Stop: make sure the tank is empty ...
Of course, the old mechanical overfill prevention devices work properly. At least if they are handled responsibly. Unfortunately, there are environmental offenses committed when the operator decides to boost the pressure and the last bit flows out of the tank.
Unfortunately (or fortunately): That can no longer happen
It is late afternoon and the truck's tank is nearly empty. Sometimes, it is tempting to the unload the last bit. Driving back to the depot could be an unattractive alternative for both the driver and the transporter. Should a driver be tempted: the waTch system ensures that "the door" after emptying the hose can not be opened whether one pushes the envelope or not.
Environmentally
The tanker drivers who replenish your system work with the waTch help to follow the guidelines. Even the drivers are more aware that they are pumping environmentally hazardous substances and thereby contribute to an environmentally friendly way.
Therein, everyone benefits.