Thermal equilibrium is impossible. Thermal change is an exponential, never achieving the end-point.
The test is to determine that some part does not exceed its temperature rating. Plotting the (exponential) curve shows whether the temperature in question is or is not going to exceed its rating. (Curve-fitting would be nice, but impossible.) As soon as this determination is made, the test can be concluded. Defining thermal equilibrium as ±2°C (3.6°F) is not achievable as thermal change is an exponential. And, it is tight. For a small (low thermal mass) part, air currents will change the temperature beyond this limit. The statement does not reflect knowledge of primitive thermodynamics. Best wishes for the New Year! Rich From: Doug Powell [mailto:doug...@gmail.com] Sent: Thursday, January 05, 2017 2:47 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Thermal equilibrium - 10% rule All, The following has always been confusing for me, For those tests that require the DUT to reach thermal equilibrium, thermal equilibrium is considered to be achieved if after three consecutive temperature measurements taken at intervals of 10% of the previously elapsed duration of the test but not less than 15 min, indicate no change in temperature greater than ±2°C (3.6°F). The portion of text "taken at intervals of 10% of the previously elapsed duration of the test" has been around for some time and it exists in several safety standards, the "not less than 15 min" can be other values like 10 minutes, 5 minutes, etc. I believe the intent is to minimize the number of data points being recorded for extremely long thermal tests. The idea makes sense when I think back to the days of manual data logging. Nowadays, with the advent of automated data logging, this point probably becomes moot. Still, the phrase bugs me and I would like to understand how this might be interpreted. First the "previously elapsed duration" could indicate either the duration of the entire test or the duration of the last logging interval. Second, over a long test 10% of the previous interval very quickly shortens the logging intervals to something ridiculously small. For example, in three data points interval of 1440 minutes (24 hours) will be reduced to 144 minutes (2.4 hours) and then 14.4 minutes, at which point the 15 minute minimum takes over. I generally record at a much shorter intervals, even if for several days and check for equilibrium at a modulus of 15 minutes, as required. Thanks, Doug -- Douglas E Powell doug...@gmail.com <mailto:doug...@gmail.com> http://www.linkedin.com/in/dougp01 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
