An OPD Valve is easily identified by having a triangular knob with the words OPD marked on the valve itself. Overfill Prevention Device fitted valves are the result of extensive research into improving gas safety for you and your gas tanks. The new OPD valves prevent overfilling of gas cylinders, making filling and using gas cylinders safer than ever before. They are also compatible with your current gas appliances so it is easy to upgrade. Older style valves were more susceptible to overfilling which leaves no room for the liquid propane to expand. As a result gas may escape, creating a potentially hazardous situation. OPD valves solve this problem by using a special float (shown right) which rises during refilling to block the filling process when the tank is 80% full. The 1998 revision of the National Fire Protection Association's Pamphlet 58 requires that all cylinders, 4 lb. (1.8 kg) through 40 lb. (18 kg), fabricated after September 30, 1998 shall be equipped with an OPD valve. Further, any cylinders of these sizes that are re-qualified must be equipped with an OPD valve. If your cylinder is over 12 years old, it must be re-qualified before it can be refilled. It will be retrofitted with an OPD valve at the time of re-qualification. You can find the date of manufacture on the upper collar of your 20# cylinder. Effective April 1, 2002, no cylinder will be filled unless it is equipped with an OPD.