Weight of Fuel

[bertiethebadger]

I should probably know this:

What is the weight of a gallon of AvGas?

Thanks,

[Fly Stimulator]

African or European?

[FlyingForFun]

Water is easy to remember. 1 litre weighs 1kg, 1 gallon weighs 10lb.

Fuel weighs about 0.72 times as much as water. So a gallon of fuel weighs a little over 7lb.

Rather embarrassingly, though, I can't remember whether this is an American or an Imperial gallon! I'm going to stick my neck out and say that it's an Imperial gallon.

(Exact weight would depend on the temperature, of course, which is why airliners measure fuel in terms of its weight rather than its volume.)

FFF
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[RodgerF]

Imperial Gallon. The weight of water is the definition.

[FlyingForFun]

Thanks Roger - thought so, but wasn't sure!

FFF
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[bertiethebadger]

Thanks guys.:ok:

[englishal]

US Gal is 6lbs.....

[flynverted]

Actually, a U.S. gallon of water weighs 8.3452641 lbs. :cool:

http://www.onlineconversion.com/waterweight.htm

[parris50]

Flying for fun...

Why does the weight of the water depend on temperature? Surely, 10 lb of water weighs 10 lb even if its temperature changes.

[PhilD]

parris

When fuel (or water or any other liquid) is heated it expands, so your 6lbs of fuel occupies more than 1 US gallon (which is a unit of volume). The other way of looking at this is that a gallon gets lighter as you heat it up.

[FlyingForFun]

Why does the weight of the water depend on temperature?Ok - splitting hairs now.

I agree that if you change the temperature of water, its weight will not change - its volume will.

But the question asked how much a gallon of (some liquid) weighs. Measure out a gallon of water at 5 degrees, and weigh it. Then increase the temperature to 25 degrees, and weigh it again. Of course its weight won't have changed (ignoring the effect of evaporation). But it's no longer a gallon - it's now more than a gallon. So yes, if you think about it, a gallon of water at 5 degrees weighs more than a gallon of water at 25 degrees. :ok:

FFF
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[englishal]

Hmm...[thinking cap on, trying to remember what I learned in A level physics many summers ago....]....

Isn't water one of those strange things where the density decreases [volume increases] with decrease in temperature? Hence the reason why your pipes crack in the winter if they freeze. I also seem to remember something about water being least dense just above freezing [2-3°c] ......

EA:D

[FlyingForFun]

Yes, englishal - but only around freezing point, as you say. Once it's above about 2 degrees, it behaves much more normally! :D

(Seem to remember it being to do with big gaps in the molecular structure of the ice crystals, but I didn't understand it when I learnt it, so there's no chance of me understanding it now!)

FFF
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[Mariner9]

Splitting hairs even finer here....

The anomalous expansion of water (with cooling) occurs from 4deg C to 0 deg C

Avgas always expands with increasing temperature (and vice-versa). Density is generally quoted at 15 deg C (Metric system) or Specific Gravity at 60 deg F (imperial system)

Therefore to accurately determine weight, you need to measure volume, temperature, and Density/SG. Then apply a correction for buoyancy. (-0.0011 g/cc) for density) Easy isnt it!!!

Simple answer:

1 US gallon weighs 6lb
1 Imperial gallon weighs 7lb

[foghorn]

FFF

You're right with water - the relatively large dipole moment (difference in charge) between the Oxygen and Hydrogen atoms in Water produce very strong intermolecular forces known as Van-der-Waals forces. These keep it a liquid at room temperature and keep its liquid structure really quite compact, making it quite dense.

However ice has a regular cubic structure, which is not very efficient in packing the molecules in - there are really quite large interstices in the crystalline structure. This makes water at 0 degrees more dense than ice at 0 degrees, and for a good few degrees above zero as well, giving the anomolous expansion upon freezing.

No prizes for guessing which degree Foggy did at Uni...

[28thJuly2001]

....bet Bertie wishes he never asked.
28th,,

[Flyin'Dutch']

Take it that Bertie wanted to have this information as an aide to flightplanning and more in particular the weight and balance thing.

Bertie, if you have a look in the W&B part of your POH you are likely to find a table with volumes, weights and moments which will help (as others have mentioned keep an eye out for the US/IMP gallon trap!)

And if you forget what the weight was (as I always do as it comes under the heading of rarely immediately needed info) have a look in the info section of any good VFR guide. A fond of knowledge.

Have fun!

FD