A chiller water sampling device includes a pair of flow meters and a proportional valve to provide a constant flow rate of sample water containing peroxyacetic acid from a chiller to a mixing tank. Acid can be added to reduce the pH of sample water in the mixing tank to bring the pH within the operating range of a peroxyacetic acid sensor. The sensed level of peroxyacetic acid can be used to control further addition of peroxyacetic acid to the chiller.