An inkjet printing system, fluid ejection system and method thereof are disclosed. The fluid ejection system includes a fluid ejection device and a determination module to determine a supply condition based on the count value output by the converter module. The fluid ejection device includes a fluid supply chamber to store fluid, an ejection chamber including a nozzle and a corresponding ejection member to selectively eject the fluid through the nozzle, a pressure sensor unit having a sensor plate to output a voltage value corresponding to a cross-sectional area of an amount of fluid in the ejection chamber. The fluid ejection system also includes a converter module to output a count value corresponding to the voltage value output by the pressure sensor unit.