big of a difference.
I see hundreds of vague explanations online when it comes to the core differences such as:

• "A balanced first stage routes medium pressure air to both sides of the valve-spring assembly, so that the effort to move the spring is consistent and unaffected by variations in external conditions"
• "A balanced first stage regulator offers consistent air flow regardless of depth and tank pressure."
• "the spring and high pressure seat system in the first stage of an unbalanced regulator responds (opens and closes to allow air through the first stage into the second stage) to tank pressure/ambient pressure instead of the controlled intermediate pressure flow of a balanced first stage."
• "With unbalanced piston first stages, the air from the tank pushes on the valve, adding to the amount of force required to close the valve. As the tank empties, there is less force pushing on the valve, and less force is needed to close the valve."

can someone who really understands the inner working give a detailed explanation of the physical differences in how differential pressure changes between the two below designs? Please no vague explanations as there are plenty online.

 

[inquis]

I'm not sure what to add, as the bullets seem to tell the story (especially the first and last ones).

In the balanced reg, the HP air is shut off as soon as the force in the IP chamber (IP pressure * piston area) equals the sum of ambient + spring forces. Just those two quantities. Taking a breath lowers the IP pressure (dropping the force since the area is unchanged), opening the piston allowing a bit more pressure into the IP chamber until equalized, at which point the piston shuts off the HP path (no more pressure increase). That's why IP is always a fixed amount over ambient.

For unbalanced, the force to overcome -- and therefore the IP force at equilibrium -- is due to ambient and spring forces, PLUS the HP inlet force on the seat (tank pressure*seat area). The latter pressure (and therefore force) decreases throughout the dive, so the IP force (and therefore pressure) must also decrease throughout the dive.

 

[Eric Sedletzky]

Look at the balanced design and how the HP air comes in and pushes onto the sides of the piston stem, it can’t open or put any force on the piston to try and make it open. What opens the HP seat is when you lower the LP side by inhaling on a second stage, then the piston slides back letting in air. As soon as pressure hits the IP it closes. So spring pressure is not affected as much on balanced pistons just by the way air is introduced into the 1st stage. But they do go down slightly from full tank to empty, they are not actually 100% balanced.
Still, It’s a brilliant design with still only one moving part. If you notice, all balanced piston designs have to use both ends of the piston, that’s why the air inlet is always on the side.

Now look at the unbalanced design and how the HP air comes straight into the piston HP seat. The spring in the unbalanced design is the only thing that’s regulating and maintaining the IP pressure. As the tank pressure drops the spring tension stays the same so it’s harder for decreasing tank pressure to push back the piston and you get decreased flow and lower IP. In order for the unbalanced design (as it is) to become balanced, the spring would have to lighen at the same rate as the tank goes down to maintain equality, but that’s just theoretical, there would be no way to make a variable spring that changes tension.

Balanced diaphragm models are even more interesting with their two opposing springs and a balance chamber.

 

[diverui85]

Thank you, both explanations finally made it clear to me. I have not found similar explanations online so thank you.