Quote:
Originally Posted by gerrym51
by law a 20 pound tank can only be filled to 16 pounds.

We were just talking about this same thing a few weeks ago in another forum I frequent.
Actually, this is a falsehood that has been told over by the attendants exchanging the cyclinders from Amerigas and Blue Rhino.......as they have actually been shortchanging people. When you use their exchanges, you may only be getting a 75% filled tank.
A 20LB cylinder meant to hold 20 lbs..... will hold 20 lbs of propane.
There are lot's of news article all over the internet about Blue Rhino and Amerigas doing this. Their labels do state 15 lbs of propane are in the bottles, but they could fill them to 20lbs if they wanted. They used to put 17lbs in the bottles, but now it's 15lbs.
Here is how to tell how many pounds your tanks is supposed to hold:
Below is information you need to know when getting your cylinder bottles checked/refilled by weight:
First off, most small propane tanks will have two important numbers stamped onto the cylinder itself (usually around the handle area).
One number is the tare weight, usually stamped TW. This is the weight of the cylinder when empty.
(EX: TW 18 means the cylinder weighs 18 pounds when empty)
The other number is the water capacity, stamped WC, which tells how many pounds of water the cylinder will hold. (EX: WC 47.6 means the cylinder will hold 47.6 pounds of water)
A cylinder will hold 42% of propane compared to the WC. (Ex WC 47.6 X .42 = 19.992 pounds of propane)
So a cylinder with a WC of 47.6 holds basically 20 pounds of propane.
The bottle filling station will generally have a cylinder filling chart that converts water capacity (WC) to pounds of propane that the filler will refer to before filling the bottle. The chart will show that 47.6 pounds of water converts to 20 pounds of propane. In other words, a propane bottle that will hold 47.6 pounds of water will hold 20 pounds of propane. This indicates that the scale needs to be set a little over 38 pounds (20 lbs propane + 18 lbs tare weight = 38 lbs) 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 a link to an online propane tank fill calculator where you will enter the TW and WC numbers to get the amount of propane, along with how much your tank should weigh full.
Propane Tank Fill Calculator  Measurement Technology

Now that you know the process for having your tanks refilled, you can also take bathroom scales (I use a handheld luggage scale) and check to see how much you have in your tanks.
Just take the weight of your tank and subtract the tare weight(TW).
Here is a quick approximation for the various common size tanks (but you really need to read the TW and WC numbers to make sure):
How much should the tank weigh?
38 lbs. for a full 20 lb. size propane tank, 28 lbs half, 18 lbs. if empty.
54 lbs. for a full 30 lb. size propane tank, 39 lbs half, 24 lbs. if empty.
70 lbs. for a full 40 lb. size propane tank, 49.5 lbs half, 29 lbs. if empty.