[CentOS] died again

Warren Young warren at etr-usa.com
Thu Dec 5 03:50:18 UTC 2013

On Dec 4, 2013, at 1:49 PM, John R Pierce <pierce at hogranch.com> wrote:

> On 12/4/2013 10:44 AM, Warren Young wrote:
>> Bottom line: 70% is too impure for this task.
> Huh?!?  I've cleaned numerous CPU-heatsink surfaces with 70% isopropyl, 
> never had any problem….

I'm not saying it's impossible.  An expert can ameliorate many risks through skill.  That is no reason to recommend a risky process.

I listed the theoretical risks in my previous post.  Now for some experimental data.

I filled a plastic bowl to a depth of 1 inch with the contents of a 21-year-old bottle of 70% rubbing alcohol from my medicine cabinet.  (Ah, 1992…it was a good year.)  I then inserted my bench DMM's probes into the bowl, spacing them approximately 1 inch apart.  I read 377 kΩ.

I sanity checked this measurement by moving the probes closer to each other, then farther apart, and observed that the resistance changed as expected.

I poured the rubbing alcohol back into the bottle, dried the bowl with a towel, poured the contents of my electronics bench's Menda bottle -- nominally 99+% pure isopropyl alcohol -- into the bowl, then tested again.  This time I read over 2 MΩ.

I was expecting a higher value, or even an overload indication.  I decided to refill the Menda bottle straight from the cubitainer in order to rule out impurities in the bowl, and also the natural concentration of impurities due to evaporation from the Menda bottle.  This time I read over 8 MΩ.

You questioned someone else's DMM in another post, so I will pre-defend mine.  It's a general purpose bench meter, not a dedicated insulation tester, but it will go up to 1 GΩ, and the company that made the meter isn't the sort that publishes bogus specs.  It's been a while since it was calibrated, but for 3 digit readings, I'm confident enough in its quality of design and manufacture to trust those readings anyway.

> I clean optical lenses with it, in the 
> form of those eyeglass wipes you buy by the crate at Costco.

I use my tee shirt.  :)

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