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Old 19th March 2013, 01:11 AM   #111
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I can borrow nanoammeter but what I could of actually measure is a surface resistivity. To measure bulk one need a thin film I guess plus pretty good controlled environment to exclude surface water and such and that's rather hard to get your hands on.
EDIT
On the other end you can submerge a long spool into a salted water and measure it for yourself but, and I do not like that word, you need to make sure that isolation is defect free...

Last edited by alexberg; 19th March 2013 at 01:13 AM.
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Old 19th March 2013, 12:01 PM   #112
Calvin is offline Calvin  Germany
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Hi,

I wondered the same as bolserst. Different sources, different formulations/dopings, different measurement setups, different numbers.
The only reliable thing I could find is that PVC and PU are rather on the lower side of volume and surface resistivity and PE, PP and various FEPs and PTFEs are rather on the high side.

jauu
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Old 7th February 2014, 08:29 PM   #113
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Quote:
Originally Posted by portreathbeach View Post
I can get .5mm^2 or .6mm^2 PVC coated wire pretty cheap. The PVC insulation is 0.3mm, which is about 12mills. PVC is said to withstand 500v per mill, so hopefully this should work well for a first prototype build
Considering you want some of the electrical charge to escape the insulation and be in the air gap, can your sprayed on insulation, or wire insulation be too thick.
I know you can just up the voltage on the membrane to compensate, but wouldn't it be just as good or better to have a lower voltage, and a thinner coating/PVC.
For example I see 600 V PVC wire (thicker PVC jacket) and 300 V PVC wire (thinner PVC jacket). If the 300 V. wire has enough thickness to not arc at my high voltage working point it will be preferred to the 600 V. product right?
Thanks,
Paul
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Old 8th February 2014, 06:42 AM   #114
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I am thinking this is why you want a material that has a high Dielectric constant.

At least that is what Rodger Sanders had explained to me, But I have not grasped the whole picture of that subject yet.

jer
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Old 8th February 2014, 07:24 AM   #115
Calvin is offline Calvin  Germany
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Hi,

the issue is explained quite well, I think, in the 73s patent of Beveridge.
In that a stator is described, which is claimed to be alot more flashover secure and efficient at the same.
In short does it view at the behaviour of the electrical field and the flashover behaviour independently.
For a high flashover treshold the thickness of the stator counts, as this reduces the voltage stress gradient [V/mm or V/in] within the stator crossection.
This also regards the Q of the resistive value of the stator material and the airgap.
In a schematic this would be represented by two series connected
This implies at first glance that the volume resistivity of the stator isn't an issue at all, as a too low voltage gradient in the airgap just required to raise the Bias voltage.
On second thought, one will realize that the volume resistance should be prefferrably low to keep the voltage gradient within the stator material sufficiently low and to reduce overlad recovery time.
AC-wise -or in terms of the electrical field- the equivalent circuit is that of two series connected caps.
They form a AC voltage divider where the voltage distribution is inverse proportional to the cap value.
To aquire as much voltage over the airgap, and to loose as few Volts over the stator, the dielectric constant must be high, preferrably >10 times the airgap capacitance.
Beveridge suggested to mix a blend of conductive and high-K dielectrics with a (fluid) base material of high resistivity and low K, that could be cast into the desired shape to form a material with the improved R and K.
In thecequivalent schematic the resistor chain and the cap chain were paralleled.
In other words, this forms a frequency dependent resistor with a -not too high- DC resistance and low AC resistance.
A matter which J.Strickland also has written a note about.

jauu
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Old 9th February 2014, 02:03 AM   #116
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Thanks Calvin,
I (usually) always enjoy your answers as they show me how little I know.
If we could boil it down just a little...

If I'm planning on HV running at ~5000V.
PVC~ 550 V / mil
300 V PVC is 16 mils thick = ~8800V.

This should be more than adequate.

600V PVC wire at 32 mils thick = ~ 17000V

I Re read Jim Stricklands white paper, and he describes the charge being held like a ballast in the thickness of the PVC. Not sure if thats good or bad, so ...
If 300 V wire is thick enough to prevent arc overs, is it better than 600 V rated wire? (or visa versa).
Thanks,
Paul
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