Method and system to measure and monitor the grey water content in a rotating concrete mixer drum mixer truck using a sensor attached to the interior of the concrete mixer drum. By measuring the grey water content before a batching process takes place, the batched water (and cement content and admixture type and content) can be modified in order to maintain expected performance of the batched load in terms of both strength and rheology, or more simply, some or all of the grey water can be discharged from the concrete mixer drum before batching.