[volt-nuts] Resistance standard
Brent Gordon
volt-nuts at adobe-labs.com
Tue Dec 15 07:53:49 UTC 2009
Rob Klein wrote:
> feedthrough capacitors of low capacitance. After mounting the resistors
> 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.
Brent
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