The WL601 water level alarm is designed to give an alarm output when the level in an automotive radiator falls below a predetermined level. The unit operates by measuring a resistance to ground through the water in the radiator, and using this reading to hold the relay contact open. If radiator level drops below the probe, this is read as a high resistance and the relay contact is closed, giving an alarm output to ground. This module is designed to alarm on falling water level. Other units are available to switch on, or alarm, on rising water level or to control the level in a container between two probes.