Breakdown voltage of air

[wgoldfarb]

I am trying to calculate the minimum distance I can have between two electrodes with very high potential difference while still preventing arcing (for something I am trying to build).

The emitter (negative electrode) is made from copper and ends in a sharp point. The open circuit voltage at this emitter is -20KV DC at about 50 uamps.

The grounding "plate" is actually a short piece of copper pipe, arranged so that the axis of the pipe points towards the emitter. This geometry is based on the ion generator shown in the middle of this page.

Right now I have the grounding pipe set so that the edge closest to the emitter is about 4cm away from the emitter. I know this is more than enough to prevent arcing, but I'd like to reduce this distance as much as possible while avoiding arcing under the entire range of conditions (i.e. air pressures, temperatures, humidity levels, etc) that a person may encounter at home, regardless of their location, weather, time of year, etc.

I know that the breakdown voltage of air at STP is 33KV/cm. If I understand this correctly, at -20KV and STP, arcing should not occur if the distance is over 6mm. Is this correct?

How are these distances affected by the geometry of the grounding plates and/or the emitter? Is the distance affected by the fact that I am using a pipe instead of a flat grounding plate?

Would enclosing the assembly in a dielectric material have any impact? (I am thinking of putting the emitter-ground assembly inside an acrylic tube with a diameter just slightly larger than the diameter of the copper pipe). I don't think it should, but just want to make sure.

I am also thinking of having more than one emitter point, each one with its own "grounding pipe" (like they did in the device linked above). Will the number/geometry of emitters affect the distance significantly?

Using a distance of 1.2 cm would be compact enough for what I want, and provide a "safety margin" of 100% over the 6mm "theoretical" distance (I would need 40kv to produce an arc across 1.2cm, which is twice the voltage I am using). Would this margin be sufficient to take into account all these issues and avoid arcing?

If anyone has any links to more info about how to estimate the voltage required for arcing under different conditions, it would be greatly appreciated!

PS: yes, I know the precautions needed to work with high voltages

 

[Triumph]

What you have is basically a capacitor with air as the dielectric material; the capacitance of your set up is determined by geometry and if you want to figure that out, you are going to have to open up a physics book and determine some Gaussian fields and electric charges. However I do not believe that the dielectric strength (breakdown voltage) is dependent upon geometry, and I think you are on the right path, all you need to worry about is how a material's breakdown voltage varies with temperature, pressure, and humidity. I don't have the answer to that, but a quick google search brings up a bunch of results. Or you could try to figure it out empirically.

 

[silverpig]

You could encase it in something air tight and then pump it down to low pressure.

 

[wwswimming]

Originally posted by: wgoldfarb

I am trying to calculate the minimum distance I can have between two electrodes with very high potential difference while still preventing arcing (for something I am trying to build).

will you be blowing air through the assembly ?

building an ozone generator ?

 

[Modelworks]

An easy solution is to use some high voltage tape, either rubber or silicone.
Its good to 60KV+

http://www.aplussupply.com/plymouth/tape/tape.htm

Self bonding, highly conformable 30 mil EPR rubber - With Liner. Use as primary electrical insulation for applications through 69 KV. Rated for 90 deg. continuous operating temperature. Intermittent to 130 deg. C.
Will not crack at -40 deg. C.
EWHR307530L High-Voltage 3/4 x 30 ft $6.21

The problem with an ion generator is that the copper pipes have to be just outside the spark range. If they are too close, it sparks, too far it doesn't work well.
Here is another article on home made ion generators.
http://inventgeek.com/Projects/IonCooler3/overview.aspx

 

[wwswimming]

Kapton, polyimide-imide (?), a common material for flexible circuit
boards (and bulletproof vests), has a breakdown/standoff of about -

first .001" - 5000 volts
second .001" - 4000 volts
third .001" - 3000 volts
fourth-tenth .001" - 2500 volts

so a piece of .010 tape, between planes with rounded edges,
will stand off about 7 x 2500 + 12000 volts, approx. 30 KV.

Kapton tape is a good insulator for HV, with exceptions. we
used it everywhere, seemed like, at a HV power supply place
i worked at.

in some geometries & exposure conditions, though ... not a
good material choice.

kapton used as an insulator on round wires will sometimes
degrade over time - not good in airplane fuel tanks. ends up
acting almost like a firecracker fuse.

 

[wgoldfarb]

Originally posted by: Modelworks
The problem with an ion generator is that the copper pipes have to be just outside the spark range. If they are too close, it sparks, too far it doesn't work well.

LOL, exactly my predicament right now. I have my ground pipes set too far, I know I can get them closer. I am trying to determine how much to move them and stay outside of the spark range.

I did find an interesting Journal article about optimizing the geometry of a generator like the one I am using (nails and copper rings) so I will take a careful read and use their information to try and reduce the size of my setup.

Thanks!