The Federal government prohibited the sale and use of POL valves found on older DoT type propane cylinders. (POL is an abbreviation for Prest-O-Lite, for the company that first produced the valve). These valves have the familiar female left-hand-thread in the valve and are the type most of us currently have. Cylinders with this type valve were discontinued beginning in the year 2000, and may may no longer be refilled.
The old POL valves were replaced by a new Acme type which uses a QCC (Quick Closing Coupling) connector. This connector has an external (male) right-hand thread on the valve. A pigtail hose can be connected and disconnected by hand... no wrench required. These valves also have the old-style POL internal left-hand thread, so they can be used directly in place of the old valves without modifying your RV. If you like the no-tools convenience of new style connector you may want to replace your pigtail hoses with new ones having the QCC connectors.
The new ACME or QCC valve also contains an OPD (Overfill Protection Device). So you may find them called ACME valves, OPD valves or QCC valves, depending on the supplier. The OPD feature prevents accidental overfill of the cylinder. An internal float mechanism shuts off the valve when the propane tank is 80% filled. The 20% empty space is necessary to prevent the cylinder from venting large amounts of propane when the temperature rises. (ASME type cylinders in most motorhomes have had this OPD feature for many years). The new OPD valve also contains another safety feature - it will not release gas unless the pigtail hose is properly connected, even with the valve open.
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