I was asked if I could put a simple description of how a balanced and an unbalanced diaphragm first stage work and how are they different.
Below are some diagrams of the Royal Aqua Master
(the pictures of the parts above may also help).
The green represents the HP seat.
The gold is the spring block / balancing chamber.
The red is the high pressure air.
The light blue is the low pressure air.
Notice that inside the seat (in green) there is a thin line of light blue low pressure air. That small hole down the seat fids the low pressure air to the chamber inside the spring block (notice the blue chamber).
There is a small O-ring (shown in black) in the spring block that doesn’t allow the high pressure air to leak into the LP chamber.
The seat closes on the bottom against the edge of a volcano shaped orifice.
Notice that the diameter (and therefore the area) of the orifice on the bottom is the same as the diameter of the chamber on the top of the seat assembly. This allows the seat to move up and down with the same area of low pressure air on top and below. The pressure times the area produces a force and since they are equal they balanced each other.
The high pressure of the tank does not affect the opening or closing of the valve seat. The HP air only presses around it not up or down on it.
The seat motion is only controlled by the push pin which connects the diaphragm to the seat and the springs (the pin is not shown, but it would be in the white area and then down to the diaphragm). The diaphragm motion is only affected by the low pressure air, ambient pressure, and spring forces. The IP is therefore kept constant independent of tank pressure.
The Aqua Master
The big difference in the Aqua Master is that there is no balancing low pressure chamber behind (on top of) the seat. The spring block in an Aqua Master just has a hole through it and no sealing O-ring. The seat has no hole down the middle as in the RAM.
A portion of the seat is exposed to LP air (inside the boundary of the volcano orifice), but the back side of it is being pushed by a constantly changing tank pressure.
The forces on the top and the bottom of the seat are not balanced and will change as tank pressure changes. Therefore the intermediate pressure which controls the motion of the 1st stage diaphragm will change to compensate for tank pressure changes.
Since the valve motion is up stream (into the high pressure), a lower intermediate will close the valve when the tank pressure is high (at the beginning of the dive). As the tank pressure goes down, a higher intermediate pressure will be needed to push the diaphragm and close the valve since the tank pressure is not pushing down on the seat as hard.
In a unbalanced piston first stage the seat moves down stream so the IP change is reversed as a function of tank pressure.
One method (used by US Divers) to reduce the amount IP change was to us a smaller volcano orifice. Since the amount of unbalance is a function of the difference in pressure times the area, by reducing the area it reduces the unbalancing forces. The problem is that is also the area of air flow. Therefore, there was a tradeoff.
Well this was longer than I planned. I just hope it is clear.
Any questions, something is not clear, or you see any mistakes (including spelling) you are welcome to send me a PM.
Royal Aqua Master (1st stage details):
Royal Aqua Master: