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...

Modern dielectric oil (aka transformer oil) is refined mineral oil
with an antioxidant (tocopherol, BHA, or BHT) added to retard
spoiling.

Mineral oil does not "spoil" in the sense that animal or vegetable fats
(long chain carboxylic acids) do. Tocopherol (Vitamin E),
butylated-hydroxyanisole and butylated-hydroxytoluene are used (in the low
part per million range) to prevent oxidation of the oil which produces
"sludge" and "varnish" (here used as referring to deposits from
hydrocarbons such as gasoline on fuel tanks and carburetters). All three
compounds are also used in the food industry - to prevent spoilage. :-)

Shell Diala AX
ExxonMobil Univolt 65

are the two major kinds (there's one from Castrol,too)

Both are sold (in 5 gallon pails, typically, as a minimum quantity) by
"jobbers" which can be found on the mfr's website (you enter a zip
code and gives you the distributors within X miles), or by looking in
the yellow pages under Oil,Lubricants-Jobbers. Used to be in the $4-5/
gallon range, but I just was talking to someone who had to pay around
$50 for a 5gallon pail. (probably a hangover from $100/bbl crude
prices)

You *can* use USP White Mineral Oil (laxative) available in pints at
the drugstore, gallons at the feedstore (If you've got a colicky
horse, gallons are the quantity wanted), but it's
a) more expensive
b) not water content controlled

For HV dielectric purposes water content (in the ppm range) is
important. So is particulate contamination. For a dummy load,
probably not so much.

The partioning coefficient of agricultural grade white mineral oil is
greater than one million. Dissolved water will have no measurable effect
on the dummy load.

Another inexpensive source of mineral oil without many additives is
hydraulic oil (as used in, say, tractors, etc.).

Even "straight weight" motor oil without additives/detergents can work
(look for the SAE 10,15, or 20 viscosities).. it can be VERY cheap on
sale as a "loss leader" to get folks into the store (since nobody in
their right mind would actually run this in an engine)

For good natural convection around and through the Kanthal-Globar silicon
carbide resistor used in the Cantenna, I would use no higher a viscosity
oil than SAE 5.

For a dummy load, viscosity IS important, because convective flow is
important. (viscosity change with temperature, too...)

Most organic oils are less viscous at higher temperatures. This is good in
the Cantenna application. The lubricating properties of the oil are
unimportant here.

Silicone would be massive overkill, and we won't even get into
Fluorinert. BTW, if you spill mineral oil, it cleans up nicely with
detergent and water.. the same cannot be said of silicone or FC-xx..
Silicone oils are almost impossible to remove.

This is an excellent point. Freon TF®
(1,1,2-trichloro-1,2,2-trifluoroethane) was excellent for removing silicone
oils and greases, and it had a number of physical properties that made it
ideal for many electronic applications. It is too bad that it is banned as
a ozone destroying chlorofluorocarbon. In looking at some of the
alternatives for cleaning away silicone oils, I would suggest looking at
Caig Laboratories (makers of DeoxIT®) CaiKleen NF which is mainly
1,1,1,3,3-pentafluoropropane. Unfortunately its boiling point is 15 C (59
F) and it is quite expensive.

Good post, Jim!

73, Dr. Barry L. Ornitz WA4VZQ