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Re: dielectric constants




From: 	Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
Sent: 	Tuesday, November 04, 1997 10:20 AM
To: 	Tesla List
Subject: 	Re: dielectric constants


>  As can be seen there is a wide range, can't figure the high motor oil
> reading though, possible additives causing high readings?  Any comments
or
> suggestions?
 How did you measure the C? If you used a digital multimeter, it might be
that the oil is actually conductive or has water in it. This would give you
a polarization effect much like an electrolytic capacitor (which also has
Al electrodes).

The best way to measure dielectric properties is to put the substance in as
the dielectric in a transmission line then measure the transmission line
complex Z over frequency. This will give you not only epsilon (diel
constant) but also the loss coefficient.  And, you can make the conductors
of your transmission line insulated, and just fill the space between with
your unknown. You could either use a parallel strip line configuration or a
coaxial line.


>  I'll perform break down voltages for each oil on another day.  Of coarse
a
> high dielectric constant does not necessarily make it a good choice for
> capacitor construction.  The flash points may make some dangerous to use
if a
> spark occured in a sealed capacitor with any air in the enclosure.  Also
the
> thicker the oil the more easily the high voltage electric field will
force it
> away from the foil edges of a capacitor as noted in other experiments of
> past. 

To make the measurements compatible with handbook values, you should use
ASTM standard gap (.1") and electrodes.  BTW, the breakdown voltage is not
usually a linear function of the gap. Smaller distances have higher
breakdown voltages per length (i.e. the kV/inch is bigger). Another test
setup for breakdown that is useful is a sphere of reasonable size (say 1")
against a larger flat plane (say a few inches).