1.  Field of Invention
This invention relates to a protective cap for a threaded valve stem.  In particular, the application relates to a tamper resistant, easily removable, protective cap for a propane gas tank valve stem, that provides the purchaser with a convenient visual identification that the tank is full. 
2.  Description of Related Art
Conventionally, for propane valves, for example, new QCC propane valves, labels are used to indicate when a propane tank has not been used or has been filled with propane.  With the tanks stored outside, for example, the labels are exposed to rain, snow, wind and the like, which may cause the labels to eventually degrade or fall off.  In addition, the labels can be easily manually removed from the valve.  Therefore, it is difficult to make sure that the propane in the tank has not been used.  Furthermore, with such labels, if the tanks are stored outside, dust and/or dirt may build up on the valve.  As such, more protection is needed for the propane valves for security and contamination purposes. 
U.S. Pat.  No.  4,712,705 to Fuehrer discloses a tamper indicating cap seal for container valves.  The cap includes an upper body portion which is securely engagable over a valve and completely covers a plug of the valve, a lower skirt portion attached to the upper body that covers lower part of the valve, and a plurality of teeth that are formed inside the upper body to fit in inclined surfaces of the valve to provide locking of the cap onto the valve.  The cap also includes a tear strip member constituted by a pair of spaced and parallel weakened grooves formed inside the cap, and a tab attached to the tear strip member, to indicate that the cap has not been removed or tampered and to allow easy removal of the cap from the valve by pulling the tab to break the weakened grooves.  The grooves provide vents for escape of air to avoid the trapping of air in the end of the cap when the cap is rapidly pushed into the plug. 
However, because the teeth must be made to fit on the inclined surfaces of the valve, the cap must be specifically produced for the valve.