How can you tell how full a propane tank is?

When do you fill the tank? I know by weight, but what are the weight by volume?

I don鈥檛 see that anyone really answered your weight by volume question. Although you don鈥檛 state the size of your cylinder, for the sake of explanation we will assume you have a 20lb. BBQ type cylinder.

All portable propane cylinders are required to have certain markings stamped into the cylinder usually on the collar around the valve, the marking you need to be concerned with is (Tare Weight). You need to look for (T.W.) followed by a number stamped into the cylinder collar. In the case of a 20lb. cylinder you should see something like (T.W. 11.5); tare weight is quite simply the weight of the cylinder when it contains no gas.

The common reference of cylinder sizes like 20lb. and 30lb. refers to the amount of propane by weight that a particular cylinder will hold when filled to 80% of its total capacity. This 20 and 30 pound reference refers only to the weight of the propane and not the weight of the tank itself.

So, if we use the example tare weight of our grill cylinder above of 11.5 we add the weight of the propane 20 to get the total weight of our filled cylinder 31.5 pounds. So, to judge the amount of propane left in the cylinder by weight, a person would figure that 31.5 pounds is full and 11.5 pounds is empty.

Just a side note here to keep in mind, most of the cylinder exchange services that you see at Home stores and Wal-mart, are filled in an automated assembly line type facility. Because the process is automated and the machines can鈥檛 account for tare weight variations in cylinders, they are generically filled to around 17 pounds of propane.

Regarding the devices mentioned in some of the other answers here, those that estimate volume using tank pressure are notoriously inaccurate. The decal or magnetic strip placed on the side of the tank are the best, as well as the least expensive option.

In reality the quickest way to estimate the amount of propane in a cylinder is to stand above the cylinder, bend your upper body forward, use both hands and lift the cylinder just off the ground using the handle in the collar. Once it鈥檚 just off the ground gently swing it just a bit from side to side. When you stop swinging the cylinder you will feel the weight of the propane sloshing side to side in the tank.

Sorry Calvin,
The Flame King device is nothing more than a pressure gauge and in no way has the ability to indicate the actual volume of propane in the tank. It is simply measuring the working pressure in the tank at any given point in time. It could give a false reading of full or empty under various circumstances. But, with that being said I know many people use this type of gauge, one just needs to understand that they are not a true mesure of the propane remaining in the tank.

If it is a 100lb tank pour hot water on it and it will leave a frost line. If it is another size there are gauges. The 100lb is the only one they do not make a gauge for

Duka is almost right, Pour hot water down the side of the tank. Frost will form, or almost frost, at the level of propane. 1/4 tank or less have it filled or exchange it for a full one. Most barbecue types hold 20 lbs

when I used to get them filled, I had a 30 pound cylinder. that's how it was filled, on a scale.
I never knew when it was empty, only when the stove wouldn't work and the pilot lights went out.
I think you can put a gauge on them, though.

Stickers containing liquid crystals are available. They measure the temperature difference that occurs due to evaporation when the tank is in use. Some tank manufacturers now offer built-in level guages. At least one model has two windows -- one for warm weather and another for cold (outside temperature affects rate of propane consumption) -- and lets you know when there's less than an hour's worth of fuel.

Propane tank Gage that works for less than $20.00!!
http://www.flameking.com/

if you let a little out of it the tank will freeze up to the level of the propane in it.. you will see the frost all the way up to the level...also there is a tape that you can stick on your tank that changes colors as the level goes down,

You are correct. Weight and volume. I did a Google search with key words 'weight of propane'and found the calcuator below. It might be of help to you.

There should be a Gage just inside the cap which will give you the percentage of the fuel that you have left in it.

you did not say how big of a tank that makes a differences. there are 3 different ways depending on the size

Feel it, as it will be cold

they sell a magnetic thermometer that sticks to the side of the tank witch reads the temp difference at the fuel line will show the fuel level.

Propane weighs about 4 #lbs. per gal. give or take a hair. There are stick on "temperature labels" you can buy that will tell you the level . You fill it up when it runs out==have another full tank to switch over to. Certain propane gauges will also tell you how much is left. A "5 gal. tank is called a " 20 pounder"-- which would be the net weight.

You need a gauge to read the psi. The full psi of the tank should be n the tank somewhere- usually a sticker near the top. Then just read the gauge and do the math.

NO I CAN'T TELL YOU, IT'S A SECRET !

You check the weight.6lb. or 100 ib. etc. If its cold and in use you'll see a line on the tank. A frost line

Sorry, I actually don't have the answer to that however we got this really neat little device that somehow hooks up to our propane tank and when it gets low, it starts to make little chirping noises.

You will need to know the tare weight of the tank (its weight when empty.) A gallon of propane weighs 4.237 lbs. Thus the weight of the tank in pounds would be:
wt. of tank = ( 4.237 x (no. of gallons in tank) ) + (tare weight)
so the number of gallons in the tank will be equal to:
( (weight of tank) - (tare weight) ) 梅 4.237.