how important is a balanced reg?

[DA Aquamaster]

The other issue which you might run into with an unbalanced first is that tuning on the 2nd stage could become more problematic. Probably not a big deal on the rig you are looking at because you have a way to tune/detune the 2nd, but if you are looking at a rig where the 2nd does not have a diver adjustable control, getting the tuning just right can be a bit of a pain. The reason is because the 2nd is also unbalanced, and it is subject to perform differently at differing IP pressures. If you get a heavy fill, for example, 3200 lbs instead of 3000, you are likely to lose most of the "extra" free air in freeflow, because the 2nd will have been tuned to work at 3000 lps, which translates to a higher IP pressure for an unbalanced reg.

That is the real difference you will see in performance - in an unbalanced reg the pressure in the hose that connects the 1st and 2nd stage will have a greater variation in pressure, and it is why tuning can be more difficult.

Having said that, I've dove all over the world on and old Scubapro Mark III - -an unbalanced piston reg, with an unadjustable 2nd stage, so your choice should work just fine!

A drop in IP of nearly 20 psi is normal in a Mk 3 as the tank pressure falls from 3000 psi to 300 psi. When I first started diving, the technician / owner of the LDS used to tune my Mk 3's High Performance second stage (essentially a smaller metal cased predecessor to the R190) while the first stage was connected to a full steel 72 at 2250 psi. So on the first dive after an annual service I arrived at the dive site, attached the reg to my new AL 80 at 3000 psi and was greated by a fairly noticeable freeflow. I had watched him adjust the reg while he explained the process, so I pretty much knew the drill and in about 5 minutes, despite lacking the specialty tools that make it much easier, I had managed to make my first DIY adjustment.

20 years later as an SP certified tech, I am always careful to make the final adjustment on a second stage attached to an unbalanced piston reg like the Mk 3 or Mk 2 with a supply pressure of 3300 psi.

With an unbalanced diaphragm first stage the seat is on the upstream side of the orifice so the IP increases as tank pressure falls so freeflows will occur at low tank pressures. So with an unbalanced diaphragm reg, care needs to be taken to adjust the second stage with a supply pressure of 300 psi.