parameters such as the pressure in the liquid ampoule 12 as measured by the pressure sensor 37 and an actual flow rate of the liquid as measured by the liquid flow meter 40 the controller 60 may also control set points for the pressure control valves 22 and 32 and opening and closing operation of the valves 24 34 38 42 48 and 56 vaporized gas output by the vaporizer 58 is input to a substrate processing system 90 the controller 60 may also control operation of the temperature adjusting device 54 referring now to fig2 an example of a method 150 for operating the pressure based liquid flow control system is shown at 160 control determines whether there is a flow request for the liquid precursor if not control returns to 160 otherwise control continues at 162 and determines whether the temperature of the capillary tube t cap is equal to a desired temperature t cap _ des if not control adjusts t cap to t cap _ des at 168 control measures pressure in the liquid ampoule at 172 control adjusts the pressure p act in the liquid ampoule to a desired pressure p des corresponding to the desired flow rate fr des.