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Author: Subject: Determining gasoline volume - best method
RogueRose
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[*] posted on 26-12-2018 at 17:37
Determining gasoline volume - best method


I'd like to check to see if a local gas station is pumping the right amount of gas b/c I got almost 2 gallons more in my tank than what it should hold and it was right after the gas light came on.

I was thinking of getting .9 or 1 gallon of gas in a gallon jug (heavy duty HDPE jug for 50% H2O2) and weighing it (taking into account the tare weight of the jug) but then I started worrying about the density of gas from season to season as it can be .71-.78g/ml for pure gasoline and when you add in some percent of ethanol @ .79g/ml (usually 10% or less) then that makes the weighing method very difficult to get an accurate answer.

I do have 250ml glass graduated cylinders as well as a 1L plastic grad cylinder (IDK if it is resistant to gas, don't know the plastic composition) but measuring 3785ml gas would require filling the 250 15+ times which seems very cumbersome and possibly inaccurate.

Would making a mark on the jug with known amounts of water be more accurate as I can weight it to know the amount of water in it. The top of the jug narrows a lot for the last 200-300ml and would give fairly accurate measurement as it is about a 5.5" rise for the 200-300ml, so it is KIND of like a volumetric flask in a way.



On the other hand, I might be able to find (buy/order) some 2L, 3L, or 4L volumetric flasks which I could use instead but I'd rather not spend $$ to prove this.





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Ubya
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[*] posted on 27-12-2018 at 02:43


well if your car has a 15 gallons tank, and you think that they sold you just 13 gallons, this means that the pump station is selling 0.866 gallons as it was 1 gallon. to verify this you just need to measure with a better accuracy than 0.5L, so even the 15+ measurings using your 250 ml measuring cylinder is more than enough

[Edited on 27-12-2018 by Ubya]





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wg48
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[*] posted on 27-12-2018 at 04:11


In the UK they have a government department called Weights and Measures. They routinely check the accuracy of fuel pumps used for selling petrol and lots of other things. If they receive a few complaints about a particular garage they go and check the pumps. I guess they have the similar departments in all the EU and the US.

If you really think its that far out report it and if a few others do as well it will probably be checked.

In the UK at one time many petrol pumps blended two grades of petrol to produce a third grade. This was done with a special valve in the pump. Which on many pumps had failed. So the wrong grade of petrol was dispensed. Surprisingly there where not that many complaints from customers or garage managers compared to the number of faulty pumps. Apparently the valve usually failed in the position that dispensed the usually purchased low priced grade or that was the reason given why so few people had noticed the problem.

I guess the garage managers where happy to not worry that they were dispensing the cheap stuff and getting paid for the more expensive grade LOL.


[Edited on 27-12-2018 by wg48]




Borosilicate glass:
Good temperature resistance and good thermal shock resistance but finite.
For normal, standard service typically 200-230°C, for short-term (minutes) service max 400°C
Maximum thermal shock resistance is 160°C
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Tsjerk
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[*] posted on 27-12-2018 at 05:02


Weigh a ""gallon", and afterwards determine density by weighing a known volume.
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Sulaiman
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[*] posted on 27-12-2018 at 05:39


If your plastic volumetric flask is anything like the cheap one that I bought via eBay from China,
then I would bin it ... now. It is of no use for anything 'scientific',
(maybe an ugly flower vase ? ).
If normal volumetric grading is A or B, these are F****** useless.

The volumetric marking should be used a guide only as it is inaccurate
and varies with temperature, not just thermal expansion, shape-deformation.
Just in case, test vs. 4x 250ml before binning :D
...................................................................
Here in UK we are not supposed to use glass containers at a pump,
only 'petrol cans' are allowed - usually made of plastic.
....................................................................
Whatever measurements you make will not be usable as evidence so you only need enough accuracy to be sure that cheating is occuring before you report your suspicions.
....................................................................
As the fuel is sold by volume it is best to measure by volume.
Buy a certain ammount, take home and measure as accurately as you can,
given the importance to you of not making a false statement/accusation you should take whatever time is required to be as accurate as required,
even if it means 4.5l/0.25l= 18 measurings.

Obvious but I'll write it anyway,
Keep the fuel clean and you can use it after measuring it




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Tsjerk
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[*] posted on 27-12-2018 at 06:07


18 measurements means 18 times the inaccuracy. Weighing is as precise as your balance is.
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Ubya
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[*] posted on 27-12-2018 at 07:02


Quote: Originally posted by Tsjerk  
18 measurements means 18 times the inaccuracy. Weighing is as precise as your balance is.


a graduated cylinder should have an error of +-1ml, even if 18 measurements are needed the error is still +-18ml, given the fact the the fuel pump is probably selling 0.86 gallons as 1 gallon, an error of 18ml is negligible

plus we need volume not mass, we would need to know the density first, and at this point might as well measure everything by volume

[Edited on 27-12-2018 by Ubya]





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Sulaiman
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[*] posted on 27-12-2018 at 07:13
Although I prefer to weigh reactants for molar ratios,


In this case;

(accurate weight) / (inaccurate density) = inaccurate volume.

The 250 ml measuring cylinder will have to be used at least once
the pecentage error of the measuring cylinder is the critical factor
unless the scales can not measure to whatever bulk volume of petrol is weighed
with the resolution required for calibrating the 250ml sufficiently accurately.
e.g. 250 ml measuring cylinder = +/- 1ml = 0.4%
To weigh one gallon scales of 3.5 kg capacity would be required,
So your scales would need to be >=(3.5 kg plus container weight) capacity with <0.75g absolute error
- to match the measuing cylinder accuracy.
(Unless you have more than one set of scales)

You will need suitable calibration weights also. ($n)

[Edited on 27-12-2018 by Sulaiman]




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