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Inaccurate Gas Station Gas Pumps . . . Overestimating The Gallons Pump

By datsa, June 25, 2008

A few days ago, a local man went to his neighborhood 76 gas station to fill up his 5 gallon gas can after his lawn mower ran out of gas. To his surprise, the gas pump stated that he had put in over 7 gallons. After talking to the gas station manager, he also called the police. Later, he repeated his test at three other gas stations, and two of them also showed higher amounts than what his gas can could hold. Only one station was accurate at 4.982 gallons. True, hot weather does cause gas to expand, but the police suspect that the gas station pumps are not calibrated, and that consumers are being gouged.

Has this happened in your area?

There were some that complained that they pumped in a gallon more than what the gas tank is supposed to hold. They called a hotline that was posted next to the pumps - someone came out later than week and checked all the pumps - all were within spec.

There was a huge number of violaters in NJ earlier this year. Something like 1300 stations checked, 350 had some violations. Some are minor - like changing the price multiple times a day, to mismarked prices, wrong octane, pumps not calibrated, etc.

This happened in North Texas. Same type of situation. A friend filled up a small tank, realized what happened, went inside and spoke with the manager. He demanded a refund before he called the police.

The expansion of the liquid should not matter. The gas expands in the tank (as the temperature rises), but the meter (upstream from the tank) should still read the volume of liquid that passes. This is a good reason to fillup in the morning, when it is cooler.

That is true - well known as the Hot Fuel Premium - got some attention when it came up in a Oversight and Government Reform Committee in the House of Representatives last year.

But we area taking a very small fraction of the volume change due to temperature differences. To give you an idea of what the differences are:

Gasoline is stored, wholesaled, and distributed at a temperature compensated standard of 60 degrees F.

Gasoline has a thermal expansion coefficient (?) of 0.00069 F or 0.00095 C

The change in volume (?V) for a given change in temperature from the 60F standard (?T) follows this relationship

?V = V0??T

For a gallon of gas that is 10 degrees (F) hotter than the storage temperature

?V = (1gallon)(0.00069)(10)

?V = 0.0069 gallons

Say you pump 5 gallons of gas that is 100 degrees hotter than what is in the storage tanks - then the change in volume due to temperature is ~ 0.345 of a gallon more. So if you fill a 5 gallon gas can with 7 gallons of gas - it is either due to the gas can actually holding more than 5 gallons, or the pump is WAY off. No way thermal expansion can account for that much offset - regardless of how hot it is outside. Any station owner that claims that thermal expansion is causing gross pumping inaccuracies is full of BS.

But....if the tanks are burried well below the frost level. The outside temperature shouldn't really affect them, would it?

muzak - absolutely correct.

EPA has done tests ad nauseum - and found that the temperature of the underground tanks stay fairly constant (assuming they were installed correctly). Plus the system that pumps the gasoline into the car is also highly temperature controlled. Very tough to get pumps to read incorrectly, unless the equipment was sabotaged, or older design.

this reminds me of my 37mpg fluke of a tank of fuel. the pump clicked off at 6.5 gallons in, and i've filled up several times with the needle near the same position, 7.5 to 7.8 gallons of fuel. i think that one pump clicked off prematurely.

once in michigan i put in 13 gallons to a 13 gallon tank that had 1/8th indicated on the gauge, which is NEVER more than 11 gallons in. it was 2AM and i didnt want to fight.

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