[volt-nuts] Resistance standard
cfharris at erols.com
Tue Dec 15 19:41:23 UTC 2009
Brent Gordon wrote:
> Rob Klein wrote:
>> feedthrough capacitors of low capacitance. After mounting the
>> and a thorough cleaning, the whole thing will be baked at ~85°C
>> overnight to get rid of any moisture, then filled with oil and
>> soldered shut.
> I tried something similar to this about 15 years ago. I needed a 10 KW
> dummy load so I bought a bunch of 300 W air-cooled dummy loads, removed
> the resistors, and connected them together in a series-parallel
> combination to get 50 Ohms. I measured the value just to make sure. I
> then got a small drum and filled it with baby oil and put the resistor
> assembly in. The next day the resistance had changed by more than 50% (
> I don't remember if it increased or decreased.). The value continued to
> drift and every time the resistor heated up the value changed even
> more. Tried cleaning, washing in solvent, and baking at high
> temperatures in various combinations but I could never get the value of
> any of the resistors even close to 50 Ohms. Ended up throwing the whole
> mess away.
> The resistors were uncoated, tubular types so this warning probably does
> not apply to your molded resistors.
> Another post in this topic mentions baby oil. Even though I had
> problems with my dummy load, it wasn't because of the baby oil. I also
> tried some lab-grade mineral oil and had the same problem. I recall
> reading on a mailing list somewhere (probably Time Nuts) that at Los
> Alamos National Lab a group had to buy baby oil because mineral oil was
> considered a hazardous material.
+/- some suggestive perfume, they are the same thing
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