A process for determining the quantity of a gaseous fluid expended to operate a process control device of a control system comprising: providing a control system having a process control instrument and at least one process control device; providing a controlled fluid source in selective communication with the process control instrument; providing a means for measuring pressure at an outlet of the process control instrument for measuring an outlet pressure from the process control instrument and communicating a measured pressure signal corresponding to the measured outlet pressure; determining an initial pressure of the controlled fluid source; supplying the fluid to the process control device; operating the control system for a predetermined period of time; characterized in that the process includes: determining a pressure of the fluid from the controlled fluid source after the predetermined length of time; communicating to a processor at least one of a signal corresponding to a difference between the pressure of the fluid from the controlled fluid source after the predetermined length of time and the initial pressure, and a first signal corresponding to the initial pressure and a second signal corresponding to the pressure of the fluid from the fluid source after the predetermined length of time; transmitting data corresponding to the fluid pressure at the outlet to the process control instrument; and calculating a difference between a value corresponding to a pressure of the fluid supplied from the controlled fluid source during the predetermined length of time and a value corresponding to the outlet pressure from the process control instrument measured by the means for measuring pressure at the outlet of the process control instrument. providing a control system having a process control instrument and at least one process control device; providing a controlled fluid source in selective communication with the process control instrument; providing a means for measuring pressure at an outlet of the process control instrument for measuring an outlet pressure from the process control instrument and communicating a measured pressure signal corresponding to the measured outlet pressure; determining an initial pressure of the controlled fluid source; supplying the fluid to the process control device; operating the control system for a predetermined period of time; characterized in that the process includes: determining a pressure of the fluid from the controlled fluid source after the predetermined length of time; communicating to a processor at least one of a signal corresponding to a difference between the pressure of the fluid from the controlled fluid source after the predetermined length of time and the initial pressure, and a first signal corresponding to the initial pressure and a second signal corresponding to the pressure of the fluid from the fluid source after the predetermined length of time; transmitting data corresponding to the fluid pressure at the outlet to the process control instrument; and calculating a difference between a value corresponding to a pressure of the fluid supplied from the controlled fluid source during the predetermined length of time and a value corresponding to the outlet pressure from the process control instrument measured by the means for measuring pressure at the outlet of the process control instrument. determining a pressure of the fluid from the controlled fluid source after the predetermined length of time; communicating to a processor at least one of a signal corresponding to a difference between the pressure of the fluid from the controlled fluid source after the predetermined length of time and the initial pressure, and a first signal corresponding to the initial pressure and a second signal corresponding to the pressure of the fluid from the fluid source after the predetermined length of time; transmitting data corresponding to the fluid pressure at the outlet to the process control instrument; and calculating a difference between a value corresponding to a pressure of the fluid supplied from the controlled fluid source during the predetermined length of time and a value corresponding to the outlet pressure from the process control instrument measured by the means for measuring pressure at the outlet of the process control instrument.