AEL                                 Atmospheric pumping (a packing hazard)

Most people think that they know the basics of good packing.  Secure the contents of a package so that if it is rolled or dropped, the delicate devices are not subject to unreasonable forces.  Make certain that the box (or whatever) is proof against the rain should the package be left in the open.  Abuses like having the box dropped into a river would be considered to be beyond reasonable abuse-prevention.  But there is one aspect of damage-prevention that most people would not consider, as they would probably be unaware of what I am going to tell them.

Atmospheric pumping.  You've never heard of it!  I don't think Plessey had either.  That is, until they had one delivery of equipment sent to Africa, that was quite literally chucked into the jungle as scrap.  I'll explain the basic physics and how the problem may be overcome.

If you make a waterproof package you will only be able to do so much to keep the contents dry.  Unless you are prepared to spend an enormous amount of money on hermetic sealing of the package, such as making a metal or glass enclosure so that air is totally excluded, you will only be partially successful.  If you use plastic sheeting and good quality tape, the package will NOT be airtight.  Plastic sheeting breaths a little..  And as you are probably aware; air pressure, humidity and temperature vary during the day, especially if the package is on the open deck of a ship sailing from Britain to tropical Africa..  This is what Plessey found..

As the air pressure falls it removes some air from the package and when the pressure rises again that excluded air is replaced by new air.  Now as the temperature changes, the amount of water that the air can hold changes.  I do not know the reason that condensation that occurs inside a "sealed" package  when the temperature falls, is not re-absorbed by the air when the temperature rises again.  But I know it doesn't happen.  So over a period of a number of cycles, condensation slowly provides more and more water inside a so-called sealed box.

I had this exact case demonstrated to me in Hampton when I lived there.  I had a bulkhead electrical fitting outside the side door of my brick garage.  The wiring was in galvanised screwed barrel with a watertight switch below the lighting fitting.  One day the fuse blew and when I investigated the cause I found that the metal-clad switch had about half an inch of water in it.  The water level had covered the two terminals of the switch, and a short-circuit to earth blew the fuse.  Although the watertight switch was hermetically sealed, it was open to the air via the conduit carrying the wiring.  I did what I should have done initially, and drilled a small hole at the bottom of the metal case to allow any condensation to drain out.

In the case of a so-called sealed package containing electrical equipment, one needs to have a hygroscopic material within the package.  The stuff usually used is silica-gel.  But you have to expect that the desiccant (note the spelling) to become exhausted after a period that depends on how humid the conditions are.  I expect there are formulae available to calculate the quantity of silica gel required, but to fail to use any is asking for trouble.  Plessey learned the hard way

Now have a look at another packing hazard that Plessey learned by experience, on page AEM