Since 1998, ASME tanks or any RV propane tank installed on motor homes, as well as standard DOT propane cylinders, are required to come equipped with overfill protection devices that prevent a dangerous amount of propane from being filled into tanks.
Because of the nature of propane as a volatile gas that expands under increased temperatures, the overfill protection device, or OPD valve, only allows an RV propane tank to be filled to 80 percent capacity. Dave Solberg opens up an RV propane cylinder that would typically be mounted to a trailer to show you how the overfill protection device works in typical RV propane tanks. He demonstrates how the mechanism operates, and discusses the importance of such a simple component.
The OPD valve automatically causes the RV propane valve to stop filling once it has reached its allotted 80 percent in order to excess expansion and dangerous explosions that could occur when you are least expecting it.
All RV propane tank models constructed since 1998 utilize a triangular knob, which signifies that the appliance is equipped with an overfill protection device. If you take your RV propane tank into a certified filling station, the technicians are required to refuse to fill any model that has a round or otherwise shaped knob.
Thus, when purchasing a used RV, you should check your model to determine whether your model has a working OPD valve and swap it out for a newer, more modern unit if it was built before 1998. Likewise, if you notice that your OPD valve is malfunctioning or you hear any strange noises coming from inside the ASME tank, you should consider having the system repaired prior to visiting a certified refill station. Remember, this applies to all propane tanks, including DOT cylinders used for standard backyard grilling units.