This thread is intended to hopefully correct some misconceptions about what constitutes a fully filled propane cylinder.
We are going to focus primarily on what is generically referred to as a "20 pound" cylinder but the same would apply to larger propane cylinders (ie 30, 40 pounds). The 20 pound is probably the most common, and is used for other purposes besides the RV industry. This is also the cylinder you see in the exchange cages at retail stores, gas stations, hardware stores, etc.
Quite simply, the 20 pound propane cylinder is designed to hold approximately 20 pounds of liquid propane. Where the confusion starts is with the Overfill Protection Device (OPD) and when people start saying that a propane cylinder has to have 80% propane with a 20% vapor expansion space. This is also where they start doing the math incorrect, and start trying to take 20% from the 20 pounds of propane which then puts only 16 pounds of propane in the cylinder. (20 pounds - 20% or 4 pounds = 16 pounds.
This sounds plausible, but is not correct. Here is how it works below in the following posts:
These propane cylinders are sized according to their WATER CAPACITY. This water capacity or WC as you will find stamped onto the cylinder, is the amount of water (in pounds) that the cylinder could hold when completely filled with water. You will find this WC number on the small cylinders and it is key to understanding all of this.
Another number you are going to find on the cylinder is the tare weight or TW. This is what the cylinder weighs when completely empty of propane. It's these TW and WC numbers that determine the amount of propane to put into the cylinder as well as the amount of propane that might be left in the cylinder.
Here is an example of the WC and TW numbers:
Now by regulations, a propane cylinder does indeed require a 20% expansion space for the propane, but this expansion space is taken from the cylinders overall water capacity(aka liquid), not the propane capacity.
The OPD float lever inside the cylinder will shut off the flow of incoming liquid propane once the cylinder is filled to 80% of it's water capacity with liquid propane (which is approximately 20 pounds for this cylinder). See the diagram below:
The tare weight (empty weight) indicated by a "TW" is also followed by a number such as "18" meaning the cylinder weighs 18 pounds when empty. When you go to have your cylinder refilled, the station will have a Cylinder Filling Capacity Chart like below. This will show the approximate pounds of propane that is allowable inside the cylinder as it relates to the water capacity(WC). FYI propane weighs on average about 4.24 pounds per gallon, while water weighs on average over 8 pounds per gallon.
The chart below will show that 47.6 pounds of water capacity converts to approximately 20 pounds of propane capacity (which already takes into account the OPD). In other words, a propane bottle that will hold 47.6 pounds of water without the OPD will hold approximately 20 pounds of propane with the OPD. This indicates that the scale needs to be set a little over 38 pounds (20 lbs of propane + 18 lbs Tare Weight = 38 lbs of a filled with propane cylinder) to obtain the weight of the bottle when it reaches its allowable capacity.
After the scale is set and the hose end nozzle is hooked up to the cylinder, the attendant may open the bleeder valve, reset the meter and begin pumping propane into the bottle. The attendant will stop the pump once:
- The bleeder valve starts to spew liquid
- The scale indicates the cylinder has reached its legal filling capacity
- The OPD valve stops the flow of propane into the cylinder (if equipped with an OPD valve)
Here is also a chart taken from the Manchester Tank website that confirms the above numbers:
Propane Bottle and LP Gas Cylinder Filling
Manchester Tank: 5#-420# Steel Cylinders
(continued in next post)