A dialysis machine useful in hemodialysis can process or treat a reverse osmosis water flow received through the machine to prepare a dialysate. The machine can include an additive source to introduce an additive, such as bicarbonate, to the reverse osmosis water flow. The machine can include a sensor in fluid communication with the additive introduction point that can measure the conductivity or similar characteristic of the solution. During a first time period when additive is actively introduced to the reverse osmosis water flow, the sensor can measure a relatively high conductivity value. During a second time period when additive is not introduced to the reverse osmosis water flow, the sensor can measure a relatively low conductivity value. The dialysis machine can include a controller that processes these measurements to assist control and operation of the machine.
