awap:
While cracking pressure pressure can be adjusted to be the same for a balanced or unbalance 2nd, doesn't the lighter spring associated with a balanced 2nd stage result in lower WOB. I'm using both R109 Adjustables and R109 Balanced Adjustables and the size and compression resistance is readily apparent when examined side by side.
I prefer the unbalanced adjustable in heavy current situations as it seems to be more resistant to freeflow initiated by current against the diaphram.
Both of these seconds are being used on Mk5s and Mk10s.
In any second stage the downstream force of the intermediate pressure air from the first stage pressing against the soft seat has to be overcome to get the soft seat to seal against the orifice.
In a standard unbalanced downstream demand valve second stage like the Adjustable (109) second stage the pressure needed to overcome the downstream force and close the valve comes soley from the mainspring in the second stage which presses the seat and poppet assembly upstream against the airflow. So a heavy mainspring is required.
The Scubapro Adjustable (109) regulator was designed in part to enable the user to adapt the reg to differing conditions (such as diving into a current, in unusual positions, etc) but also to provide a solution to the potential change in IP by enabling the spring pressure, and in turn cracking effort, to be increased or decreased to compensate for the change in downstream force.
Balancing the second stage poppet was a slightly more complex but much more elegant and fully automatic way to address the problem with changes in IP. In a balanced second stage, air is allowed to pass through the seat and poppet assembly to a balance chamber on the other side of the poppet. The IP air then can press both ways against the poppet and essentially cancel out the downstream force and virtually eliminate the effect of any change in intermediate pressure.
Since the downstream force is cancelled by a near equal force on the balance chamber side of the poppet, a much less powerful mainspring can be used.
However, the important thing to remember is that the total forces involved are still the same as they would be in an unbalanced second stage, the difference is just that in a balanced second stage, the spring force is augmented by an assist from the IP air acting on the poppet on the balance chamber side.
On center balanced designs like the Air 1, D300, D350 and D400 the air enters the poppet assembly in the middle and pushes both ways and these first stages have an extremely light mainspring as very little spring pressure is required for stable and balanced operation.
In practice, balanced second stages of both designs are never totally balanced as there is still a requirement for the poppet to open and vent excess pressure in the event the HP seat fails. So the area of the balance chamber is always slightly smaller than the area of the orifice to provide a very slight downstream action to the valve.
The various balanced poppets used in Balanced Adjustable have normally presented less resistance to air flowing through the air barrel than the various unbalanced poppets used in the Adjustable. The current S-wing poppet used in the Balanced Adjustable (as well as every other balanced second stage made by Scubapro ecept the X650 which uses a shorter version of the S-wing poppet) is quite steamlined and offers the potential for increased flow rates compared to the dura poppet used in the Adjustable. That, in combination with the elimination of any effects from changing IP accounts for a lower WOB.
As for freeflow resistance differneces, there are several varaibles to consider.
SP has also used about 4 different types of rubber purge covers on the 109 second stage and they vary somewhat in their resistance to current induced freeflow.
The levers on some of the oldest 109's also have a slightly different profile (and need to be changed if they are upgraded to BA status.)
The increased spring pressure on the Adjustable tends to increase the amount of set that occurs in the soft seat, particularly if the storage key is not used, and this inevitably means the cracking effort is increased as the knob is adjusted in to eliminate the slight freeflow that occurs as the seat takes a deeper set.
Then there is the effect of the decrease in IP that occurs. If the second stage is adjusted to a suitable cracking effort at the beginining of the dive and not adjusted again, the cracking effort (and freeflow reistance) will increase as the dive progresses as the IP will fall 4-8 psi during the dive as neither the MK 5 nor Mk 10 were perfectly balanced designs. The balanced adjustable on the other hand will maintain virtually the same cracking effort throughout the same dive.
Another possible difference is orifice adjustment which affects lever height and in turn both airflow and freeflow resistance in a current.
