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If the tank is sealed then any evaporation would condense at night, (when it cools) and the positive pressure would help to pump.
And, over the course of a year, assuming the gas is pumped from the above ground tank, a definitive percentage of profit would be achieved.
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The real key here is, as was mentioned earlier, gas is stored in underground tanks where the temperature doesn't vary too much. The fact that it expands or contracts a bit when it's in your tank doesn't mean squat because you've already paid for it.