I know! I know! LP tanks are boring!!! Well... they may be BUT since you'll find them on all sorts of rv's including motorhomes, fold down campers, truck campers, travel trailers, and 5th wheels it only stands to reason that since we're using them we should also understand them.

REMEMBER, liquid propane is a gas, it is stored under pressure, it is flammable, and IT CAN BE DANGEROUS. There are not only rules which must be followed in their use but also laws have been enacted to help insure the safe use of liquid propane. All of this is especially pertinent for older units and many purchasers of these units who thought they "got a steal" on that old trailer get a surprise when they learn the LP cylinders are "out of date" and subsequently "illegal".

OK, lets review some basics.
First, there are two types of tanks or cylinders.

DOT(Dept. of Transportation)

Dept. of Transportation container is the one most of us are familiar with. They are vertical in style and are referred to as DOT cylinders.

- used primarily on fold down campers, truck campers, travel trailers, and 5th wheels.

- can be removed from the recreational vehicle

- usually used in the vertical position, BUT not always. Some may be used in the horizontal position but each cylinder MUST be used in the manner for which it was designed.

(they are usually marked for the position in which they should be used)

Pre l998 cylinders without OPD devices could be used both ways but this doesn't apply today as OPD devices are required on ALL cylinders.


-primary valve for the cylinder

-used for both filling and vapor withdrawal

-also called a "shutoff" valve

ASME (American Society of Mechanical Engineers)

-manufactured to the code of the American Society of Mechianical Engineers

-most commonly referred to as "tanks"

-usually found in motorhomes

-are mounted permanently


-marked "vapor withdrawal" (knob on top right of tank)

-used only for vapor withdrawal

(ASME tanks have a separate fill valve-(one with the yellow cap)

NOTE: This ASME tank has the capability of liquid withdrawal which is labelled on the right side of tank


Three types have been available for RV's in recent history

1) Most common was the POL CG 510 (Presto Lite)

-identified by its 5 or 6 prong twist handle and its metal left handed hose connect


2) OPD service valve with "quick disconnect" which is primarily used by the BBQ industry and is NOT permitted for use on rv's

Quick Disconnect fittings can be used downline on rv's on such things as gas grills but are not permitted at the LP cylinder

3) Type I OPD service valve which is now mandatory (May l, 2002) and has a 1 5/16" ACME thread and is easily distinguished with its black or green connector for the high pressure hose connection. (See top photo) The open/closing twist handle will also only have three prongs which distinguishes it as the new style.


There is an exception however. You can run into the "POL style" with the left handed brass hose connectors with OPD devices. It is easily distinguishable as the open/closing twist handle will NOT have the old style 5 or 6 prongs but will have the new style 3 prong handle which will be clearly labelled OPD. You can rest assured you are not only up to date but also legal.

OPD DEVICE (Overfill prevention device)

OK, so what is the big deal about the OPD device we've been referring to? OPD devices are now required on all recreational vehicles cylinder and tanks are are a safety device to insure that cylinders and tanks are not filled above the 80% level.

(OPD devices have been required on ASME tanks since l984 and DOT cylinders since l998)
The device utilizes an internal float connected to the fill valve which engages a piston which in turn restricts the flow of liquid propane into the tank once it reaches the 80% full range.

While they are not foolproof, for the most part they accomplish their intended purpose. Obviously the float devices differ between the horizontal and vertical containers so cylinders and tanks with OPD devices MUST be used in the position (horizontal or vertical) for which they were designed.


The fixed maximum liquid level gauge is another device which all rv'ers should be familiar with and understand its purpose. The following will help distinguish it from other devices.

-used to determine the 80% level in the container
-part of the service valve on DOT cylinders
Note: on POL SERVICE VALVE photo the small round opening on left side of valve above threads going into the tank. This is a screwdriver slot to open and close the fixed maximum liquid level gauge

-connects to a dip tube which goes into cylinder to the 80% level

-on ASME tanks it is physically located at the 80% full level or it is equipped with an internal tube at the 80% level.

How it is used

-should always be OPEN (turned out) when container is being filled

-during the filling process vapor will be emitted until liquid reaches the 80% level when a "white mist" (liquid propane) will be emitted signifying the container is FULL.

If the container is filled past this level the excess propane should be "bled off" until only vapor is emitted to insure container is not overfilled and to avert any possible future problems with the container.

For instance, an overfilled container could become dangerous if left in a vehicle or in the sun as temperature increases the pressure inside the tank.

Now you have a basic understanding of what you are looking at when you see a LP tank or cylinder and you will know if it is up to date. One more thing to note is the condition of the tank. Tanks that are either rusted or missing paint should be closely inspected for any other possible damage. If you question anything on the tank or cylinder have it inspected by a professional.

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