> On Oct 26, 2015, at 10:45 AM, Jerry Weiss <[email protected]> wrote:
>
> Water in the dummy load? Water cooled - sure. Water immersed? Even
> though I see it posted on the web, i have may doubts it would be a stable
> method.
How stable do you need?
>
> Oil is the usual medium here.
Transformer oil is a nice dielectric, so for high voltage transformers that's
what you want.
But for low voltage dummy loads, or for medium power ham transmitters, the
voltages involved are not that high. Plain tap water is slightly conductive,
but nowhere near as much as the resistors you're using. And water is a MUCH
better coolant than oil. This is why machine tools typically use water as the
coolant, or more precisely, water with a small amount of "soluble oil" mixed
into it.
It's true that resistors are not designed to sit in water indefinitely. But
that means that, after a year or so, they may have been damaged. Hours or days
of immersion doesn't bother them at all.
If you want a good UHF dummy load, or one with 1% accuracy, this is probably
not the best answer. If it needs to dissipate multiple kW, or last for a
decade, ditto. But for a basic solution at near-zero cost, it works great. If
you're not confident the load is still good 6 months from now, just spend 50
cents making another. Or lift the resistors out of the water-filled jar when
done with the day's testing.
paul, ni1d
