The subject itself can easily be a presentation at ISPCE. Doug, what you and
Gert have done should be a formal paper. Please consider collaborating and
making it happen.
John
________________________________
From: Pete Perkins <00000061f3f32d0c-dmarc-requ...@ieee.org>
Sent: Sunday, January 8, 2017 9:33 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Thermal equilibrium - 10% rule
Doug,
A great approach; it would make for an interesting ISPCE/PSES
presentation and a paper. Go for it.
:>) br, Pete
Peter E Perkins, PE
Principal Product Safety & Regulatory Affairs Consultant
PO Box 23427
Tigard, ORe 97281-3427
503/452-1201
p.perk...@ieee.org<mailto:p.perk...@ieee.org>
From: Doug Powell [mailto:doug...@gmail.com]
Sent: Sunday, January 8, 2017 10:39 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Thermal equilibrium - 10% rule
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<mailto:doug...@gmail.com>
http://www.linkedin.com/in/dougp01
On Sat, Jan 7, 2017 at 12:51 PM, Richard Nute
<ri...@ieee.org<mailto: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
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