What is the definition of a water sensor on an AC unit? A water sensor is a device that detects the presence of water in the air and sends an alert to the AC unit in order to activate certain safety measures. This could include shutting off the AC unit to prevent further damage, as well as activating an alarm to notify the user of the situation. Water sensors typically come in the form of a small plastic disc, which is placed in the air stream of the unit. When the disc comes in contact with water, it will send a signal to the AC unit. Water sensors are an important part of AC systems, as they provide an early warning of the possibility of water damage or other safety issues. They can be used for both residential and commercial applications, as a way to help protect people and property against water damage. Water sensors are an important part of any AC system, and should always be installed and regularly maintained in order to ensure the safety of the user.