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Old 05-12-2017, 09:47 AM   #1
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Propane Cylinders (Refilling vs Exchanging)

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:

Click image for larger version

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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:


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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)

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.


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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:
  1. The bleeder valve starts to spew liquid
  2. The scale indicates the cylinder has reached its legal filling capacity
  3. 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:


Click image for larger version

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More resources:

Propane Bottle and LP Gas Cylinder Filling

Manchester Tank: 5#-420# Steel Cylinders

https://www.gasteconline.com/faqs.php

(continued in next post)
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Old 05-12-2017, 03:31 PM   #2
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OK, now that the process for determining the cylinder size and CORRECT amount of propane that goes into it has been established for the refilling process, let's talk about exchanging cylinders.

You see 20 pound propane cylinder exchange cages just about everywhere now. They are at retail stores, gas stations, drugstores, hardware stores, and many other businesses.

They are definitely convenient, and real easy to just swap out your empty cylinder for a "full" one. But we need to understand that the exchanges most likely are not swapping you for a cylinder that is truly full of allowable propane. Actually, in probably the vast majority, their cylinder is going to only be 75% filled..or about 3/4ths filled with allowable propane. This equates to 15 pounds of propane.

Several years ago, the two biggest distributors dropped the amount of propane that they were putting into the exchange cylinders. They had been putting 17 pounds (still not a full cylinder) but kinda silently dropped it to 15 pounds while keeping the price the same. A class action lawsuit as well as federal agency actions followed (which the gas companies lost), and although they still are only placing 15 pounds in them, the signage on the cage is supposed to be more indicative of this. Next time you walk by a cage, see if you can spot the amount of propane in the cylinders.....since you know that a true correctly filled one has 20 pounds of propane.



It seems that a lot of people who didn't know better at the time, were telling others who complained of the propane shortage in these exchange cylinders, that the reason for the shortage was due to the OPD law and you had to have a 20% vapor expansion space, so the most you could put in them was 16 pounds. If you did the fuzzy math by taking the 20% off the cylinders liquid propane capacity with it's OPD instead of the cylinders full water capacity, you could make it sound plausible.

This mistruth has been promulgated over the years by many people, and is a lot of the reason for this thread, in order to straighten it out. It was an incorrect statement that gained legs, so to speak. It not only walked, it ran...so fast that it started beating the truth out in marathons. LOL

You can do a lot of internet searches and find the lawsuits and such, which explain the details.

Here is also a graphic which may put it into better perspective,

Click image for larger version

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There is nothing wrong with exchanges, and they are handy. Just know going in, you are most likely only getting a cylinder filled with 75% of allowable propane, which will run out quicker. The missing 25% of propane you can consider the convenience fee for the process. You also will need to factor this in, when pricing the difference between exchanging vs refilling so you can compare the price per pound (apples to apples).

More resources:

http://www.lexology.com/library/deta...d-ac7baeb6ab12

http://www.elivermore.com/propane.htm

https://www.tractorsupply.com/know-h...ooling_propane

http://movinginsider.com/2012/06/22/propaneprices/

(continued in next post)
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Old 05-12-2017, 07:57 PM   #3
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Now that you are familiar with identifying the tare weight (TW) on the propane cylinder, 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.

Here is a great link to a propane filling calculator, that lets you input the tare weight and water capacity numbers to see how much propane your cylinder should hold.

Propane Tank Fill Calculator | Measurement Technology


Also, if you want to do just simple math, there is a ratio that takes into account the proper propane for a cylinder, factoring in the 80% propane liquid fill level for the OPD. Just take the WC and multiply by 42% (0.42).

47.6 X .42 = 19.992 pounds of propane (rounded to 20 pounds)

We keep a continuing thread where you may can get propane cylinders refilled, especially if on the road or on weekends:

Propane refills on Sunday

Hope the above posts help everyone with a better understanding of refilling vs exchanging the small cylinders, as well as what all the numbers mean.
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Old 05-13-2017, 09:08 AM   #4
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Metric cylinders

For our members who may use the metric system, the cylinder basics are the same.

However the water capacity may be listed in liters (liquid volume), while the tare weight could be in kilograms (weight)

This is an example of the cylinder markings from Canada.

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More resources:

https://propane.ca/wp-content/upload...ugust-2016.pdf
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