The Pure Light™ is a go/no-go indicator for the quality of pure water from purification systems including service deionization, reverse osmosis, and distillation. Under normal conditions, the green light is illuminated, indicating that the water is in the acceptable purity range. When the purity falls below the user selected set point, the indicator light switches from green to red. The user can activate the timer function to also alarm based on time and prevent organic or biological fouling of the system. This easy to interpret, go/no-go, signal alerts the user to take appropriate action.