The question of how much LPG can I safely put in my tank continues to arise. You are of course limited to an 80% fill by volume to allow for thermal expansion, but the actual weight depends on the LPG mixture that you are using..

Here is some math to make that easier for you, using a 50# tank as an example.  The 50# rating is what the tanks internal volume full of water would weigh:

Assume:

50# of Water weighing  .0361 lbs/in3

50# divided by .0361 = 1385 in3

80% fill by volume or .80 X 1385 = 1108 in3 available.

Specific gravity of butane is .601 or .0361 X .601 = .021696 lb/in3

Specific gravity of Isobutane is .563 or .0361 X .563 = .0203243 lb/in3

Specific gravity of propane is .495 or .0361 X .495 = .0178695 lbs/in3

Therefore:

100% n-Butane = 1108in3 X .021696 = 24 lbs.

100% Iso-butane = 1108 X .0203243 = 22.5 lbs.

100% n-propane = 1108in3 X .0178695 = 19.8 lbs

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70:30 = (.7 X  1108 in3) = 775.6 in3 butane + (.3 X 1108 in3) = 332.4 in3 propane

(775.6 X .021696) + (332.4 X .0178695)  =  16.8 lbs + 5.9 = 22.7 lbs total

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50:50 = (.50 X 1108 in3) = 554 in3 butane and 554 in3 propane

(554 X .021696 lb/in3) + ( 554 x .0178695 lb/in3) = 12 lbs butane + 9.9 lbs propane = 21.9 lbs total.

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20:20:60 = (.20 X 1108 in3) = 221.6 in3 and (.60 X 1108) = 664.8 in3

(221.6 X .0179695) = 3.98 lbs propane + (221.6 X .0203243) = 4.5 lbs Isobutane + ( 664.8 X .021696) =14.4 lbs butane or 22.88 lbs total.