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Filling Propane Tanks

by Peter

Most grills and small campers use standard 20 lb propane tanks. A 20 lb tanks holds 20 pounds of propane, right? Not necessarily. "Blue Rhino" and similar exchange tanks are charged with only 15 to 16.5 pounds of propane. If you notice, none of their displays say that you are getting 20 pounds of propane. You're not.

If you go to a gas dealer or hardware store, where they actually fill your personally owned, 20 lb tank, they will fill the tank completely - until OPD (overfill prevention device) clicks off - and charge you for 20 pounds. So you get - and pay for - 20 pounds of propane, right? Not necessarily. Most tanks that are refilled are not completely empty but, if they're filling a "20 lb tank" they will charge you for "20 pounds" whether they actually delivered 20 pounds or not. And they usually are not.

The only place where you get what you pay for is a dealer or hardware store that charges you for the actual amount of propane delivered - usually in gallons - as reflected on the the meter on their delivery system.

How do you know if the dealer that you just walked into charges by the pound or gallon? You ask them when you first walk in the door. You can say something like, "I have a tank that's about one quarter full but I want it filled up. Do you charge by the gallon or just by the size of the tank?" If the person has no idea what you're talking about, they charge by the size of the tank. Go elsewhere unless you're desperate

Comments for Filling Propane Tanks

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Nov 12, 2014
The 80% Rule is Previously Built in to Tank Design per ASME!
by: Anonymous

NO KUDOS SILLY! The physical dimensions of a 20 pound propane tank are designed to a dimensional standard such that the tank can technically hold 25 pounds of propane at 72 degrees F by volume but is only filled to 20 pounds to comply with the federal DOT mandated 80% rule for highway transport of a highly flamable and thus hazardous material. Eighty percent of the design volume of 25 pounds equals the 20 pounds and the customer should get 20 pounds, PERIOD!

The reason for this built in "factor of safety" in the original tank size is to make allowance for the change in volume and pressure (and even state, from liquid to vapor, depending on ambient temperature) is because propane has a very high thermal expansion rate, meaning that as the temperature in close proximity of the tank vastly affects the pressure in the tank. The higher the temperture of the air around the tank, the higher the pressure of the propane in the tank, much like the air pressure in your tires is affected by temperature -- lower when cold than when hot.

This 80% rule accounts for the possibility that the tank can conceivably be filled on a much colder day than the tank will live to see. If a tank size is designed to fit 25 pounds of propane, but is never filled beyond 20 pounds there is enough storage space left in the tank so that even if if gets very warm out, there is enough room left to allow for expansion without producing an unsafe pressure.

If you don't trust what I am saying, fill your tires to operating pressure during a polar vortex and then park the vehicle in a heated garage and take new tire pressure reading sometime later when the vehicle has warmed to that temperature.

The reader is invited to read up on the physics of the beast by searching the term "Boyle's Gas Law".

Dec 14, 2012
30-lb.Tank vs. 20-lb. Tank
by: Anonymous

I have both 20 and 30 lb. tanks. When I get them refilled, each get the same amount of fuel (6.8 gallons]. Why? two different size tanks same amount of fuel. I'm confused. What should a 20-lb or 30-lb tank hold? It's getting expensive and confusing and I'd like the answer.

Oct 27, 2011
Propane Refills
by: Anonymous

The propane company I have do my refills uses a scale and charges you by the weight of the propane added. Simple!

Dec 30, 2009
Overfilling and underfilling propane tanks
by: Anonymous

"New" (triangular valve handle) propane tanks have a built in overfill prevention valve. "Old" (round valve handle) tanks, without this feature, are no longer refillable and haven't been for some time. Folks fill the tank until the overfill valve closes and no more can go in. This insures the maximum, correct fill quantity and no more. No thinking involved. Blue Rhino tanks have this same safety valve but are filled with less than this quantity. A person pays for a "20 lb" tank of gas (what ever the actual weight is) and expects he/she got the correct amount; ie he maximum, safe amount the tank can hold. This is not what you get with Blue Rhino. But, to say the truth, none of their literature says what you are getting for your dough. The assumption is that the tank is full but it is not.

Dec 27, 2009
20Lb tank
by: Anonymous

Regulations state that pressure vessels can only be charged to 80% of their capacity. Blue Rhino is doing what they are supposed to do. The people who fill your tank until the relief valve blows are putting your safety at risk. Pressure vessel codes and regulations are written to keep people from being hurt or killed.
Kudos for Blue Rhino...

Dec 17, 2009
Propane Gas
by: Anonymous

They most all charge you a minimum 5 gallons also. Never fill a half empty tank as the price goes up for what you get. Here it's now at $3.15 a gallon with most asking $5.00 at the least. I have 3 tanks and only fill the one that is 100% empty.

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