An Associated Press article claims that increased temperatures cost consumers money at the gas pump. (the link is to MSN Money Central.) The reason is not the usual seasonal fluctuations in gas prices, but rather the fact that gas is priced by volume, but the chemical reactions that fuel a car don't care about the volume of fuel you put in, but rather the mass. A United States liquid gallon has historically been defined as 231 cubic inches (although I suspect that now it, like most other units in our system, is defined as some exact multiple of some metric unit, in this case the cubic meter, liter, or cubic centimeter). But since gasoline, like other materials, expands as the temperature goes up. The article says that gas pumps "always dispense fuel as if it's 60 degrees". I assume this means that when it's warmer than 60 degrees, the "gallon" of gas you get from the pump is still one gallon by volume but it's slightly less mass -- about one percent less at 80 degrees than at 60 -- than it would be at 60 degrees and therefore your car doesn't go as far.
Consumer advocates are up in arms about this.
Personally, I'm a bit suspicious. For one thing, gas prices fluctuate wildly. If you are not blind, you can see this. (I mean this literally -- anyone in this country with eyes has some idea what gas costs, even if they don't buy any. I personally don't buy any -- I don't drive -- but I know that regular gas costs $2.96 a gallon right outside my apartment. Then again, I live across the street from a gas station.) I have no doubt that gas station owners are aware of this effect. Their margins on gasoline, depending on who you believe, are anywhere from 2% to 10%; this effect could seriously eat into their margins if they didn't know about this. The real money in running a gas station is from the attached convenience store.
I suspect that the news media over-reports on gas price fluctuations, though. "Gas prices went up this week" or "gas prices went down this week" gets people's attention; "gas prices stay the same this week" doesn't.
Second, a gallon is a unit of volume. In fact, I'm not totally sure what this calibration to 60 degrees even means. The case would seem a lot less silly to me if gas were being priced by the kilogram. (This leads me to wonder -- if gas were priced by the pound, would people complain that at higher altitudes gravity is weaker so a pound of gas has greater mass?)
Third, fitting the pumps to correct for temperature would be expensive -- a few thousand dollars a pump.
Fourth, 60 degrees was probably chosen because it's pretty close to the average annual temperature in much of the United States. So this might help people in the summer -- but it'll hurt them in the winter. (Of course, it is easy for me to say this, as both a non-driver and someone from a place where the average annual temperature is 54.3 degrees Fahrenheit.)
Still, it wouldn't surprise me if there are some people avoiding buying gas on hot days so that they can save a few cents a gallon. I'm not sure how viable such a strategy is, in part because the gas is stored in underground holding tanks and underground temperatures don't vary nearly as quickly as air temperatures. Second, what kind of savings are we talking here, a couple cents a gallon? Is it really worth stressing out over this to save thirty or forty cents on a tank of gas?
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3 comments:
a gallon is a unit of volume. In fact, I'm not totally sure what this calibration to 60 degrees even means. The case would seem a lot less silly to me if gas were being priced by the kilogram
They're implicitly converting the amount of gasoline purchased to the amount of chemical energy it contains. The density of chemical energy in any fuel is a function of its mass.
Second, a gallon is a unit of volume. In fact, I'm not totally sure what this calibration to 60 degrees even means. The case would seem a lot less silly to me if gas were being priced by the kilogram.
You have this exactly backwards. The energy content is proprtional to the mass. If gas were priced by the kilogram, you would pay the same in dollars per joule regardless of temperature.
The issue is that gas is priced by volume. At higher temperatures, the same volume, and hence the same price, corresponds to gas at lower density, and hence to a lower mass, and energy content, per unit volume and per dollar.
it wouldn't surprise me if there are some people avoiding buying gas on hot days so that they can save a few cents a gallon.
This is backwards also; you want to buy the gas at colder temperatures, when it is more dense, and so the gallon you buy and pay for at a constant rate contains greater mass and so greater energy.
Natural gas bills are usually assessed in terms of energy content, typically BTUs or therms, for this reason.
Look it up. Gasoline expands by 1% for every 15 degress F. So if you were to pump gasoline at 90 Degress F (not outside temperature but the gas temperature) you would get 2% less gas by volume. So if you got 20 gallons by the Meter in the pump you would actually get 2% less or about 19.6 gallons based on the 60 Degree calibration and price. Based on a $3.5 / gallon price you would pay about ( 3.5 * .4 = $1.40) for gas you did not get!!!!
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