Abstract: Control apparatus for swimming pools to automatically regulate chlorinator and acidifier operation, thereby maintaining a desired chlorine residual and pH level. A first liquid sample from a swimming pool along with a chlorine reagent substance and a second liquid sample from the pool along with a pH reagent are sequentially introduced into a sample cell with a light source projecting light through the sample cell onto first and second photoelectric cells. The first photocell produces a signal having a magnitude that is directly proportional to the chlorine residual in the first liquid sample, and the second photocell produces a signal functionally related to the pH level of the second liquid sample.