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Old 01-08-2006, 08:38 AM   #1
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volume correction of gas sales

interesting page about volume correction in fuel sales: Buying Gasoline in Arizona

covers things like
  • the concept of buying gasoline by energy unit, rather than by volume, because they're not the same thing
  • why someone paying $3/gal for fuel in alaska is getting a better deal than someone buying $3/gal gas at the same time in arizona
  • why canadians (and hawaiians) don't normally receive the amount (volume) of gas their receipt says they pumped, yet this approach is arguably the most fair to consumers
  • why gas retailers who don't volume correct may be "ripping off" consumers, depending on a number of issues (that's the main point of the page)

why? because...

Quote:
Gasoline volume changes approximately 0.125% for every 1C change in temperature (0.058%/F) .
but the energy density of a given mass of gasoline is constant regardless of its volume.

(though you could argue it's harder to extract that energy as efficiently at lower temperatures...)

for some reason my brain is slightly resistant to absorbing this concept. e.g. how does volume correction, or lack of it, affect MPG calculations? hmmm?? think about that.

for one thing, it means if you calculate your MPG on the back of your fuel purchase receipts based on volume reported, it's going to be a bit different than "real" MPG calculated electronically by your car (or scangauge). how it compares depends on the temperature of the fuel and whether it's volume corrected or not.

for another thing, it means that according to my fuel receipt, at 0 degrees C i can put 40.75L of gas into my 40L tank (not talking about filling the neck either)
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Old 01-08-2006, 08:56 AM   #2
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After reading this, the one

After reading this, the one thing I don't understand is why they complain about rounding up .5, I was tought since whenever to always round up .5 and have never been told otherwise.
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Old 01-08-2006, 09:14 AM   #3
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Gasoline energy

Density isn't the only thing that changes the amount of energy in gasoline. The types of chemicals making up gasoline has a bigger effect than density:

Some gasolines ingredients are "aliphatic", some are "olefinic", and some are "aromatic". Each of these types produces different amounts of energy when burned, and you never know which ones you're getting in a gallon.

Then there are the "oxygenates" (methanol, ethanol or methyl tert-butyl ether). They have less energy than straight gas, especially methanol.

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Old 01-08-2006, 09:42 AM   #4
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true, sludgy, but let's be

true, sludgy, but let's be careful not to use the fact that chemistry is also important to dismiss the importance of density changes. (not saying that you were; just raising the point.)

it's clearly significant enough that entire countries and some states have legislated to try to compensate for it.

consider this: until i read that page, i never understood the common fuel saving tip that "it's better to buy gas in the morning". to canadians (and hawaiians), it's irellevant when fuel is bought (assuming steady price of course) because it's volume corrected to account for changes in temp. i didn't know that most U.S. states (is hawaii actually the only one?) don't volume correct fuel sales.

also consider this: all else being equal (which clearly is rarely the case), if you're doing mpg calculations based on non-corrected fuel purchases, you'll go less far on a gallon of gas purchased warm than on a gallon pumped in cool temps. it alters winter vs summer mpg calculations.
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Old 01-08-2006, 09:56 AM   #5
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Quote:consider this: until i

Quote:
consider this: until i read that page, i never understood the common fuel saving tip that "it's better to buy gas in the morning". to canadians (and hawaiians), it's irellevant when fuel is bought (assuming steady price of course) because it's volume corrected to account for changes in temp. i didn't know that most U.S. states (is hawaii actually the only one?) don't volume correct fuel sales.
Yes, I considered saying something about the morning thing. This somewhat validates that idea. I dunno if you saw my way old thread about it, but the answer was sort of up in the air.
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Old 01-08-2006, 10:05 AM   #6
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nope, didn't see that

nope, didn't see that thread.

you might as well post the link to it

(no point in a bunch of "dead end streets" in the forum - where they overlap we should try to make it more of an interconnected neighbourhood concept, for people who "move in" later, like me )

it also raises the question: is it really fair or correct to compare the MPG of a flex-fuel vehicle to a gasoline powered vehicle if the fuels have different energy densities?

or the mpg of diesels to gasoline, for that matter?
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Old 01-08-2006, 10:23 AM   #7
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<a href=http://www.gassavers.org/forum_topic/time_of_day_to_fill_up.html target=_blank>http://www.gassavers.org/forum_topic/time_of_day_to_fill_up.html</a>

There you go, sir.
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Old 01-08-2006, 04:16 PM   #8
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yes i can see how that tip

yes i can see how that tip got started (from the days of above-ground tanks) and agree it's likely that underground tanks have much more temperature stability.

that said, underground tanks are often installed directly beneath asphalt paving as well.

we know what the thermal rate of expansion is, so the experiment would be to simply measure it. bring along a digital thermometer and a small container to the gas station one day. of course this experiment won't benefit me at all...
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