[volt-nuts] Resistance standard

Robert Atkinson robert8rpi at yahoo.co.uk
Tue Dec 15 20:00:52 UTC 2009

As Chuck mentioned, Baby oil is mostly refined mineral oil, liquid paraffin in fact, which is also a laxative. So "generic" pharmaceutical grade liquid paraffin is a good refined oil. Had a problem in a biotech lab once, customer sent a sample of lab grade (GPR) liquid paraffin complete with MSDS that said to use a full face shield and mask. The safety officer would not let us use it. They were quite happy with some locally purchased baby oil though!
Don't know why it affected your resistors unless they were some kind of unfired ceramic and it was getting between the particles.
Robert G8RPI.

--- On Tue, 15/12/09, Brent Gordon <volt-nuts at adobe-labs.com> wrote:

From: Brent Gordon <volt-nuts at adobe-labs.com>
Subject: Re: [volt-nuts] Resistance standard
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Date: Tuesday, 15 December, 2009, 7:53

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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