We discovered that the calibration for transducer #3 (the low-pressure ~7 psi side of the gas supply) was wrong.  I suspect that it was doing integer division.  The graph below is the result after fixing the setup.

The pressure on the tank regulator (pressure 2) started dropping as soon as it was valved off.

The alarm level is raised as soon as pressure 2 drops below 100 psi.

Shortly after that we opened a vent valve up on the rack.  You see pressure 2 and 3 drop quickly.

We closed the vent valve. Pressure 3 drops more slowly at this point and gets down to about 1 psi.

I tried to valve the tank on slowly so see the pressure rise.  On this timescale it happens pretty quickly.

During the entire exercise the values shown here closely matched the readings on the regulator gauges.