Opti-Therm Rack Monitors. The OPW Opti-Therm rack monitors are used to automatically recognize the type of overfill system (optic or thermistor signal technology) that is being used on a transport. When connected to the transport, the monitor checks the status of sensors within tanks or compartments. Overfill detection systems such as the 8460 provide automatic warning of product overfill detection at predetermined levels and warn of a pending overfill condition.