Allan asked for the dielectric constant of avgas
for use with capacitive fuel quantity sensors

It varies a bit with frequency, and a lot with temperature

at 1 Khz it is appx 1.94
  it goes up a bit and
at 100 KHz it is again 1.94
at 10 MHz it is 1.92

all at 20 deg.C

only using a 555 as an oscillator is not very precise unless you make some
compensation for water content.
Remember to take the propagation delay of the 555 into account as it becomes
non linear with higher frequencies.
As dielectric strength of AVGAS varies with temperature, a temp sensor or
compensation capacitor would also improve the result.


Regards
Villi H. Seemann
Senior Engineer
Infrastructure Network
Phone  (+45) 3333 2101
FAX      (+45) 3333 1130
CellPhn (+45)2220 7690