I should know better than to disagree with
@day_trippr , but I disagree. The bank of 4 regulators look like secondaries to me.
The information below assumes you are looking at the regulators straight on as in the IMG_0956 pic.
The primary regulator, the one attached to your CO2 tank, has a high-pressure input at 3 o'clock. That's the pressure directly from the tank, and that high pressure runs through the primary regulator to the 9 o'clock position where the high pressure gauge is installed. That high pressure gauge will read 500 PSI until the tank is nearly empty, and the reason for that is beyond this thread. For now just accept that the high pressure gauge will read about 500 PSI until the tank is close to empty. The primary regulator regulates down the high pressure to the range of 0-60 PSI, which is called the "secondary pressure". The secondary pressure is present at 12 o-clock where the secondary gauge is installed (it probably ranges 0-60 PSI) and is also present at 6 o-clock where the Y and the two shutoff valves are located. If you shut off the valve on the tank, then close the secondary valve on the left, then open the secondary valve on the right you'll bleed off the pressure from the secondary part of the regulator. At this point the secondary valve (12 o-clock) should read 0. Now open the tank valve and CO2 will hiss out the secondary port on the right. Quick close the secondary valve on the right because you don't want to waste CO2. The secondary valve will jump up to a number, probably 20 PSI since you said that's where it was set. This means there is 20 PSI of CO2 (secondary pressure, or low pressure) available at those two ports at the Y.
The bank of regulators you have I believe are secondary regulators. This means they take low pressure from the secondary side of the regulator on your tank and allow you to independently regulate down the pressure to each output on the secondary regulator bank. Looking at the bank of secondary regulators, the secondary pressure from the Y at the tank regulator feeds through the bank of regulators, from the 3 o'clock position to the 9 o'clock position. This provides secondary pressure to all of the secondary regulators in the bank. If you put a
regulator gauge at the 9 o'clock position on the far left side of the secondary bank it should read the same as the secondary gauge on the tank regulator. Because that should be the same pressure. This is where the fun begins. Each of those regulators on the bank has a gauge at 12 o'clock, a valve/output at
3 6 o'clock, and an adjustment screw in the middle. The adjustment screw allows you to adjust the output of that regulator independent of the other regulators. You should be able to adjust each of the regulators in the bank from 0 PSI to n PSI, where n is the secondary or low pressure from the regulator attached to your tank (shown on the 12 o'clock gauge on the tank regulator).
Be aware that adjusting CO2 pressure with regulators (squishy) is not like adjusting tension with a turnbuckle (firm). It will take some time for adjustments to settle, and there is hysterisis. Let's say you pressurize a keg to 12 PSI for a few weeks then start tapping off beer. The pressure might drop to 10 PSI before the diaphram opens up and equalizes the pressure to 12 PSI. Just like your home heating system where the set point is 68 degrees. The furnace may kick on at 68 and kick off at 70. This is a long way of saying, make small changes to secondary pressures for your kegs or you will overshoot and undershoot your desired settings.
Didn't expect this to be so long, but there it is.
Made some
edits edits because I wrote too quckly then read it later. Sorry.