Vacuum Variables

[K9ACT]

According to the attached, the dielectric strength of air at sea level and vacuum are the same.

We know that the dielectric constants are, for all practical purposes, the same.

So the question is, why do vacuum variables exist?

Seems like oil is a much better option for both reasons.

js

[W3RSW]

The ionization of gas molocules in air make for a much easier path for electric discharges.

...and once a path or potential path is establshed, then....

[N3DRB The Derb]

a vacuum is by definition not air. It's the absence of air and the molecules that make it up.
One of the places I worked for years ago as a engineering assistant had a vacuum chamber that could pump down to a pretty good nothing. The round walls of this thing were 3" thick steel.

The boss came by and asked if there was anything in there, and the dude I was working with said "no, not even air". I didn't get that he had made a funny in his dry engineering sense of humor way for a day or two. 

ionization cannot take place in a good vacuum. there's nothing to start the spark.

[k4kyv]

Plus, oil probably has more rf loss that a vacuum, or even air.  And there is no guarantee that the dielectric constant of the oil would be stable with temperature, so an rf oil capacitor might have a positive or negative temperature coefficient.  It would be a real pain to have to re-dip the plate every time you had transmitted for a short period.

The drawback with vacuum capacitors is that they don't have an infinite life expectancy, as do bread slicers.  Air slowly leaks into the envelope, just like it does with a tube that has lain on the  shelf for a long period of time.  So theoretically, the capacitor would lose its vacuum after many decades of use, although I use a WW2 vintage fixed vacuum from a BC-610 in one of my antenna tuners, and seems to work as well as new. This might be a greater problem with a vacuum variable, since the mechanical configuration is more complicated, and the thin bellows will eventually wear out from metal fatigue and spring a leak.

A slightly leaky vacuum is worse than no vacuum at all.  At low atmospheric pressure, the gas tends to ionise easily.  That's why a gassy tube turns blue inside at normal plate voltage, even though it may not arc over between the elements.

[K9ACT]

ionization cannot take place in a good vacuum. there's nothing to start the spark.

So then, why is the measured/published  dielectric strength the same?

js

[KD6VXI]

ionization cannot take place in a good vacuum. there's nothing to start the spark.

So then, why is the measured/published  dielectric strength the same?

js

The absence of any impurities?  Just a theoretical guesstimate, but maybe the absence of air means no humidity or other "elements" in the air.

PURE vacuum takes variables out of atmosphere...  After all, atmosphere at 1K feet is a LOT different than at 11K feet.

When I lived at 6300 feet, I had to move a LOT more air across my tubes to keep them at rated temp...

Maybe that's the idea behind a vacuum cap... Take the "variables" out of the equation to make a cap that works everywhere, and doesn't matter the rH of the air, and other variables.

--Shane

[W1ATR]

As already said, it totally has to do with keeping the ratings uniform so a piece of equipment can be designed around parts whose tolerance is known. As far as Vac Variables go, most are used at voltage potentials that could destroy equipment. Under a vacuum, we know that it will take x volts to form a corona strong enough to ionize x gap (spacing between plates). If that component is used at different altitudes, the tolerance would not be the same. (Voltage de-rating), but under a vacuum, the tolerance doesn't change.

That 20-75kv rating in your post just indicates the corona discharge across a given distance. The closer you are to sea level, the higher the kv withstand voltage is. (WHAT?, bullshit!, no really, it isn't) It would make sense that the closer you are to sea level, the thicker the air would be and the easier an arc would form. It's actually the opposite. According to Paschens Law, as you go higher in altitude, the air thins out and there is less molecules for the arc to conduct across, therefore it ionizes a path easier than it would at sea level where there are more molecules to jump thru. Oil has a shitload of molecules for a given distance compared to air, so it's dielectric breakdown is much higher.

73

[w4bfs]

the ionization potential of a gas (example: nitrogen) is essentially the same as air as described .... as the pressure is reduced by either a vac pump or higher altitude the ionization potential drops .... at a partial pressure of 1 Torr ( which is as I recall is about 10E-6 mm mercury column) 2500 V will easily ionize several feet of nitrogen between two electrodes ... as the pressure is further decreased, the ionization potential goes back up ....hope this helps, Jack ....73....John

[N0WEK]

A slightly leaky vacuum is worse than no vacuum at all.  At low atmospheric pressure, the gas tends to ionise easily.  That's why a gassy tube turns blue inside at normal plate voltage, even though it may not arc over between the elements.

I used to see this in the Neon shop. We'd hook them up to a 20,000 volt @ 1 amp transformer and start the vacuum pump, when it got down to a partial vacuum the remaining air would ionize and heat the tube, at that point we'd stop the pump. When the small piece of paper we placed on the tube started to burn we'd shut off the power and restart the pump and pump down the tube while it cooled off and then add the neon or argon (plus mercury).

The EPA and the lawyers would go nuts now, we laid out the designs on asbestos paper, added mercury to the argon tubes and played with very high voltages; all of this in a nearly unsupervised college art studio. Of course this was in the 1970s.