I agree with Ted's explanation; that is the physical basis for the change in temperature at higher elevation.
Just a word of warning that that rule-of-thumb for derating product performance at sea level (41 + 4 = 45C) can be very imprecise since convection cooling is only one of the three heat transfer mechanisms. It may be wise to use a thermal chamber capable of simulating high altitude if you have any concerns. This is common in the NEBS world. ...Marko From: [email protected] [mailto:[email protected]] Sent: Friday, September 28, 2007 5:17 AM To: [email protected] Subject: RE: Max. ambient temperature The actual temperature may not change with an increase in altitude, but the air pressure does go down. As the air gets thinner, heat dissipations through convection goes down. We commonly use a vacuum for thermal insulation as in a thermos or vacuum flask. As the density of the air goes down, you depend more on radiation for heat dissipation. The temperature adjustment is a compensation factor. A heat dissipating device that reaches 41C at sea level would reach approximately 45C at 2000 meters altitude due to the reduction in heat dissipation ability. Ted Eckert American Power Conversion/MGE http://www.apc.com/ The items contained in this e-mail reflect the personal opinions of the writer and are only provided for the assistance of the reader. The writer is not speaking in an official capacity for APC-MGE or Schneider Electric. The speaker does not represent APC-MGE's or Schneider Electric's official position on any matter. "Paul Lovell" <paul.lovell@hunt leigh-technology. To com> <[email protected]> Sent by: cc [email protected] Subject RE: Max. ambient temperature 09/28/2007 03:39 AM I'm not sure I follow the argument here. I'm talking about products which would be used in a controlled environment such as an office, hospital or central office/telephone exchange, so the ambient temperature will be the same as at sea level. I would also hope that manufacturers finding products within 4C of a limit would do something to increase this margin - my concern is that currently the Agencies do not take this thermal effect into account during their testing at all currently, so a device tested and found to be close to the limit at max ambient, sea level will be certified as being suitable at max ambient, 2000m elevation when it may not. Mind you, the Standards themselves give no guidance on this matter when perhaps they should. Best regards, Paul Paul Lovell Group Compliance Manager Huntleigh Technology Tel: +44 (0)1582 745891 Mob: +44 (0)791 956 5599 From: [email protected] [mailto:[email protected]] On Behalf Of Pettit, Ghery Sent: 07 September 2007 18:18 To: John Woodgate; [email protected] Subject: RE: Max. ambient temperature And keep in mind that this is the standard lapse rate. It can vary. Heck, you can actually have higher temperatures in a layer above. Temperature inversion. Traps pollution down low. But, for the purposes of this thread, standard lapse rate should be fine. Ghery Pettit From: [email protected] [mailto:[email protected]] On Behalf Of John Woodgate Sent: Friday, September 07, 2007 10:03 AM To: [email protected] Subject: Re: Max. ambient temperature In message <[email protected]>, dated Fri, 7 Sep 2007, Ralph McDiarmid <[email protected]> writes: >One hopes that the 4C difference would be accounted for in the >temperature limits imposed by most standards. If I found a spot within >4C of a limit, I'd take a harder look at things anyway. > >If it's 35C at MSL, what's the air temperature at 2000m? Perhaps it's >down by more than 4C. From http://en.wikipedia.org/wiki/Lapse_rate it decreases at 6.5 C per 1000 m. Note that the 'official' lapse rate is the NEGATIVE of the raw figure. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk There are benefits from being irrational - just ask the square root of 2. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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