Liquid level reading of spheric, circular or not regular section tanks needs a multi-point linearization of sensor signal.  
ATR500 relates 16 level sensor values (0…10V/4…20mA) to 16 free values of water column / liters / kg to adapt the measurement to the particular tank section.  
4 on/off relay outputs with hysteresys can be related to tank levels as alarms / indicators / actuators.