gas pumps

[DrPizza]

I'm sure I asked this somewhere before, but couldn't find it. With the weird pricing on gasoline - that extra 9/10ths of a cent; are pumps that precise?

I mean, suppose gasoline is 1.799 for a gallon of gasoline. If I could measure 1.0000 gallons of gasoline (by mass, since the volume will vary slightly with temperature), how close to 1.799 should I expect the pump to read?

Would I see a range of values between 1.78 and 1.82 from gallon to gallon, averaging about 1.799? 1.75 to 1.85? Is it a crapshoot and hope that it's within a nickel? When the county/state seals the pumps, what is the tolerance?

 

[nismotigerwvu]

I'm not certain how precise they are but I know I've had a few that read (and charged me for) ~19 gallons despite the fact that I managed to drive to the station itself and my tank is rated to hold 17.2 gallons.
Even if I had run out and pushed the car in there would still have been something in the tank that the fuel pump simply couldn't get out.
Understandably I went inside and asked the clerk for an explanation (bringing my car's manual with the capacity clearly stated).
All 3 times (at 3 different stations) they gave me the cash value for the transaction back (not that I was asking for that much, I just said I wasn't paying for more than my tank can hold).

 

[LostUte]

I think the maintenance tolerance is 0.5% (I am not aware of a temperature correction being applied). So, assuming reference temperature, the price should come out to be $1.79 to $1.81.

 

[CycloWizard]

There are a few ways that they could measure the flowrate, but I'm not sure which is actually used. Probably the simplest would be to compute the volumetric flowrate based on the pressure difference between the storage tank (known, constant pressure due to the pump) and your car (known pretty well, almost constant during pumping). A simple thermocouple could allow conversion from the volumetric flowrate to a mass flowrate, but I don't think that this is done, since you are literally buying a volume of gas rather than a mass. The energy density changes depending on ambient conditions due to formulation changes in anticipation of ambient conditions (e.g. google Reid vapor pressure), so even a known mass would have different energy content from place to place and season to season. I could go on, but I won't.

 

[Rubycon]

Text

These devices are regulated and inspected by your state/province department of weights and measures.

 

[sethkinast]

In Oklahoma where I go to school, they have to be +/- 5 cubic inches per 5 gallons. In Texas it's +/- 6 cubic inches per 5 gallons. Usually when I look at the seals, the inspector has marked how much over or under the pump is.

 

[L00PY]

From some random Googled site, "o, in a business where price is measured in 9/10ths of a penny, what does all of this mean in dollars and cents? If a 5-gallon test is off by 5 cubic inches -- just within the allowable range -- and gas is $4 a gallon, the loser is out about 34 cents on a 20-gallon fill-up."

So plus or minus a penny or so on 1 gallon at $1.7999.

 

[DrPizza]

Thanks all, that pretty much answers my question: The .9 cents is absolutely meaningless & should simply be rounded off to the next cent.

 

[SsupernovaE]

Originally posted by: DrPizza
Thanks all, that pretty much answers my question: The .9 cents is absolutely meaningless & should simply be rounded off to the next cent.

Of course, you know it's for purely psychological marketing purposes.

 

[sao123]

Originally posted by: Rubycon
Text

These devices are regulated and inspected by your state/province department of weights and measures.

It has been calibrated by top members of the state and federal Departments of Weights and Measures, to be dead-on balls accurate.

 

[bobdole369]

A gas station where my roommate works - has installed illegally fast pumping gas pumps. THey are illegally fast because instead of a certain tolerance, the fast mode has a tolerance thats ever so slightly less than legal - i.e. if legal was +/-0.5% than these faster pumps would be +/- 1%. They have a switch under the cash register for when the state comes by to measure.

 

[kpb]

Originally posted by: nismotigerwvu
I'm not certain how precise they are but I know I've had a few that read (and charged me for) ~19 gallons despite the fact that I managed to drive to the station itself and my tank is rated to hold 17.2 gallons.
Even if I had run out and pushed the car in there would still have been something in the tank that the fuel pump simply couldn't get out.
Understandably I went inside and asked the clerk for an explanation (bringing my car's manual with the capacity clearly stated).
All 3 times (at 3 different stations) they gave me the cash value for the transaction back (not that I was asking for that much, I just said I wasn't paying for more than my tank can hold).

I would avoid going back to those stations ever and if I knew how report them. Some gas stations will cheat and run their pumps so that they read higher than the actual amount of gas pumped into the car. with a 17.2 gallon tank its probably possible to put close to 18 gallons in from bone try to actually filling the hose leading to the tank but even then you are ~5% more than possible. Odds are they are doing it more like 10% figuring most people won't fill up on that empty of a tank to catch the fact. Essentially it allows them to charge a higher price than they advertise which is of corse illegal.

 

[blahblah99]

Originally posted by: DrPizza
I'm sure I asked this somewhere before, but couldn't find it. With the weird pricing on gasoline - that extra 9/10ths of a cent; are pumps that precise?

I mean, suppose gasoline is 1.799 for a gallon of gasoline. If I could measure 1.0000 gallons of gasoline (by mass, since the volume will vary slightly with temperature), how close to 1.799 should I expect the pump to read?

Would I see a range of values between 1.78 and 1.82 from gallon to gallon, averaging about 1.799? 1.75 to 1.85? Is it a crapshoot and hope that it's within a nickel? When the county/state seals the pumps, what is the tolerance?

I'm not sure how old mechanical pumps worked, but I would guess the newer ones are based on modern electronics.

Each pump would have to be able to calculate gas down to the tenth of a cent at the very least. Say gas is $5 a gallon. That means the pump would have to resolve $5.000 per gallon, or 0.002 gallons per tenth of a cent.

The average tank holds (lets say) 20 gallons. That means the dynamic range of the electronics would have to be 10000:1, or 80dB.

If the pump can only resolve 0.002 gallons per tenth of a cent, there would be rounding errors, so it would obviously have to be able to measure much better than 0.002 gallons.

The best AUDIO ADC does about 120dB dynamic range (1000000:1) while most general purpose ADCs are around 80-110, an order of magnitude lower.

If the DR is at 100, the pump can measure down to 0.0002 gallons. At that resolution, the max. error can be 0.0001 gallons / tenth of a cent. If you're pumping 20 gallons @ $5 a gal, the total can be off by +-$10. at the resolution above.... but fortunately there's calibration.

 

[hanoverphist]

Originally posted by: kpb

Originally posted by: nismotigerwvu
I'm not certain how precise they are but I know I've had a few that read (and charged me for) ~19 gallons despite the fact that I managed to drive to the station itself and my tank is rated to hold 17.2 gallons.
Even if I had run out and pushed the car in there would still have been something in the tank that the fuel pump simply couldn't get out.
Understandably I went inside and asked the clerk for an explanation (bringing my car's manual with the capacity clearly stated).
All 3 times (at 3 different stations) they gave me the cash value for the transaction back (not that I was asking for that much, I just said I wasn't paying for more than my tank can hold).

I would avoid going back to those stations ever and if I knew how report them. Some gas stations will cheat and run their pumps so that they read higher than the actual amount of gas pumped into the car. with a 17.2 gallon tank its probably possible to put close to 18 gallons in from bone try to actually filling the hose leading to the tank but even then you are ~5% more than possible. Odds are they are doing it more like 10% figuring most people won't fill up on that empty of a tank to catch the fact. Essentially it allows them to charge a higher price than they advertise which is of corse illegal.

i was thinking along the same lines, but i also know that many owners manuals dont list the reserves in the tank capacity. my truck is rated at 26 gal but that is with a 2 gal reserve.

 

[schneiderguy]

nvm

 

[CryHavoc]

Originally posted by: sao123

Originally posted by: Rubycon
Text

These devices are regulated and inspected by your state/province department of weights and measures.

It has been calibrated by top members of the state and federal Departments of Weights and Measures, to be dead-on balls accurate.

two utes
Two what?
Utes
Whats a ute?
TWO YOUTHS

LOL

 

[crazychicken]

Just because the tank is rated to be 17.2 gallons doesn't mean it will only hold 17.2 gallons, right? Just like on a lawn mower gas tank, there is a "fill" line which gets you to the "rated" 2 gallons, but you can certainly put another 1/4 gallon in there if you want.

Dave

 

[OCedHrt]

Originally posted by: crazychicken
Just because the tank is rated to be 17.2 gallons doesn't mean it will only hold 17.2 gallons, right? Just like on a lawn mower gas tank, there is a "fill" line which gets you to the "rated" 2 gallons, but you can certainly put another 1/4 gallon in there if you want.

Dave

That is usually only if you top off.

 

[Zepper]

Yup, we can certainly trust our State or County weights & measures flunkies for accuracy and actually doing anythng more than just slapping the sticker on the pump... :roll: How much of our tax dollars has disappeared into the labyrinth at the Dept. of Defense with no accounting? Where did the first dose of bailout money to the banks go? How about the accountability of the gov't schools to at least turn out citizens who can actually find their butts with both hands? Good to remember that YOU are accountable, not the gov't flunkies. I'd prefer it if gas was sold by weight not by volume as it has quite a coefficient of thermal expansion. While we wait for that, buy your gas as early in the morning as possible (or whenever is the coolest time of the day in your neck of the woods).

.bh.

 

[Rubycon]

Originally posted by: Zepper
While we wait for that, buy your gas as early in the morning as possible (or whenever is the coolest time of the day in your neck of the woods).

.bh.

Aren't the tanks underground? That would be a tremendous buffer to ambient temperatures. Subterranean temperatures in temperate climates vary little between winter and summer seasons let alone the highs and lows of a single day.

 

[KIAman]

Originally posted by: Rubycon

Originally posted by: Zepper
While we wait for that, buy your gas as early in the morning as possible (or whenever is the coolest time of the day in your neck of the woods).

.bh.

Aren't the tanks underground? That would be a tremendous buffer to ambient temperatures. Subterranean temperatures in temperate climates vary little between winter and summer seasons let alone the highs and lows of a single day.

Exactly. Significant change in ground temperatures (over 2c) is a months-long process and in that time, I am guessing the tank has had its content refilled several times.

If you truly want to save gas, lose weight.

 

[Zepper]

Yes, but the plumbing between the tank and the pump (at least AFAIK) may not be well insulated. It would be an interesting experiment on some summer day to test some samples from 3 or 4 am against some samples from 3 or 4 pm from both the beginning and end of the fillups (perhaps use two 5 gal. gas cans so the fillups would be the same volume). You'd also want to know when the ground tanks received a tanker load as that could significantly alter the results. IAC, I know that advice has been out there for a long-long time - if it really is an "old hubbys'" tale, then that would be useful knowledge as well.

.bh.