It's all about part #4 in the schematic John posted. US Divers calls it a balance chamber. That chamber puts tank pressure on both sides of the HP seat, so that whatever pressure is in the tank, it's BALANCED. The springs are there in any diaphragm 1st stage. Pistons have springs too.
I adjust the IP at full pressure, but always like to check it at around 300-500 psi as well.
Good Dives, Charlie
Ok, I see that now. My old man eyes couldn't tell what part#4 was and the description just says, "Retainer - 1st Stage." Guess I wasn't very artful in my description. Balanced it is!
I was talking with David Doubilet (Nat Geo photo guy) at a party last night (yeah, I know, name-dropping ). Apparently he used the MR12 quite a bit back in the day.
I SEE HOW IT IS. WHEN OTHER PEOPLE GO OUTSIDE IT'S "GOOD FOR YOUR HEALTH" AND "HIGHLY RECOMMENDED." BUT WHEN I GO OUTSIDE IT'S A "CONTAINMENT BREACH" AND A "HIGH-LEVEL THREAT TO PUBLIC SAFETY." OK, BE THAT WAY.
It's balanced, you don't have to have springs on both sides of the balance chamber like the RAM. The problem you have here is the oring in the balance chamber (part 6 in assembly 3)is a pita to replace. Voit intended for you to replace the assembly rather than the oring like you do in a USD reg (same oring). You can set it at any tank pressure but if you intent to set the second stage the very edge, then do so at low tank pressure. Even though it's balanced, it's not perfect and will vary some. Unlike piston regs, diaphragm regs increase IP as tank pressure gets lower....until they meet.
CJ and I were having this conversation yesterday. Hopefully, I have it right.
On a RAM, a balance chamber block replaces the DA style crown block. Although they look very similar, the DA crown block has no o-ring and no balance chamber drilled up into it. It's drilled through for the HP soft seat stem to fit into . The RAM balance chamber has drillings up into it and has a groove machined into the end for the o-ring, but the top is solid.
The RAM HP soft seat has a hole drilled all the way through the stem, but the DA's HP soft seat stem is solid. The DA's HP pin simply presses into the divot on the end of the soft seat and lifts it off the orifice. The HP pin on the RAM penetrates up into the soft seat and lifts it via an internal step. Different step depth drillings on different soft seats creates the long/short pin length issue.
As I understand it, HP air pressure finds its way around the pin, up through the soft seat stem drilling and into the balance chamber, pressurizing the tiny space to tank pressure. Booyah! You have equal air pressure on both sides. Kinda.