[ARC5] Ant. Series Matching Caps- I Just Don't Get It.

[Bruce Long via ARC5]

capacitors and humidity
When I was an undergrad I had a summer job as a research assistant at a lab on campus that did civil engineering research.  One project i was associated with did coefficient of friction testing on road surfaces.  I would get to work early check out the electronics in the friction coefficient tester mounted in a pick up truck.  I'd bless the electronics and send the CE technicians off to do their testing.
More times than not they would be back at lunch having found the electronics non-functional when they arrived at the test site.
This went on most of the summer,  I was looking bad.   Then I noticed the electronics seemed more likely to fail on the worst, hot muggy days.  i suspected temp-co and spent a week or so chasing that.
The problem turned out to be a humidity sensitive yellow film capacitors.
Bruce KJ3Z

      From: Brian <brianclarke01 at optusnet.com.au>
 To: David Stinson <arc5 at ix.netcom.com>; milsurplus at mailman.qth.net; ARC-5 <arc5 at mailman.qth.net> 
 Sent: Thursday, January 15, 2015 4:20 AM
 Subject: Re: [ARC5] Ant. Series Matching Caps- I Just Don't Get It.
   
Hello Dave,

Measurement is only one of many environmental events.

Were the capacitors you measured last year then tagged and bagged in humidi-seal zip-lok bags? 

Let’s have a look at humidity. 

Water ingress is seldom a specification you will see on capacitor manufacturer's spec sheets. And consequently, there will be some variation between capacitors from the same manufacturer - hence the need for individual tagging, which is what the Australian military does.

As temperature falls, if you don't get rain, then the relative humidity rises. [That's why frost forms on stuff put in your fridge.] When capacitors that are warm cool down, they draw in the local air + humidity. The increased humidity now inside the capacitors may start forming deleterious electrolytes that eat the insulation and solutes that spread out. So, warming up the capacitors a year later may not reverse the effect of the humid ingress. Hence, the need for bagging.

So, you may think that you have measured the same thing at the same time, but changes in internal humidity and chemical composition of the capacitors may have occurred. Such effects may be seen as increased capacitance, possibly because the distance between the plates has decreased and possibly because a conducting solution has migrated to a larger area over the insulation, but also you may see a fall in the breakdown Voltage, again because of the reduced separation of the capacitor plates.

73 de Brian, VK2GCE

On Thursday, January 15, 2015 2:04 PM , you said: 

<snip>
This evening after work,  I decided to go through several kinds
of caps and record just what was best/worst etc.
Remember how some of us have puzzled over how we can
make a test one day, get a certain result, then days or weeks or
years later, do it exactly the same way and get radically 
different
results?  I know-- sounds crazy, but....

<snip>
I don't understand it.  How can two sets of basicly identical 
tests
a year and change apart produce such radically different 
results??
I'm starting to believe in gremlins and magic "spooks"
in our radios.  I don't know what else to think.

<snip>


73 DE Dave AB5S
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