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Is my R109 Scubapro 2nd stage balanced? The back has # 4901210. It has the all metal housing with the flow control. I'm thinking it's from the mid 80's. What are the benefits to having a balanced 2nd stage? Does one need a balanced 2nd stage if the 1st stage is balanced like an MK16. What if one has a piston style 1st stage like the MK10. I haven't purchased the 1st stage yet, so I'm trying to see what I have, and what I need for rec diving, with some diving between 75 and 100 feet.
The R109 is an unbalanced adjustable. It looks the same as the balanced adjustable version (I believe it was refered to as an R156). The only outward difference is the cover which is plain in the 109 and says "BALANCED ADJUSTABLE" in the 156. Internally, the difference is the poppet, sping, and cap or balance chamber. Converting a 109 to a 156 is a $5 spring and a service kit, cheap as long as you don't change the cover also.
Most of my rigs have an R156 for a primary and an R109 for an alternate. I believe the R109 is a little less sensetive to current and surge, therefore less likely to freeflow. For river dives, I only use the R109.
Properly tuned and adjusted, I can not distinguish between the two on performance. But I can't tell Coke from Pepsi either.
It's hard to tell from a serial number and in the mid eighties, both were available. They seem to all generically get called "109's" when, as AWAP points out, only the Adjustables are properly called "109's" as the Balanced Adjustables are 156's.
Due to the assist from the pressure on the other side of the balanced poppet, the BA uses a much lighter spring and tends to engrave the seat a lot less over time - unless of course you use the little blue purge button depressor (or a quarter wedged betwene the rubber and metal purge covers) during storage, in which case the seat is lifted off the orifice anyway and spring pressure makes no difference.
If properly adjusted, there really is not much discernable performance difference between a 109 or 156. But regs like the MK 5 and MK 10 are not totally balanced as the piston stem is the same diamater as the knife edge, so the area of the knife edge itself is actually not balanced, which causes a 4-6 psi drop in pressure as the tank empties. So theoretically 156 would need adjusting slightly less than a 109 to maintain peak breathing performance.
Personally, I like to upgrade mine to Balanced Adjustable status as they then use the current S-wing poppet as used in the S600, G250HP etc and offer performance comparable to either. The only parts needed for the update are a G250/S600 annual service kit (the older ones included a balance chamber and most dealers will have a bunch sitting around) and a spring with a dealer cost of $1.00.
Whether you decide to upgrade it or not depends on what your LDS is charging to do it. Annual service parts will run $15.00 for the annual service kit and no more labor is required for the coversion than for an annual service ($25.00 per stage at most) so the additional cost in nearly all cases is $1.00 for the spring. But...I have seen LDS's charge as much a2 $75 to upgrade a 109 to 156 status and that is basically a rip off.