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Re: cap oil

Bob Volk,

I've done a little experimenting with oils.  90 weight gear oil is
probably not what you want.  In general, all oils formulated for
lubrication should be suspect.  Graphite, and buffers can be added that
will make the stuff conductive.  Gear oils can contain animal fat.

Graingers sells some "dielectric" oils, but I have no experience with
them.  White Kerosene works well, but I wouldn't recommend it for its
flammability.  You're better off staying with the Drugstore variety,
mineral oil.  

Copier oil is good stuff (a little on the thin side) if you can buy the
oil without the toner added.

Paraffin wax is a very good dielectric (but hard to pull a vacuum on and
heat at the same time).  Paraffin wax shrinks a great deal, upon
cooling.  Wax has to be added to the cooling wax, or the level has to be
well over the capacitor plates to begin with.

You can test oil:

If you have a source of high voltage it isn't very hard to test the oil
you plan to use.  Take a small sample in a plastic or glass container
and run some well insulated leads into the oil and a small spark gap,
bring the gap closer and closer until a spark jumps.

I use a 10 ma transformer, and use a variac to bring up voltage slowly. 
Keep a 20 milliamp lamp (voltage doesn't matter) in series with one
lead.  My gap is a ring of some one inch PVC pipe, that has machine
screws (ends covered with acorn nuts) driven from opposite sides.  I
shut down the power and bring the gap closer between tests.  Start with
a 0.1 inch gap.

The lamp should not glow at all unless/until the gap fires.  If you can
close the gap to 0.01-0.02 with 10 KV across it, the oil is very good. 
I do a further test of conductivity (assuming the lamp remains out).  

First, open the gap until there is no possibility of it firing. Then put
a well insulated (mechanical-not digital) 0-1 AC milliameter in series
with the high voltage, there should be no or slight deflection of the
meter needle.  If your test set-up contains long wire runs and/or the
wires are spaced close together, capacitive reactance may be enough to
cause a slight current flow (I haven't seen it).  Avoid meters with
styrene plastic cases, they pick up a static charge that will deflect
the needle.

Perform the tests outdoors and well away from flammable structures. 
Stay as far away from the test vessel as you can.  (you can hear the gap
fire)  Limit current to the gap, to less than 10 milliamps.  Keep a
suitable fire extinguisher handy. Shut off the power before attempting
to extinguish a fire. Keep the volume of oil under test small.  Put the
test set-up in a metal basin to contain a fire or oil should the test
vessel break.  

I haven't started a fire using the above method, but the device does
tend to spit a little oil when the gap fires.  Removing an oil-covered
gap from the test container, and then firing it, is a guaranteed fire.

If your only NST is a high current one, you can drop the current by
wiring a 7 watt, 120 volt lamp in series with the mains power (assuming
a 120 volt mains).  Tungsten lamps have low resistance until the
filament heats.  This technique won't work, if the unloaded current is
high enough to cause the filament to heat.

Stay safe.  Testing oil in the above manner is inherently dangerous. 
The lamp in series with the HV lead to the spark gap is necessary for
safety.  Keep the lamp well away from the test vessel (it can shatter if
the current is too high).  Limit the current to 10 ma or less at the
gap.  I accept no liability for your actions!

Good luck.

bob misiura