# Re: air bubbles (fwd)

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---------- Forwarded message ----------
Date: Tue, 19 May 1998 19:43:36 -0700
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: air bubbles (fwd)

The horrible thing is that the voltage across the section with the lower
dielectric constant (air) is higher, and it has the lower breakdown field
strength. Consider it as a set of series capacitors. The capacitance is
proportional to the dielectric constant. In a set of series capacitors, the
voltages divide in inverse proportion to the capacitance, i.e. less
capacitance is more voltage.

Let's use a bubble 1/3 of the thickness for an example. PE has a dielectric
constant of roughly 2 (to one significant figure), so it will look like a
series string of capacitors with values 2:1:2.  The voltages will divide
out as 0.25 : 0.5 : 0.25.

Take 30 kV/cm as the limiting voltage for air. This means that the
breakdown E field  for the laminate of PE:air:PE would be 7.5 kV/cm + 15
kV/cm + 7.5 kV/cm (where the length is the TOTAL length of all three
layers) or 30 kV/cm. Oddly, for this example, the breakdown is the same as
if you used air alone as the dielectric. That's because I chose a
dielectric constant of 2 for the PE.

The fact that bubble is spherical changes it all a bit, but you get the
idea. In general, bubbles are bad. They lead to field concentrations, which

This is reflected in the dramatic difference in breakdown strengths between
"PURE" liquids (megavolts /cm) and ordinary liquids with contaminants (tens
of kV/cm)

>
> ---------- Forwarded message ----------
> Date: Wed, 20 May 1998 00:12:02 +0300
> From: Harri Suomalainen <haba-at-cc.hut.fi>
> To: tesla-at-pupman-dot-com
> Subject: Re: air bubbles (fwd)
>
> >I am in the process of building one of two large capcitors that I
> >will use in series on the new coil I'm building. The problem is two of
> >the six sheets of poly I ordered have small air bubbles in them. Does
> >anyone know If this will cause the cap to breakdown? I'm using two
>
> It will cause problems. Depending on size and shape of bubble the
> electrical field in the bubble is many times the field on insulator.
>
> The answer is that instead of normal 20kV/mm (PE) you might
> need to derate it to perhaps 2-4kV/mm or it will fail pretty soon.
> Note that air is better insulator than this :)
>

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