Beware- Meth cooks are getting the propane tanks from the tank exchange stores and emptying them of the propane. They are filling them with Anhydrous Ammonia which is used in the 'crank' cooking process. After they are finished with them they return them to the store. They then get refilled with propane and sent back for you and me to buy. Anhydrous Ammonia is very corrosive and weakens the structure of the tank and valve. It can be very dangerous when mixed with propane and hooked up to our RV's or grills. So always inspect propane tanks for any blue/greenish residue around the valve areas. If it is present, refuse to purchase that one. The pic below shows a valve from a tank that was corrupted by ammonia. Note: Sherwood valves contain a green coated valve stem. Additionally, a green thread sealing compound is used on some valves. These valves should not be confused with those that have been exposed to Anhydrous Ammonia.