hamsiss:
What is a balanced second stage?
what is "unbalanced"
what is "mechanically balanced"
what is "pneumatically balanced"
Aren't all second stages have water in front of the diaphram to BALANCE the pressure?
If not how they are working?
In all second stages, the pressure of the air in the low pressure hose coming from the first stage (called downstream force) exerts pressure against the seat that is sealing against the orifice to stop the flow. When you inhale the lever adds the extra force needed to pull the seat away from the orifice to let the air flow out.
So...in essence, the forces acting on each side of the valve seat (often called a poppet) are "balanced" the difference is in how this is done.
In an "unbalanced" poppet design, all of the pressure holding the poppet against the downstream force comes from mechanical spring pressure. This works quite well as long as the pressure of the air coming from the first stage is constant. However, if the pressure of this air drops (as can happen at lower tank pressures with an unbalanced first stage), the spring force becomes greater than the downstream force and the lever needs to exert more force to start the airflow. To get the extra force you have to suck in harder to move the diaphragm and in turn move the lever. This is what is referred to as "mechanically balanced" - even though the second stage is considered an "unbalanced" second stage.
In a "balanced" poppet design, there is a hole going through the seat to allow air to pass all the way through the poppet to a balance chamber on the other side. The area of this chamber is nearly as large as the orifice so that the air pressure from the first stage is almost equal on both sides of the poppet. This means that the downstream force from the first stage is balanced by the air pressure in the balance chamber. The result is that very little spring pressure is needed to hold the seat against the orifice, and in turn very little force from the lever is needed to get the air flowing. Also, since both sides are connected by the hole, the pressure is the same on each side, so inhalation resistance is not affected by changes in the pressure of the air coming from the first stage. This is also what is referred to "pneumatically balanced".
Balanced poppets should however be designed so that they have a downstream bias to ensure they will vent excess pressure in the event the high pressure seat in the first stage fails. If they cannot vent the pressure, a hose could rupture. So in practice the area of the balance chamber is slightly smaller than the area of the orifice so that if the pressure of the air from the first stage increases it will, due to the greater area of the orifice, eventually have enough force to to open the poppet - the same way it does in an unbalanced design where the extra pressure over powers the mechanical spring pressure.
The diaphragm in the secodn stage just ensures that the air inside the second stage stays at the same pressure as the water outside, but it does not have anything to do with balancing the poppet assembly in the second stage.
