Roy Lewallen wrote in message ...
Brian Kelly wrote:

In the case of a paint can being used for the housing for a dummy load
the outer surface of the can needs to be able to radiate as much power
as possible. A Teflon liner between the oil and inner surface of the
metal can is a thermal insulator and will degrade the ability of the
can and the dummy load to transfer the BTUs in the oil to the air
surrounding the can.
. . .

Don't tell my wife that. She's been thinking all along that the food
she's been putting in the Teflon lined frying pan has actually been cooking!

Hee! Couple weeks ago I got stupid and tried to fry a couple eggs in a
bare aluminum pan. I wound up with one edible egg and one which
alloyed itself with the pan.


Seriously, what you've got is a series of "thermal resistances" in
series. This is a legitimate if simplified way of evaluating temperature
drop, and is widely used in determining chip temperature of transistors
and ICs. From the resistor, you've got the thermal resistance of the
oil, in series with the thermal resistance of the Teflon, then the metal
can, then finally the resistance of the can-to-air transfer, or
"connection". The temperature difference between the resistor and the
air is calculated like the voltage in a circuit, where the circuit's
"current" is the amount of power being transferred. That is, DT = THETA
* P, where DT is the temperature differential, P the power, and THETA
all the thermal resistances in series. In electronics, DT is usually in
degrees C, P in watts, and THETA in degrees C per watt.

Yup. There isn't a steady-state thermal circuit out there which can't
be modeled on the bench with a pile of resistors. Gets hairy when
fluid dyanmics gets into the mix, laminar vs. turbulent flow, etc.
Ditto the electrical network simulation of mechanical vibrations. A
lot of grad school theses were written in the general topic area back
in analog computer days. Mother Nature could care less about the lines
between engineering disciplines.


While the thermal resistance of the Teflon layer is undoubtedly several
orders of magnitude greater than the thermal resistance of the can, I'd
be willing to bet that it's much less than the can-to-air thermal
resistance,

Blow air on the can with a fan. In the limit dunk the can in a bucket
with water running in/out.

and probably (because of its very small thickness) even much
less than the resistor-to-Teflon thermal resistance through the oil
(unless, perhaps, the oil is very rapidly stirred). So, in the big
picture, the Teflon layer doesn't make any difference.

Agreed.

Nobody ever died from an engineering troll.

Roy Lewallen, W7EL

w3rv