Dick.
The major properties that should be important for a liquid used in the
dummy load or transformers is:
Solvent = solvents will destroy the resistors.
Flammability - not a big concern for most oils in our application. Don't use
kerosene or similar
Viscosity - not important for our application as long as it flows (isn't
thick).
Dielectric Constant - mineral oils, vegetable oils and various types of
transformer oils are all approximately 2. This number affects the RF
impedance; see transmission line construction.
Voltage Insulating properties - not a big deal for this since the
greatest voltage will be 250v at 1kw across 50 ohms. Just don't use water.
Thermal conductivity - this is the big one and is a measure of how much heat
will be pulled away from the resistors; higher is better.
Silicon transformer oil = 0.150 W/mk
Synthetic transformer oil = 0.144
Mineral transformer oil = 0.126
Regular mineral oil = 0.138
Now, I don't know why mineral oil is listed with better conductivity than
mineral transformer oil , except for maybe the additives needed to improve
high voltage breakdown (not an issue for us).
See articale further down page on " regular automatic transmission fluid is a
mineral oil product"
So, the point is that Silicon transformer oil would be the best, but plain
old mineral oil (baby oil) will be only 10% less efficient. That 1 kw
should be down rated to 900 watts or operate for shorter time. I
can't imagine losing a minute of sleep or spending any significant money on
the difference.
Just don't use water = low voltage breakdown, or ethelyene glycol
(antifreeze) = dielectric constant is 30.
> Here's the relevant text from the Handbook article (posted here under
> "Fair Use" Copyright provisions):
>
> From the 1985 Radio Amateur's Hanbook, pp.34-17 to 34-19:
>
> Selection of the coolant for the resistor involved a lengthly
> investigation. The use of PCB transformer oils is illegal and involves
> a serious health hazard. The investigation did identify several
> modern-day transformer and heat transfer oils that could be used, but
> none are available for purchase in any quantity less than a 55-gallon
> drum. Transformer oil has lower viscosity. That means lower resistor
> tepmeratures, but at the penalty of using an oil with a 140 to 150 C
> flash point (vaporization temperature) . Transformer oil is also treated
> to remove water, but maintaining this dry condition is difficult and
> expensive. Calculations of the fluid heat transfer show that turbulent
> nature convection flow exists for rated power levels. The use of higher
> viscosity oil produces higher resistor temperatures, but the gain in
> flash point temperature is greater than the rise in resistor
> temperature. Oils designed for heat transfer have flash points in the
> 230 to 250 C region.
>
> Heat transfer oils are composed predominantly of mineral oil. Highly
> refined or medicinal mineral oil has a usably high flash point but at
> an undesirably high cost. Heat transfer oils contain small amounts of
> rust and oxidation inhibitors, making them preferable to mineral oils.
> Heat transfer oils are not available in small quantities, but in at
> least one case the very same product is relabeled and sold in 5 gallon
> cans for service as a "turbine oil". That product is sold by TEXACO
> distributors as Regal Oil R&O No. 46.
>
> Use of automotive and transmission oils is undesirable. They are
> composed of a wide mixture of oils with different flash points. Motor
> oil additives will damage the metal platings on an RF resistor. There
> is no danger in the short term use of automotive oils, but the
> long-term effects are a problem. Further, there are no economic
> advantages with automotive oils, as they cost as much as turbine oil.
>
> --from the "Super Dummy" construction article, designed by ARRL TA Dick
> Jansson, WD4FAB.
>
> 73
>
articles from other group, but provides a good understanding on which oil to
use.
Me i refilled mine with regular transmittion oil, after 8 years no problem. i
just dont run it at full spec.
73's
Dave
kg4uxr
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