Richard, I have tried a number of approaches in the past. Given that most products are quite complex with regard to all the potential heat sources/sinks and interfaces I decided that anything along the lines of FEA is impractical.
I also tried the time constants idea which is analogous to RC time constants. I found this works well enough if you have a good amount of history with the product itself. Otherwise, due to the non-linear nature of the problem, it is difficult to predict end time or temperature until 3 to 4 time constants have already passed. I tried using the slope of ΔT to estimate when the end of the test is pending. The next attempt was to dig in a little following the equations V = Voe -(t/RC) and V = Vo[1-e-(t/RC)] where I substitute V for the the various temperatures (Vo = the absolute value of the temperature delta from start to end), C is analogous to product mass and R is the Rtheta of the product. With a little testing history, you can assume the composition of the product is similar for other products designed by the same company (copper, steel, plastics, air, liquids, etc), I solved for RC and then rearranged the algebra to solve for t which is time. There are a couple of problems in that I am still unable to come up with a general purpose solution. First this is a simultaneous solution of several unknowns which is not conducive to quick on the fly solutions. This is especially true when you are in the early stages of a temperature run when things are still moving quickly. As you know extrapolating outside an existing dataset is risky, especially when nonlinearities are involved. I am now going back to basics. Q = Cp * m * abs(T2-T1) q = heat energy in Joules m = mass of the product Cp = specific heat of the product T1 = The initial temperature of the product at the start T2 = The final temperature of the product abs() is used to correct for heating or cooling With the start/final temperatures and mass taken from prior tests I can extract a Cp for a particular product. Understanding one watt is Joules/second you can factor into the equation time. My thought is that the composition of a product from the same engineering group with have similar ratios of copper, iron, plastics, etc. And then I may be able to solve for total test time or final temperature. Not forgetting that the air mass and equipment of the environmental chamber is part of the big picture. I have not fully tested this method yet, but so far I remain hopeful. If this works, I plan to build a small database of product Cp values. I would be interested to know if anyone else gives this a go and how as yet undiscovered problems are overcome. -Doug Douglas E Powell Laporte, Colorado USA doug...@gmail.com http://www.linkedin.com/in/dougp01 On Sat, Jan 7, 2017 at 12:51 PM, Richard Nute <ri...@ieee.org> wrote: > > We have to consider that the temperatures sought are not of > > metrological value, but to > > to establish a safe/non-safe result. > > Yes! > > > The mathematical limit of an exponential rise is easy to > > estimate, once a few timed samples are available, > > I haven't been able to come up with an equation, even though I have tried > and sought help from folks who are more knowledgeable than me in the field > of thermodynamics. Please tell us your methodology. > > Best wishes for the New Year! > Rich > > - > ---------------------------------------------------------------- > 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> > -- Douglas E Powell 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>
