This one takes a little explaining, but it is worth the effort.
The National Propane Gas Association recently sent an alert to backyard cooks. It seems that people who cook up meth steal anhydrous ammonia and pump it into small tanks usually used for propane. The association says: "These cylinders have been found in many states at cylinder exchange and refilling locations as well as in hotel rooms and mobile laboratories, where the manufacturing of this illegal substance takes place."
When the ammonia comes in contact with the brass valve usually used in the propane business (to reduce sparks), the ammonia can ruin the valve. This becomes a big problem when the meth user turns in the tanks to a recycler and the next poor backyard cook gets the damaged tank/valve.
One telltale sign that a brass valve has come in contact with ammonia is that it may turn blue-green. Note that some valves have a green thread sealing compound or a green-coated valve stem and aren't to be confused with valves that have been exposed to anhydrous ammonia.
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