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From: rinne@mcnc.org (Glenn A. Rinne)
Newsgroups: sci.military
Subject: Electronics in Desert Heat
Message-ID: <1990Sep18.024659.21117@cbnews.att.com>
Date: 18 Sep 90 02:46:59 GMT
Sender: military@cbnews.att.com (William B. Thacker)
Organization: MCNC; RTP, NC
Lines: 29
Approved: military@att.att.com



From: rinne@mcnc.org (Glenn A. Rinne)

I heard on NPR (National Public Radio) that there have been 
significant electronic equipment problems plaguing our forces
in Saudi Arabia.  The report mentioned placing wet rags over
transmitters to cool them and keep them operating.

My meager experience with mil spec'd components reminds me that
all components are spec'd to operate at a temperature of 125C (257F).
I'm assuming that the same  specificatiion applies to equipment.
Why, then, is equipment failing at the desert temperatures of
49C (120F)?  Granted, the temperature of the air flowing over
the circuit boards does increase, but the largest increase I've
ever seen is on the order of 20C (36F) for fan-cooled equipment.
This would indicate that the maximum air temperature seen by the
_downwind_ component would be [T_ambient + T_rise] 69C (156F).  Am I 
missing something, or does the equipment not meet specification?


[mod.note:  A couple of possibilities:
	1) If it's 120 in the desert, it will be hotter than that inside
	a closed vehicle.
	2) If the temperature rise of a piece of electronics is 36F at
	room temperature, it will be greater than that at 120F ambient,
	as the cooling depends on the delta-T between the equipment and
	the environment.  
Still, 257F seems like a safe margin...  - Bill ]