Is this JUNK MATH?

[W2PFY]

A person is selling a capacitor on ebay. Here is his methout to prove it"s a good cap:

Gudeman 8178 high voltage 4 kilovolt kV capacitor at 60 microfarads uF. Likely a filter capacitor for a HV plate supply or energy storage capacitor for an x-ray machine. First $20 of shipping is free with BuyItNow.

I do not have a sufficient bench to test this at 4 kV.  Leakage resistance to case overflows meter at 2 Giga Ohms. (With one meter lead on the case and the other on each of the terminals I waited several minutes while the electrode--case charged from the internal battery of the digital ohmmeter and overflowed the 2000 MOhm scale) I also measured the capacitor (in parallel with warm humid Texas air) leakage resistance fairly precisely by

    1. Charging it with an old 9V battery (v0=7.98V).
    2. Waiting five minutes. (t1=300s)
    3. Measuring the voltage on the capacitor (v1=6.67V).
    4. Computing the leakage resistance v1 = v0 exp( -t1/tau) ==> tau = - t1/log(v1/v0), tau=rc ==> r=2.65*10^7 ohms.

So, the leakage resistance is 'astronomical'. (On the order of 0.84 times the number of seconds per year in ohms.)

[W3RSW]

..a great capacitor at 9 vdc. ?

Be interesting to plot a graph of typical HV capacitor resistances vs. voltage from 9 volts up to several thousand.  Then maybe we'd have a reference of sorts.

[Tom WA3KLR]

I got a similar number Terry.

The capacitor voltage falls to 83.6 % of the original value in 300 seconds. 
This is 0.179 time constant (T.C.).
1 T.C. then = 300/0.179 = 1673 seconds.
T.C. = R*C, solve for R; R = TC/C
1673/60 e-6 = 27.9 MegOhms, which is 2.79 e 7, close to 2.65 e7, not astronomical at all.

Not good I think anyhow for 8 volts.
Like Rick though, I am much more concerned about HV operation.

* You need to find insulation/leakage specs for such a capacitor to decide.

[W2PFY]

Thanks Guys, I didn't include a pic of said cap. It was all rusty and looked terrible. I think he used a nine volt battery because that's all he had.

Maybe he was using the old principal i.e. If you can baffle them with brilliance,
baffle them with BS 

[KA1ZGC]

I was all rusty and looked terrible.

We all know that, Terry... but what about the cap? 

[WBear2GCR]

Tom is now "Doctor House"??

Anyhow, the cap could work fine at some HV but breakdown just fine as some voltage threshold. This would be because of contamination of the vacuum, partial contamination. Like the gas that shows up in a transmitting tube. But in the tube there is a way to usually get rid of it by burning the tube. Afaik, no such way in a vacuum variable. It might flash over at some voltage. The question is what voltage?

Also, if there is a carbonized area inside the cap's stators that is not visible, this might also permit a flashover point. Usually there is no carbonization because it is a vacuum, but if there was some leakage, that might permit carbonization or oxidation of a surface area making an area prone to voltage breakdown - but this is speculation on my part, not based on actual experience.

Wish there was a good way to reseal vacuum caps and pump them down again... or put it another way, kinda wish there was someone who had the gear to do this trick and it was a service available. But I guess given the price of Ruskie imports, there's not much profit in such a thing... ah well...

               _-_-bear

[Tom WA3KLR]

Bear,

We're talking about a 60 microfarad foil capacitor, not a vacuum variable. 

We should have a rule on this forum that before posting, a person MUST HAVE READ THE WHOLE THREAD FIRST.

Old ARRL RULE - Receive for 5 minutes before transmitting.

[WBear2GCR]

Oh, I read it alright! 

Unfortunately the eyes and the brain were not connected at that time.
Where is my cane and seeing eye dog? Here boy! C'mon!!

Well then... I say it is a good cap.
Film/foil/oil caps are "self healing" I thought...
Unlikely that it is crapped out if the T/C is ok, imho...

So, is ZGC now Dr. House or not??

               _-_-WBear2GCR

[Opcom]

The guy should have cleaned it up.

It's a flashlamp cap. energy storage. Man it looks nasty. I'd offer $10 on a chance to see if I could clean it up and test it. His price is more like "good surplus", which that thing has not been for many many years while it rusted in a junk heap outside! It looks to have been painted silver. Should it be so shiny?

[W3RSW]

Hard to believe people take the time to list such stuff of ebay, item by item. 

A lot of us have a ton of old stuff around.   If I valued my time at 5 cents / hr. I might take the time to list it and sell it, that is if I even wanted to divert time from other stuff. 

Reminds me of the junk antique dealers who devote literally every weekend to hanging out in front of the trailer at some show or other.

I guess that's why estate sales include 'everything,'   Once you reach a certain point you just want to downsize everthing and value health and happiness far more than rusted capacitors.   

I'd be perfectly happy if I could find a little cabin in the hills with a fireplace,  a little acreage and everything in the house in working order.
.. uh, and a really neat shack outside with water and heat.
and..
and..
Well there goes the tiny cottage idea.

Say, that cap looks just like the one in my 813 transmitter.  I'd better check for leaks.

[WBear2GCR]

OMG!!!!

He polished the can with a sander or some such thing!!

The thing must have been out in the weather for the last 20 years... if the can isn't paper thin in those rust areas, I bet the cap is still ok!! Heh.

                 _-_-bear

[WD8BIL]

if the can isn't paper thin in those rust areas,

Nothing JB Weld can't handle

[Opcom]

give it the naval jelly then rustoleum, and pot it in epoxy, right up to the insulators..