Top node hotter thanothers?

David Mathog mathog at
Fri Jul 25 13:29:54 EDT 2003

We have a 20 x 2U rack and I've noticed that the
top node is always a step hotter than the other nodes.


There is a slight gradient going up the rack (see
below, 01 is on the bottom, 20 on the top) but it
doesn't explain the jump at the top node.  At first
I thought it might be due to hot air moving from
the back of the rack, over the top of the highest
node, and being sucked in by it.
However no temperature change resulted when all
side vents were blocked and cardboard pasted up
the front of the rack so that only the same cold
air as the other nodes could enter.  The only other
difference between this node and the others is
that there's hot air above 20 (two empty rack slots),
but another node above all the others. So maybe all
that hot air heats the top node's case and that
couples the heat in?  I don't have an insulating
panel handy to test that hypothesis.

node case    cpu
01   +34°C   +43°C 
02   +35°C   +44°C 
03   +37°C   +48°C 
04   +42°C   +50°C 
05   +38°C   +48°C 
06   +37°C   +50°C 
07   +36°C   +45°C 
08   +38°C   +48°C 
09   +38°C   +48°C 
10   +38°C   +48°C 
11   +36°C   +44°C 
12   +38°C   +48°C 
13   +38°C   +48°C 
14   +40°C   +49°C 
15   +38°C   +46°C 
16   +36°C   +46°C 
17   +39°C   +51°C 
18   +39°C   +48°C 
19   +39°C   +49°C 
20   +44°C   +54°C

Temperatures were measured using "sensors" on these
tyan S2466 motherboards (1 CPU on each currently.)
The case value is the temperature reading by the
diode under the socket of the absent 2nd CPU.
The temperatures jump around a degree or two.


David Mathog
mathog at
Manager, Sequence Analysis Facility, Biology Division, Caltech
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