Allan Saul asked, "Does anyone know the Electrical Dielectric Constant
of Avgas ?"

A quick search with
"Dielectric Constant" and ( Avgas or gasoline)
at Altavista returned a result with  
http://www.asiinstr.com/dc1.html
where there is a table with many dieletric constants.  They give
gasoline as:
GASOLINE (70° F ) 2.0

Also there was and article on home construction of a capacitance gas
gauge a:
http://www.rst-engr.com/
click on:
Magazine Articles
then click on:
Gauging Fuel Quantity, Cheap

RST gives the dielectric constant as 1.94.


Be very aware that water has an extremely high dielectric constant and
typically comes along with high conductivity due to ionic contaminants,
so if your tanks end up with water in the mix, the reading will quickly
be very erroneous.

Regards,
Charles R. Patton
N360JM