[volt-nuts] Resistance standard

Chuck Harris 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 
>> 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.

+/- some suggestive perfume, they are the same thing


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