IMHO:
Measurements of this type are best done with a balanced bridge for accuracy=
..
However, the most useful information is collected under actual operating=20
conditions.
I suspect that when you order 5% resistors they are going to be 5% off.=20
Fortunately, every time I have ordered a number of them they are close=20
to being matched.
Could it be that they do a production run and then sort them?
On oil cooling:
All oil cooling is not equal. In the distant past transformer oil was=20
more efficient than what we use today. I believe it contained dioxin=20
which is considered a carcinogen. The devices now commonly use mineral=20
oil which requires more oil for the same application.
Heat transfer is not a simple science.

On 2/6/2013 12:32 AM, RussellMc wrote:
>> On second thought, considering the price of 5W 0.1ohm 5% wirewound
>> resistors
>> ... to get a 0.025ohm +/-1.25% 20W resistor block
>> getting increasingly more accurate as more resistors are added.
> Nice thought.
> I understand your reasoning, probably.
> But you may want to review what is liable to happen in reality.
>
> _____________
>
> FWIW - related but unrelated, I've just today been measuring 10 x 0.1R
> connected in series so I can get a better idea of their absolute
> values and relative values :-). My two "low Ohms" black boxes did an
> excessively poor job of convincing me that I could rely on their
> claims.
>
> In this case I pass a measured current through them  and measure
> voltage drop across each resistor and across the whole string. The
> string value is completely useless. At 0.1R/resistor the leads have
> significant resistance and the  solder joints more again.  If I cared
> I could  twist the leads together over a significant [tm] distance,
> crimp them solidly and then solder. BUT for my purpose the voltage
> drop at the resistor body is a good enough measure of what I'm looking
> at.
>
> I'm using two notionally identical reasonable quality multimeters.
> After the first pass I swapped meters so the series current and V_res
> roles were swapped. Same leads as before, unplug at meter. Readings
> varied slightly but were inside errors range from other sources.
>
>
>
>     Russell

--=20
John Ferrell W8CCW
That which can be destroyed by the truth should be.
P.C. HODGELL
  =20


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