[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.


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