How is gasoline sold? Is there a Baker's Dozen effect?

Back when gas was $4/gal, a bunch of local news people busied themselves by testing gas pumps. In general, they found that pumps undercount (i.e. you get 1.00x gallons when the meter reads 1.000 gallons).

 

There is an allowable error for pumps. Most tested pumps error on the side of the station, for obvious reasons.

Tom

 

I also recall a news story about stations cheating. The inspectors measure specific amounts. So the pumps were programmed to dispense those specific amounts correctly, but cheat on values in between.

 

In the UK they are not allowed to error on the side of the station. The authorities do random checks with special containers that hold the exact amount and they have a collar round the top for spill over.

If the container is not, at least. completely full they will shut the station on the spot.

 

Assuming you really live in Virginia:

VDACS - Regulatory Services - Product and Industry Standards - Services

"The Office of Product and Industry Standards is responsible for the inspection and testing of all commercially used weighing and measuring equipment including gasoline dispensers..."

Also every gas pump repairman needs to be certified. ""Service Agency" means (i) a business or (ii) that portion of a government or political subdivision engaged in the adjustment, installation, placing in service, recommending for use, reconditioning, repairing, servicing, or selling of any weight or measure commercially used or employed (a) in establishing the size, quantity, extent, area, or measurement of quantities, things, products, or articles for distribution or consumption, purchased, offered, or submitted for sale, hire, award or (b) in computing any basic charge or payment for services rendered on the basis of something's weight or measure."
:deadhorse:

 

same with CA, one of my realatives was the head of weights and measures for the county and said that their tests are pretty strict as far as certification of pumps. Now there is nothing to really stop a station from changing the equipment between inspections but that is impossible to really enforce.

 

that would be unusual. KING 5 TV did their own test of area stations in 2006 when gas was only $3.

they found that 1 in 6 pumps were wrong with pumps favoring the station more than 5 to 1.

apparently there is a metering valve that wears out under normal conditions which actually causes this. so errors in the customer's favor would be extremely unusual.

we also have a state mandate that says every pump must be inspected at least every 3 years. in the survey, 25% had expired inspection stickers.

now there are companies here (Costco is one where i get my gas) that do independent inspections on their own. their pumps are usually done every 6 months to a year.

 

From what i've read and what I've been told, that gas pumps are supposed to be calibrated to a median temperature for that area to be perfectly balanced at that median temp. So say the median temp for your area is 70F. At 70F you will get exactly what the metered says.

If it's hotter then you will get less gas or if it's colder you will get more. If the tv news station only tested during the summer (hotter than median), it will come out in favor of the gas station. it shouldn't ever be grossly in favor of either party (consumer or gas station) but over a full year it's supposed to come to even. There are some municipalities that require the gas pumps to be calibrated for winter and summer, but most only require a calibration once per year.

 

calibration spec is like .005% so very small

 

Part of the problem is that many states no longer have the money to test pumps. The laws are on the books, but they don't get enforced.

Tom

 

true true. the dept of weights and measures only had 12 field agents who work in pairs so only 6 teams and they have to calibrate other things besides gas pumps

they only calibrate when they have free time and 75% of their time is investigating consumer complaints according to the news report

 

The "hotter comes out in favor of the gas station" as ljbad4life says is sort of true in this way--as it get hotter, gasoline expands, like most substances at most temperatures. This means you get less gas by weight in each gallon at higher temperatures.

However, gasoline is sold by volume (gallons or liters) not weight--and there's no reason I know of why the volume accuracy of pumps should change over the year.

Here's what might happen:
Gasoline is generally stored in underground tanks, where temperatures are far more stable than aboveground. (Think about the fact that an unheated basement may stay fairly stable whether its freezing or 90F out. So when you fill your tank, there should be less difference in the weight of the gas caused by differences in temperature than you might think from the differences in summer and winter air temperature. As the gas sits in your tank and reaches ambient air temperature, it may expand or shrink in volume, without changing in weight or energy content.

I suspect that there is a much bigger effect from the different formulation of "winter gas" that has been talked about in a number of threads here. However, I'm not enough of a chemistry-type guy to know.
Help anyone?

I also suspect that the comment from someone that the allowable error on the gas pumps is really small means that the difference in winter/summer formulation is far more important there too.
But I don't know that for sure either . . .

Here's what MIGHT be interesting to know--how do the sensors that the computer uses to calculate MPG work--are they based on volume in the gas tank and miles traveled, or are they based on weight vaporized and run through the ICE and miles?
Those two models MIGHT produce different MPGs depending on the ambient air temperature.
Or will the real experts tell me I'm wrong about that?

 

No, the spec is 0.5%...you are confusing percents and fractions. 0.5% is just less than a half a cup error in a 5 gallon fillup (3.2 ounces out of 640 ounces). So if you paid for 5 gallons you may end up with only ~636 ounces. At $3/gallon, you could pay $15 but only have gotten $14.93 worth of gas...worst case if the pump is in spec. Or think of it this way: if the advertised price is $2.00/gal then you might really be paying $2.01.

The standard test that various county's bureau of weights and measures does is to fillup an official 5 gallon container and they check that it is within the ~1/2 cup range. If the spec was 0.005% they would need to see that it was within about 1 ml. (There are about 20 drops in a ml -- there is no way that pumps could be economically mass produced to be that accurate over their lifetime...and there is no reason to since that would be accurate to less than 1/10th of a penny on 5 gallons).

3PriusMike