Re: [Phys-L] Hydrostatics question

[John Denker <jsd@av8n.com>]

On 07/22/2013 04:28 AM, LaMontagne, Bob wrote:
> There is an interesting practical problem related to this. One can
> find the amount of gasoline in a fuel tank (like in an airplane wing)
> by dipping a hollow glass rod into the tank all the way to the bottom
> and then cap the open end of the rod with a finger. When you pull the
> rod out, you can see how much fuel is in the tank. The surprise is
> that the air in the top of the tube only expands enough to let the
> fuel drop a fraction of a millimeter - hardly affecting the reading.

Far more significant than the effect of the weight of the fuel
is /evaporation/ of fuel into the air in the headspace in the
top of the pipette.  This can be a big deal, depending on the
ambient temperature ... and depending on what sort of high-
vapor-pressure crud they've blended into the fuel.
  http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/summer-fuel1.htm

The crafty pilot will measure the tank twice in quick succession,
to build up an equilibrium concentration of vapor in the top of
the pipette.