A dialysis machine (100) useful in hemodialysis can process or treat a reverse osmosis water flow (302) received through the machine (100) to prepare a dialysate. The machine (100) can include an additive source (140) to introduce an additive, such as bicarbonate, to the reverse osmosis water flow (302). The machine (100) can include a sensor (150) in fluid communication with the additive introduction point (144) 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 (302), the sensor (150) can measure a relatively high conductivity value (306). During a second time period when additive is not introduced to the reverse osmosis water flow (302), the sensor (150) can measure a relatively low conductivity value (310). The dialysis machine (100) can include a controller (190) that processes these measurements to assist control and operation of the machine (100).
