When you work on a boat the humidity tends to be high.  If you seal a
box with humid air in a warm lab, and then put it in the cold ocean you
get condensation.  Drawing a vacuum removes the humid air and prevents
the condensation.  Also trying to draw a vacuum tells you if you did
something stupid like leaving a drain plug out.  But vacuum introduces
problems of it own.

Filling the box with liquid means that if the sides of the box bow a
little bit, the outside pressure is transmitted to every component in
the box.  Semiconductors in metal cases or with EPROM windows can be
damaged by high pressure.  Electrolytic caps often fail.  Some resistors
are hollow and have been known to implode.  Etc...

The best thing is to draw the vacuum, then refill the box with nitrogen
(cheap & nearly inert).  We used to refill with Freon.

Sherpa Doug

> -----Original Message-----
> From: Peter L. Peres [mailto:plp@ACTCOM.CO.IL]
> Sent: Saturday, February 16, 2002 2:54 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE] TO-220 heatsinking
>
>
> >You are very right.  Even a small fan does wonders for
> cooling.  Most of
> >my stuff is in small sealed boxes so there is little or no
> air movement.
> > I love it when the customer draws a vacuum on the housing
> to "prevent
> >moisture problems" and the electronics cook to death in the resulting
> >Dewar flask!
> >
> >Sherpa Doug
>
> Why vacuum ? I understand that your customers have usually to do with
> wetness, vacuum is not a good way to seal those afaik. The
> smallest leak
> fills the box with water (completely) even if water needs to enter as
> vapor because the hole is so tiny.
>
> Don't you tell them to fill it with liquid ? (oil or special) ?
>
> Peter
>
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