A method of purging air from a tank of a refrigerant recovery unit , the method comprising the steps of: a) opening , with a controller , a purging orifice on the tank to release a gas mixture contained within the tank of a refrigerant recovery unit via a purging valve ; b) operating a timer to track multiple time intervals during which the purging orifice is open, each time interval having a beginning time and an ending time; c) determining an initial value of a system variable at each beginning time and a subsequent value of the system variable at each ending time, characterized in that , the system variable is a mass of the gas mixture: and the method further comprises d) measuring continuously a pressure of the gas mixture to derive an average pressure over each time interval; e) deriving a characteristic value of the gas mixture based on a change in the system variable from the initial value to the subsequent value measured over each time interval wherein the characteristic value is an average density of the gas mixture derived from the average pressure and the change in the system variable over the time interval or wherein the characteristic value is a volume percentage of a refrigerant in the gas mixture derived from the average pressure and the change in the system variable over the time interval; and f) closing , with the controller , the purging orifice via the purging valve if a rate of change of the characteristic value over sequential time intervals is greater than or equal to a predetermined threshold rate of change value. a) opening , with a controller , a purging orifice on the tank to release a gas mixture contained within the tank of a refrigerant recovery unit via a purging valve ; b) operating a timer to track multiple time intervals during which the purging orifice is open, each time interval having a beginning time and an ending time; c) determining an initial value of a system variable at each beginning time and a subsequent value of the system variable at each ending time, characterized in that , the system variable is a mass of the gas mixture: and the method further comprises d) measuring continuously a pressure of the gas mixture to derive an average pressure over each time interval; e) deriving a characteristic value of the gas mixture based on a change in the system variable from the initial value to the subsequent value measured over each time interval wherein the characteristic value is an average density of the gas mixture derived from the average pressure and the change in the system variable over the time interval or wherein the characteristic value is a volume percentage of a refrigerant in the gas mixture derived from the average pressure and the change in the system variable over the time interval; and wherein the characteristic value is an average density of the gas mixture derived from the average pressure and the change in the system variable over the time interval or wherein the characteristic value is a volume percentage of a refrigerant in the gas mixture derived from the average pressure and the change in the system variable over the time interval; and f) closing , with the controller , the purging orifice via the purging valve if a rate of change of the characteristic value over sequential time intervals is greater than or equal to a predetermined threshold rate of change value. <EOS>