From: hybl@umbc4.umbc.edu (Dr. Albert Hybl (UMAB-BIOPHYS))
Subject: Running a 3b1 with the cover open; was (more about my troublesome 3b1)
Organization: Univ. of MD, Baltimore County
Distribution: na
Date: Tue, 15 Sep 1992 14:16:14 GMT


This posting is spawned as a side issue from other postings
related to my troublesome 3b1.  Several respondents
point out that the fan is ineffective for cooling with
the cover opened as I described it (monitor facing the ceiling
and the fan given room to operate without much back pressure).
The issue of heating/cooling is of general interest especially
to persons planning to replace the computer's outer box.

It is true that the fan is not contributing to the cooling and it is
true that the computer P/S, motherboard and other components are
heat generators; i.e., they are exothermic.  The air around the
hot components (point sources) of the P/S expands as it absorbs
some of the heat and its density becomes less.  Some of the heated
air will diffuse away but the rest will form a bubble of hot air that
will, in a gravitation environment, rise to the ceiling--like a hot
air balloon ascension but without the fabric to prevent diffusion.
Cool air floods the P/S and it is in turn heated in a cycle
called convection.  Without the cover, convective cooling works
well; putting the cover back, lowers the ceiling and requires a
fan to assist in the cooling process.  The condensers and
cables especially the ribbon cable connector are not getting
overheated and are probably cooler without than with the cover.
The working (free air) temperature of various chips seem to range
from a minimum of 0 degrees C to a maximum of 70-125 degrees C.
I don't think that this range is being abused.

The motherboard has a low ceiling which absorbs the heat; that is,
the motherboard heats the metal plate directly above it.
Heat will flow in the plate from the hots spots to the cool
regions.  When the cover is closed, air is drawn in the slots
along the front and edges of the keyboard platform and flows
over the corresponding metal platform located below the keyboard.
This flow of air cools the metal plate and keeps the computer
from over heating.  Without the cover, the fan does not help
cool the metal platform.

Because of the space and position of the opened computer on my work
bench, I have the right corner extending over the edge of the
bench.  The exposed portion forms a right triangle (12" along
the front and 4" along the left side).  I put the keyboard onto
the metal platform with two pencils between them.  Cool air enters
from the front, flows under the keyboard and simultaneously over
the metal platform then up the chimney between the back of the
keyboard and the FD, HD and P/S.  This cools the platform so that
it is warm but not hot to the touch.  The top of the HD shield
is hotter but I can hold my hand on it without reflex or pain.
In this configuration the P/S does not cause the fan to speed up,
the machine ran continuously since it was booted until I decided
it was time to close the cover; there were no panic messages nor
keyboard lock ups.  An open machine IS AN ELECTRICAL HAZARD but
it is not in imminent danger of turning into a BLOB of overheated
plastic and metal.

WHAT ARE THE FREE AIR TEMPERATURES RANGES OF THE 68010 CPU, A
WD1010 OR WD2010 HD CONTROLLER CHIP, OR MEMORY CHIPS?  WHAT ARE
THE ACCEPTABLE MIN AND MAX OPERATING TEMPERATURES VALUES AT
VARIOUS POINTS IN OR ON THE 3B1 COMPUTER?  

I think that the platform gets hotter when the keyboard is not
there (caveat: this is a purely subjective impression and
the following is speculation).  Is it because an inversion layer
of hot-air builds up over the middle of the platform and because
the convective circulation at the perimeter can not provide enough
cooling?

The flat platform is like the face of a crystal during growth;
snow flakes for example.  Crystallization is an exothermic
process and convection is involved.  Micro-changes in concentration
of the solution is also involved.  It is a bit of a mystery
why protein crystals seem to stop growing when they reach a certain
size.  There are some photographs illustrating convection in:
1) P.J. Shlichta (1986) "Feasibility of Mapping Solution
Properties During the Growth of Protein Crystals," J. of
Crystal Growth, vol 76, pages 656-662.
2) C.E. Bugg (1986) "The Future of Protein Growth," J. of
Crystal Growth, vol 76, pages 535-544.

Regards,
Albert Hybl (hybl@umbc3.umbc.edu)