If you're looking for a basic, inexpensive second stage, the Scubapro R095 is the most economical regulator in the Scubapro lineup (but it is a solid performer under pressure).
The R095 has a classic downstream valve design and doesn't have any adjustment knobs. The regulator has been tuned for best performance straight out of the box, so it's a simple matter of plugging it in and off you go.
This regulator is popular for stage and deco tanks, as it has a reversible hose attachment!
