Every year it is estimated that there are 600 fires or explosions caused by overfilled propane cylinders. To eliminate this problem the National Fire Prevention Agency recommended to the Consumer Product Safety Commission that they require the OPD be installed on all propane tanks. So after years, here we are, scrambling to replace obsolete propane cylinders. This ruling applies to all propane cylinders from 4 to 40 pounds. So most any cylinder sold for gas grills is involved.
The OPD is a special valve that has a float inside the tank that will close the valve when the tank is 80% full. It won't measure how much propane is in the tank, but it is supposed to keep it from being overfilled. An overfilled propane tank can explode violently because of physical damage or exposure to moderate heat. Now, while the risk is relatively low, it is real.
So, how do you know if you tank needs to be replaced? The new propane cylinders have a triangle shaped valve knob. Older models had a five-prong, circular knob. If you have the triangle knob, then you're fine. Otherwise you need to get a new tank. Of course, you'll ask, why can't you just replace the valve? Any tank older then 12 years is considered too old to be used. And for those in the between years, the cost of replacing the valve is going to be about the same as exchanging the whole cylinder.
The cheapest way to exchange your tank is through a service like Blue Rhino or AmeriGas. These services will exchange your empty, obsolete tank with a new, filled tank for about $20USD. A new tank can cost about $25USD to $30USD. Old cylinders should be taken to a dealer or recycling center. Your local government can probably tell you where to take it. Some dealers might charge you a fee to take your old cylinder. Find someone who will take it for nothing. Or better yet, try on of the exchange services. You might also want to check with local hardware stores. Some stores are offering to replace you tank for you (at a fee of course).
So, if you have one of the affected cylinders, make some calls and get it replaced before your next cookout. You don't want to be in the middle of a roast or turkey and run out of gas.