Standards can't necessarily cover every possibility. For cyclical effects, I
would say that conditions are stable if two successive temperature maxima are
equal (within a reasonable tolerance). This doesn't necessarily work if more
than one cycle frequency is involved, in which case you have to look for the
repetition of the whole sequence, which might take a long time.
For example, a 10 minute cycle and a 12 minute cycle give a sequence that
repeats every 60 minutes.
If you are lucky.
With best wishes DESIGN IT IN! OOO – Own Opinions Only
<http://www.jmwa.demon.co.uk/> www.jmwa.demon.co.uk J M Woodgate and
Associates Rayleigh England
Sylvae in aeternum manent.
From: Doug Powell [mailto:[email protected]]
Sent: Friday, January 6, 2017 11:09 PM
To: [email protected]
Subject: Re: [PSES] Thermal equilibrium - 10% rule
Thanks!
I'll take a look. I generally log with LabView or direct into an Excel
spreadsheet, maybe I can get an Excel VB Script to post expected times.
One of the concerns I am dealing with now is how to determine stability when
there are cyclical operations going on. I am using the prescribed stability
criteria and using this on the minima/maxima of the temperature variations as
it moves up and down. Funny, as I sit starting at thermal data moving in this
way, I think of it "porpoise-ing" up and down.
All the best, Doug
-Doug
Douglas E Powell
Laporte, Colorado USA
<mailto:[email protected]> [email protected]
<http://www.linkedin.com/in/dougp01> http://www.linkedin.com/in/dougp01
On Fri, Jan 6, 2017 at 2:10 PM, Brian O'Connell <[email protected]
<mailto:[email protected]> > wrote:
Rough pseudo code for my transformer algorithm for logging and monitoring
temperatures for normal operating conditions:
time constant = (material ksp * mass) / (24*60)
sample interval = time constant / (mass * material kx)
breakpoint flags = false
if sample interval < min interval
sample interval = min interval
interrupts:
temperatures to circular buffer
log samples and windowed averages to network storage
loop:
for each channel
verify exponential and set breakpoint flag for each channel
update thermal lag time
adjust sample interval if time constant > thermal lag/2
find least dT/dt channel
find largest thermal lag time per ambient time per matching indices of
windowed means
if all breakpoint flags
indicate done
Brian
From: Doug Powell [mailto:[email protected] <mailto:[email protected]> ]
Sent: Friday, January 06, 2017 11:11 AM
To: [email protected] <mailto:[email protected]>
Subject: Re: [PSES] Thermal equilibrium - 10% rule
John,
I agree with the common sense approach and use it frequently. It's always
interesting to me how I can look at a screen plot of 60 thermocouples and in a
second or two decide, "yes this is stable". I can even estimate how much time
it will take to become stable as a test nears the end (usually about the time
of a lunch break). More than once I have attempted to write an algorithm to
make the same projection and have failed every time. The non-linearities and
multiple heat sources & sinks makes this nearly impossible.
On a side-bar, Voltaire is quoted as saying "Common sense is not so common",
which mean he is man with similar sensibilities as myself.
Those who know me personally have often heard me say what I consider to be a
corollary, "Common sense is usually neither.. common or sensible".
-Doug
Douglas E Powell
Laporte, Colorado USA
[email protected] <mailto:[email protected]>
http://www.linkedin.com/in/dougp01
On Fri, Jan 6, 2017 at 11:30 AM, john Allen <[email protected]
<mailto:[email protected]> > wrote:
Minor comment:
Having been the Secretary to a number of BSI committees many years ago, I
sympathise with Ralph's last para as the Chairmen can be even worse than the
other Members - they "know what they know" and it can take an awful lot of
"effort" to "persuade" them that they need to "think again" - once had to
refuse to publish a Chairman's version of a new standard because it did not
meet the basic BSI guidelines for how a product standard should be written,
until I had rewritten a large part of it to make it at least reasonably
"testable" for the EMC-related requirements - a little (sometimes a lot!) of
subject knowledge is required to set appropriate test and assessment
requirements!
Unfortunately, nowadays, a large number of Committee Secretariats appear to
employ non-SME staff to run their committees and so they can be lead by the
nose by the Chairmen.
FWIW, in the context of this thread, I used to use "commonsense" in deciding
when the temperatures appeared to have stabilized - -taking into account
where the probes were located - particularly when the observed temperatures
were substantially below the relevant Insulation Class limits.
John E Allen
W. London, UK
-----Original Message-----
From: Ralph McDiarmid [mailto:[email protected]
<mailto:[email protected]> ]
Sent: 06 January 2017 17:20
To: [email protected] <mailto:[email protected]>
Subject: Re: [PSES] Thermal equilibrium - 10% rule
For what it's worth, we measure and log temperatures at 1 minute intervals
(sometimes quicker, depending on what is being tested) and graph each data
set in Excel and look at the curves. When they go flat (even with some
ripple), we call it stable. Good enough for all the agencies we work with.
And I like the term steady-state much better, but what it is called isn't
terribly important as I see it. The temperature can be stable (no further
increase observable) over a period, even if it's oscillating slightly around
a mean value. The mean value could be the average of the last 20 or so
reading for instance. It requires some judgement.
I empathize with John's experience about it taking "many tellings" on a
committee to get something right. I have stopped just short of the Makita
Khrushchev United Nations technique at times!
Ralph McDiarmid
Product Compliance
Engineering
Solar Business
Schneider Electric
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Douglas E Powell
[email protected] <mailto:[email protected]>
http://www.linkedin.com/in/dougp01
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