An apparatus for delivering a process fluid comprises a storage vessel, a process pump for pumping the process fluid from the storage vessel, a hydraulic drive system comprising a hydraulic drive unit operatively connected to drive the process pump, a pressure sensor for measuring hydraulic fluid pressure of the pressurized hydraulic fluid in the hydraulic drive system, and, an electronic controller programmed to monitor a signal representative of hydraulic fluid pressure that is measured by the pressure sensor, process the signal to determine when the storage vessel is empty, and stop the process pump when the storage vessel is empty. The disclosed method relates to using the disclosed apparatus to measure hydraulic fluid pressure in the hydraulic drive system, process measured hydraulic fluid pressure to determine when the storage vessel is empty, and stop the process pump when the storage vessel is empty.