A method for determining the grey water content remaining in a mixer drum from a previous concrete load that has been discharged from said mixer drum to provide accurate batching of a new concrete load into said mixer drum, said method comprising: providing a concrete mixer drum having an interior volume and an axis of rotation; providing at least one sensor mounted in a location in said interior volume of said mixer drum such that during a revolution of said mixer drum, the sensor separately achieves both a submerged state in any water in said interior and an unsubmerged state and generates a signal indicative of each said state; rotating the concrete mixer drum such that the sensor separately achieves both a submerged state in any water in said interior and an unsubmerged state; determining the angle between the axis of rotation of the concrete mixer drum and horizontal; determining a submersion fraction that is the fraction of time or the fraction of a vessel rotation in which the sensor is in contact with the grey water, or an inverse fraction that is the fraction of time or the fraction of a vessel rotation in which the sensor is not in contact with the grey water, that the sensor achieves based on the rotation of Step; providing data correlating the submersion fraction or inverse fraction to the corresponding volume of water in a vessel that is substantially geometrically similar to said mixer drum; determining the grey water content in said mixer drum by comparing the submersion fraction determined in Step with said data of Step and the angle between the axis of rotation of the concrete mixer drum and horizontal determined in Step; and either creating an alert if the grey water content detected is greater than a pre-defined limit, modifying the percentage of water in said new concrete load, based upon the grey water content determined in step, discharging at least a portion of the grey water from said mixer drum based upon the grey water content determined in step, or a combination thereof, preferably wherein either: said concrete mixer drum is mounted on a truck; or said at least one sensor is an electrical resistivity sensor, an electrical capacitance sensor, an acoustic sensor, a microwave sensor, a nuclear resonance sensor, or a light sensor. providing a concrete mixer drum having an interior volume and an axis of rotation; providing at least one sensor mounted in a location in said interior volume of said mixer drum such that during a revolution of said mixer drum, the sensor separately achieves both a submerged state in any water in said interior and an unsubmerged state and generates a signal indicative of each said state; rotating the concrete mixer drum such that the sensor separately achieves both a submerged state in any water in said interior and an unsubmerged state; determining the angle between the axis of rotation of the concrete mixer drum and horizontal; determining a submersion fraction that is the fraction of time or the fraction of a vessel rotation in which the sensor is in contact with the grey water, or an inverse fraction that is the fraction of time or the fraction of a vessel rotation in which the sensor is not in contact with the grey water, that the sensor achieves based on the rotation of Step; providing data correlating the submersion fraction or inverse fraction to the corresponding volume of water in a vessel that is substantially geometrically similar to said mixer drum; determining the grey water content in said mixer drum by comparing the submersion fraction determined in Step with said data of Step and the angle between the axis of rotation of the concrete mixer drum and horizontal determined in Step; and either creating an alert if the grey water content detected is greater than a pre-defined limit, modifying the percentage of water in said new concrete load, based upon the grey water content determined in step, discharging at least a portion of the grey water from said mixer drum based upon the grey water content determined in step, or a combination thereof, preferably wherein either: said concrete mixer drum is mounted on a truck; or said at least one sensor is an electrical resistivity sensor, an electrical capacitance sensor, an acoustic sensor, a microwave sensor, a nuclear resonance sensor, or a light sensor. said concrete mixer drum is mounted on a truck; or said at least one sensor is an electrical resistivity sensor, an electrical capacitance sensor, an acoustic sensor, a microwave sensor, a nuclear resonance sensor, or a light sensor.