Fluids (including natural gasoline/petroleum) have variable volume based on the ambient temperature for the same mass of fluid. So, really, the amount of gas that you're filling your car with depends on the temperature because it's not the volume of fuel that makes your car run, but the mass which is combusted. In aircraft, aviation fuel is always measured by the kilogram. So, why is it measured by volume for cars?
$\begingroup$
$\endgroup$
9
-
$\begingroup$ i often see at filling stations x price/kg and a pressure indicator at 200bar. $\endgroup$– user28737Commented Oct 9, 2013 at 6:37
-
$\begingroup$ Link en.wikipedia.org/wiki/Gas_pump#The_metrology_of_gasoline Most pumps also indicate the fuel density. $\endgroup$– user80551Commented Oct 9, 2013 at 6:38
-
1$\begingroup$ Because it's much easier and cheaper to measure volume than it is to measure weight? $\endgroup$– John RennieCommented Oct 9, 2013 at 6:39
-
2$\begingroup$ Because a car's fuel tank has a fixed volume? $\endgroup$– phoCommented Oct 9, 2013 at 7:32
-
$\begingroup$ Well, you can apply the same argument to a aircraft's fuel tanks, though weight is usually a much bigger concern than having fully filled tanks... $\endgroup$– shortstheoryCommented Oct 9, 2013 at 10:48
|
Show 4 more comments
4 Answers
$\begingroup$
$\endgroup$
8
I take it that the question being asked by OP is :
"""...Why is gas(oline) in gas stations sold by volume (as opposed to mass)?..."""
The answer is simple. Gas stations get their gasoline delivered by "jobbers" who can supply them with gasoline that can come from a variety of different sources; even from different manufacturers. Gas stations are equipped with simple equipment that can measure fluid VOLUME, and as most liquids, are of low compressibility (not zero), that is a fairly reliable measure (for commerce).
No gas station is equipped with any kind of equipment that can measure MASS which is a rather complex physical parameter to measure, and requires highly specialized equipment.
On the other hand, If I have read the question incorrectly, then forget that I gave an answer.
-
$\begingroup$ This looks like a reasonable explanation. But I'm sure it's not impossible to make a system designed for calculating mass dispensed by taking specific gravity and volume into account. $\endgroup$ Commented Oct 10, 2013 at 16:01
-
1$\begingroup$ Mass; even under laboratory conditions, is commonly "measured" with a mass balance, which directly matches the weight of one body against the weight of some known mass body(ies). It is presumed that the gravity is the same under both balance pans, so it cancels out. As a result the method requires some digital summation set of standard masses to form any combination of mass, and that has to be done over the whole range of masses to be measured. And the mass balance needs to be able to withstand the weights of those masses. Any other method presumes knowledge of local (g) $\endgroup$– user26165Commented Oct 11, 2013 at 2:40
-
$\begingroup$ So you propose a method to dispense a known mass, by taking into account the (to be measured) volume of the specimen. Simply wonderful; why didn't I think of that. $\endgroup$– user26165Commented Oct 11, 2013 at 2:41
-
$\begingroup$ I guess it's safe to assume that the acceleration due to gravity is consistent in all places on Earth's surface. IMO, just simple weighing scales would suffice! $\endgroup$ Commented Oct 11, 2013 at 7:54
-
$\begingroup$ Well shortstheory, it most definitely is NOT safe at all to assume that; in fact it is quite untrue. In 2007 an international sailboat racing series ran into a problem, when the series moved to a sailing site in Sweden. Everyone of the boats failed measurement, when they all tested overweight; and all had to be modified to pass. The problem was the weight of the boats was specified in the rules; not the mass; and Sweden has higher gravity than tropical places. $\endgroup$– user26165Commented Oct 11, 2013 at 22:45
$\begingroup$
$\endgroup$
CNG fuel dispensers indeed measure mass not volume. Mass flow meters (coriolis)do that job. Pakistan has the biggest number of CNG vehicles as its a bit low cost(India is lagging behind Pakistan in CNG:) @Qmechanic wrongly edited your question.
$\begingroup$
$\endgroup$
Interesting question -- my wild-ass guess is that they do it this way because there is a huge installed infrastructure and technical culture around volume-based measurement and accounting, in both the oil and auto industries, and it would be a correspondingly huge inconvenience to migrate to a mass-based system.
There are now available highly-accurate online flow densitometers that derive their signal from the natural frequency of a vibrating tube immersed in the fluid. No moving parts! Google a few obvious terms to find out more. They are accepted by oil ministries and other regulators for transfer of custody of petroleum, and it's not hard to imagine them being scaled down to forecourt pumps. But the sensors and electronics didn't exist when the industry was building-out its distribution and retail infrastructure about 100 years ago, and by the time they came along (50s? 60s? later?) it was too late.
$\begingroup$
$\endgroup$
6
First of all in aviation (from my previous work in this field) fuel usually measured by "US Gallons" which is a measure of volume, anyway usually in big tenders sometimes they measuring it in Tons (for a what called Jet A-1 class of this fuel -used for aircrafts- it is 1 Ton=331.3 US Gallons if I remember it right).
And regarding natural gas, the fact that up to a very high precession, most liquids can't be compressed, thus if you compress the gas to convert it into liquid, and knowing the density, you can always calculate it's weight, and this is very comfortable because it's much more efficient to transport gases in their liquid form.
Also considering natural gas heat capacity and temperature expansion coefficient, in usual temperature conditions it's volume changes in a very small amount, and I believe that it's much easier to dial with liquid gas in the car's engine than in it's normal state.
Finally, in your place, I would worry more about the "Energy conversion efficiency" of the gas, because usually the natural gas comes with a lot of other gases, that are not useful -in sense they can't burn in usual conditions-, this may cause your car to walk 50 km instead of 200km using same amount of gas.
-
$\begingroup$ ... gases can't be compressed? $\endgroup$ Commented Oct 9, 2013 at 10:40
-
$\begingroup$ I also learned about aviation fuel being measured in kilos from this site Could be wrong though. $\endgroup$ Commented Oct 9, 2013 at 10:54
-
$\begingroup$ @EmilioPisanty: Please read more carefully, I was talking about liquids... $\endgroup$– TMSCommented Oct 9, 2013 at 19:37
-
$\begingroup$ @shortstheory I dialed with big airlines and Cargo aircrafts, I think those guys talking about private aircrafts which some times uses other classes of fuel, anyway I think it's not relevant to your question. $\endgroup$– TMSCommented Oct 9, 2013 at 19:40
-
$\begingroup$ @TMS so say liquids instead of fluids. $\endgroup$ Commented Oct 9, 2013 at 19:48