[Beowulf] Earthquakes and raised floors...

[Robert G. Brown]

On Sun, 8 Jan 2006, Alvin Starr wrote:

> A while back I found the following link about raised floor and growh of metal 
> whiskers; http://nepp.nasa.gov/whisker/
>
> I found this AFTER we put in the raised floor for running our cables but I 
> guess the upside is that we are not using it for cooling.

The meaty part of this (for those that don't want to hunt around on the
site for the particular part about zinc whiskers growing from raised
floor tiles made with a wood particleboard or concrete core, coated with
smooth galvanized steel) is here:

  http://nepp.nasa.gov/whisker/other_whisker/index.htm#zinc

In a nutshell, if you've got a raised floor with wood or concrete core
tiles (or stringers or pedestals) that have a dull grey smooth (not
shiny spangled) finish to which a magnet will stick on the underside
(zinc galvanized steel) then you MAY have a problem with zinc whiskers.
These are tiny crystals of zinc (as long as a few mm in length) that
spontaneously extrude themselves from the zinc to help alleviate
stresses induced in the coating process.  They are a few microns wide,
brittle, and are good conductors.  They are small enough that they can
easily be carried around as "conducting dust" by any sort of air
disturbance, and if they drop out in a power supply or inside a chassis,
they can short out components producing anything from transient
flakiness to catastrophic failure (if they bridge something with a high
enough voltage and current delivery that they flash into an even more
highly conducting plasma instead of just popping out the short).  They
are also (apparently) fairly common accompaniments to certain tin or
zinc coated equipment chassis lids, to tin plated contacts, and have
been observed in a variety of other metals as well as tin and zinc (but
are less common).

It does look like this is more likely to be a problem with older raised
floors, and it does take some time -- years -- for the whiskers to
ordinarily grow.  Once you've got a lot of them down there under your
floor, however, they can apparently be a serious problem.  Interesting.
I actually have worked some on stress corrosion cracking and never heard
of whiskers as a problem, let alone would have suspected them to be a
problem in electrical equipment, but apparently they are responsible for
the failure of a slew of satellites and lots of other expensive stuff in
addition to much computer hardware.  They're subtle enough that it is
very likely that lots of failures have probably been due to them that
were never actually correctly diagnosed -- written off to "gremlins".

The whiskers are too small to filter without filters that would
seriously impede airflow.  Zinc whisker dust in the quantities likely to
be breathed in while working on or under raised floors is not thought to
be dangerous to humans, as humans actually use tiny amounts of zinc as a
nutrient.

Very interesting.

     rgb

>
>

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu