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09/07/2006 10:26:46 PM · #1 |
I'm uninspired lately, so here's another General post:
Someone told me that gas pumps has a return pipe that acts to remove air from our fuel tanks (see image) as we pump in gas. At times when we fill the tank all the way up (full), there is quite an amount of fuel that actually gets seeped back into the sump through that return pipe before the fuel is cut-off. It's just the way the pumps are designed. So they were saying that we actually pay for the fuel that are seeped back through the return pipe because it has passed the charging meter/valve. The advise I was hearing is, if you wanna get the best deal, dont pump all the way to full all the time. What do you guys think?
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09/07/2006 10:31:42 PM · #2 |
Hmmm, I wonder if that's why they tell you not to top off the tank?
Interesting. |
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09/07/2006 10:33:33 PM · #3 |
Message edited by author 2006-09-07 22:34:06. |
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09/07/2006 10:45:58 PM · #4 |
Hmm, lets take a look at this from a technical standpoint. The "return hose" that you refer to is actually a vapor recovery system so we dont allow excess HC(hydrocarbons) back into the atmosphere as it adds contaminents into it. The way a fuel vapor system works on a car is such that when you put fuel in the fill tube, it displaces air that is in the tank, that goes out a hose usually at the very top of the tank and goes back to the filler neck area where that vapor recovery system collects it. In order to actually get fuel into that area, you really have to overfill a system, usually not doable on todays cars. However if you actually did, in order to get it back into their pumps you would have to have one heck of a suction on that return line(which doesnt exist). Sorry I think this is purely mythical.
MattO
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09/07/2006 10:51:55 PM · #5 |
MattO, got a question. How does the nozzle knows if the tank is full?
Thanks. |
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09/07/2006 10:54:33 PM · #6 |
Yes gas pumps have a vapor return hose. Chek here for more info.
//en.wikipedia.org/wiki/Vapor_recovery |
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09/07/2006 10:59:31 PM · #7 |
Originally posted by crayon: MattO, got a question. How does the nozzle knows if the tank is full?
Thanks. |
Here is a pretty good and almost non-technical answer.
//www.straightdope.com/classics/a1_263.html
MattO
ETA here is an explanation of why you shouldnt top off.
//www.jpsalestraining.com/topping_off.htm
MattO
Message edited by author 2006-09-07 23:02:11.
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09/07/2006 11:10:32 PM · #8 |
Fix for the 2nd URL:fixed here |
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09/07/2006 11:10:38 PM · #9 |
Thanks for all the interesting finds.
Maybe just 1 more question on the pump/nozzle thing - where is the valve/meter located? that thing that calculates how much gas has passed the pump into our fuel tanks? If it's located at the top of the hose (at the pump machine) then wouldn't we be paying for the fuel that is inside the hose (between nozzle and pump) ? |
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09/07/2006 11:34:11 PM · #10 |
The meter that tells how much gas is being pumped is not in the handle of hose, its in the big portion of the pump that sits on the island of the gas station.
MattO
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09/07/2006 11:59:13 PM · #11 |
what happens when a warm liquid (say... gasoline in a nice shiny metal tanker truck) gets pumped into a cold, underground storage tank? right... the liquid contracts, dropping the total volume...
so when that tanker pulls up filled with 1000 gallons (a nice round number) and pumps it into the ground, our poor gas station owner pays the gas company for 1000 gallons of gas. as the temperature of the gasoline falls, it contracts, right? meaning the tanks now hold 900 gallons of gasoline... meaning our station owner just got screwed out of 100 gallons of gas...
ouch, huh? (of course, i heard this from a station owner and have no idea if it's true, but it sounds plausable...)
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09/08/2006 12:22:14 AM · #12 |
Originally posted by kudzu: what happens when a warm liquid (say... gasoline in a nice shiny metal tanker truck) gets pumped into a cold, underground storage tank? right... the liquid contracts, dropping the total volume...
so when that tanker pulls up filled with 1000 gallons (a nice round number) and pumps it into the ground, our poor gas station owner pays the gas company for 1000 gallons of gas. as the temperature of the gasoline falls, it contracts, right? meaning the tanks now hold 900 gallons of gasoline... meaning our station owner just got screwed out of 100 gallons of gas...
ouch, huh? (of course, i heard this from a station owner and have no idea if it's true, but it sounds plausable...) |
Maybe they make up for it when it's winter and zero degrees and the tanks in the ground are holding some heat...
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09/08/2006 12:23:28 AM · #13 |
Originally posted by kudzu: what happens when a warm liquid (say... gasoline in a nice shiny metal tanker truck) gets pumped into a cold, underground storage tank? right... the liquid contracts, dropping the total volume...
so when that tanker pulls up filled with 1000 gallons (a nice round number) and pumps it into the ground, our poor gas station owner pays the gas company for 1000 gallons of gas. as the temperature of the gasoline falls, it contracts, right? meaning the tanks now hold 900 gallons of gasoline... meaning our station owner just got screwed out of 100 gallons of gas...
ouch, huh? (of course, i heard this from a station owner and have no idea if it's true, but it sounds plausable...) |
A 10% change in the volume of a liquid within a range of "ordinary temperatures" is quite implausible. You car's gas tank is sealed, right? so when the temperature drops overnight does your gas tank crumple inwards under the force of the vacuum created by the contracting gas?
Maybe a change of 1/2 of 1%, yielding around 995 gallons I might be persuaded to believe ... |
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09/08/2006 12:36:06 AM · #14 |
Actually, the folks that get screwed by the temp of the liquid are the consumers. Apparently the system that monitors the flow of gas into your tank is based on an average gasoline temperature of 60 degrees F. If you pump gas when it is significantly warmer than that, you really aren't getting what you pay for.
//www.kansascity.com/mld/kansascity/business/personal_finance/15370193.htm
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09/08/2006 12:39:38 AM · #15 |
Originally posted by GeneralE: Originally posted by kudzu: what happens when a warm liquid (say... gasoline in a nice shiny metal tanker truck) gets pumped into a cold, underground storage tank? right... the liquid contracts, dropping the total volume...
so when that tanker pulls up filled with 1000 gallons (a nice round number) and pumps it into the ground, our poor gas station owner pays the gas company for 1000 gallons of gas. as the temperature of the gasoline falls, it contracts, right? meaning the tanks now hold 900 gallons of gasoline... meaning our station owner just got screwed out of 100 gallons of gas...
ouch, huh? (of course, i heard this from a station owner and have no idea if it's true, but it sounds plausable...) |
A 10% change in the volume of a liquid within a range of "ordinary temperatures" is quite implausible. You car's gas tank is sealed, right? so when the temperature drops overnight does your gas tank crumple inwards under the force of the vacuum created by the contracting gas?
Maybe a change of 1/2 of 1%, yielding around 995 gallons I might be persuaded to believe ... |
Actually fuel tanks arent sealed, at least not all the time. They employ a carbon canister to trap HC in the vent system of the Evaporative emmissions system, and also allows for expansion during heat and cooling cycles. Its also left open to allow fresh air to be drawn in so that when the vapor management system is active it has a place to pull fresh air from while it removes vapors from the carbon canister and the tank. The only time a fuel tank should be fully sealed off is when an OBD II equipped vehicle is perfroming an on board test called an Evap leak test.
MattO
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09/08/2006 12:41:22 AM · #16 |
Originally posted by shamrock: Actually, the folks that get screwed by the temp of the liquid are the consumers. Apparently the system that monitors the flow of gas into your tank is based on an average gasoline temperature of 60 degrees F. If you pump gas when it is significantly warmer than that, you really aren't getting what you pay for. |
This reminds me of the advise I got - try to pump fuel when it had been cold for hours, like in the mornings before the sun heated up the ground. Wonder if it really holds merit. |
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09/08/2006 12:49:46 AM · #17 |
Originally posted by shamrock: Actually, the folks that get screwed by the temp of the liquid are the consumers. Apparently the system that monitors the flow of gas into your tank is based on an average gasoline temperature of 60 degrees F. If you pump gas when it is significantly warmer than that, you really aren't getting what you pay for.
//www.kansascity.com/mld/kansascity/business/personal_finance/15370193.htm |
FIXED LINK |
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09/08/2006 12:50:44 AM · #18 |
Originally posted by GeneralE: A 10% change in the volume of a liquid within a range of "ordinary temperatures" is quite implausible. You car's gas tank is sealed, right? so when the temperature drops overnight does your gas tank crumple inwards under the force of the vacuum created by the contracting gas?
Maybe a change of 1/2 of 1%, yielding around 995 gallons I might be persuaded to believe ... |
eh... i made up numbers to illustrate the senario...
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