"Roy Lewallen" wrote in message
...
Owen Duffy wrote:
. . .
Anyway, to your original question of how much increase in power rating
with oil immersion. In the long term, it is determined by the ability of
the load to dissipate heat from the outside for a tolerable rise in
temperature of the oil and the resistor element. In the short term, it
comes down to the resistor element, and different constructions will
behave differently (eg metal film vs composition).

Yes, I'd worry a little about grossly overdriving carbon composition
resistors because of the thermal resistance between the heat-dissipating
composition material and the oil. There's going to be a temperature
gradient across the phenolic case that might be considerable -- in other
words, the resistance part could be much hotter than the oil. Resistors
which have the actual dissipating portion very close to the outside, like
film or (noninductive) wire wound resistors would be much better in this
regard. If you overstress carbon comp resistors you'll typically get a
permanent change in resistance. If the overstress is extreme, they'll
crack, explode, or catch fire. Make sure your oil and container won't
present a hazard if this happens.

Roy Lewallen, W7EL

Thanks Roy, I too have given this some consideration and am thinking of
rebuiding the dummy load with metal film resistors heeping the brass buss
bars that came with the load. The actual measure DC resistance is near 60
ohms so I suspect some damage has already occured. The resistors have no
markings but I figure 220 ohms each. Its difficult to count the resistors
the way they are arranged. You dont pay 3 bucks for something like this at a
hamfest and expect it to 100%.