A washing machine uses a turbidimeter to measure turbidity of cleaning water for controlling the duration of its washing and cleaning cycles. Quality of this control is improved by taking measurements when the water flow is weak so that the effects of foams are negligible and waiting until turbidity drops at the beginning of the cycle to detect the initial value used in subsequent steps. Sensitivity of the turbidimeter is automatically adjusted for accuracy when the operation is temporarily stopped and restarted during a cycle.