The solenoid controls the co2 which in turn alters the PH. When the solenoid is energized (power applied) co2 flows. When it is unenergized, there is no co2 flow. If your setpoint is 7.2 then the solenoid should be energized somewhere around that PH level. I have mine set to straddle between 6.8 and 6.7. This way I know everything is working if it's either one. If the ph rises to 6.9 or drops to 6.6 then I know something is wrong. I'm not sure what the spread is between solenoid on and off but I know it's less than 1/10th of a point which is close enough for me. If your PH is varying more than that then something is wrong mechanically or you are butting up against your tanks natural buffering capacity. What is your GH and KH ? Do you know how much co2 in ppm is dissolved in the water column at 7.2 ?