John Passaneau wrote:


Great post with lots of good info. But look for a power line transformer
there are very high voltages and a lot of power. In a Ham dummy load the
max voltage on the resistor would be about 275V at 1500w. Unless your a
CB'er you will not see more than that. So hundreds, my self includeDd
Ham's have use common mineral oil from the drug store with fine results.


The issue isn't the water content or particulate contamination (which
affect the HV behavior) but the viscosity. Diala AX (for instance) is
quite low viscosity. 9.7 cSt/56 SUS at 40C, 2.3cSt/34SUS at 100C

http://www.nttworldwide.com/docs/diala-ax.pdf

9cSt (mm^2/s) is about the dividing line between SAW 20 and 30 AT 100C,
so Diala AX is a lot less viscous than even 20 weight motor oil (2.3cSt
vs 9.3 cSt)

The typical Mineral Oil, USP, heavy at a drug store is around 34cSt
viscosity (3-4 times heavier).

Diala pours pretty readily.. I'd say about like half and half, but not
as thick as whipping cream.

That's important for the heat transfer, because you need the circulation
inside the load (unless you're just pulsing it..) By the way, that's
why the rating on the Cantenna is different for USP mineral oil and
insulating oil.
http://www.mfjenterprises.com/man/pdf/MFJ-250.pdf

for 10minute duration, the rating is about 60% when using mineral oil
instead of transformer oil.

For long durations, the whole thing gets hot and reaches thermal
equilibrium.

For short durations, the lower viscosity transformer oil moves the heat
away from the resistor faster keeping it at lower temperature.


That said, most hams aren't going to run their dummy load at full power
for 10 minutes at a crack.