The Testomat is a wall-mount instrument which tests softened water for residual hardness. An indicator which changes color at a predetermined level of hardness is added to the sample of water. This color reaction is monitored photoelectrically and is also visible through a window on the front of the instrument. In the event of a color change signifying an unsatisfactory result, an alarm is initiated. The testing interval and a second alarm are switch selectable and facilities for plant control and remote control are standard features.