Yes,
My first experience with that was a 300 lb piece of equipment. I figured the thermal time constant should be many hours. It took me several hours of waiting to finally realize my ambient temps were also cycling. It was the air conditioning in my case as well. The German inspector from LGA suggested I put my ambient thermocouple in a glass vial with few cc's of machine tool oil. This did a nice job of smoothing my readings and drafts no longer had any effect.
doug --
Douglas E Powell
doug...@gmail.com http://www.linkedin.com/in/dougp01
Doug et al, Well, you don’t have to have 100K lbs of equipment to get porpoiseing. I remember a 500W product project where the test house engineer came in to witness the testing and was disturbed by the porpoiseing swings when the product temp seemed stable. He finally understood after the 2nd explanation that the swings were caused by the room AC cycling and he could see the result. Test successfully completed. :>) 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 From: Doug Powell [mailto:doug...@gmail.com]
Sent: Friday, January 6, 2017 3:09 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Thermal equilibrium - 10% rule I'll take a look. I generally log with LabView or direct into an Excel spreadsheet, maybe I can get an Excel _vbscript_ 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. On Fri, Jan 6, 2017 at 2:10 PM, Brian O'Connell <oconne...@tamuracorp.com> 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:doug...@gmail.com]
Sent: Friday, January 06, 2017 11:11 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
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
doug...@gmail.com
http://www.linkedin.com/in/dougp01
On Fri, Jan 6, 2017 at 11:30 AM, john Allen <john_e_al...@blueyonder.co.uk> 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:ralph.mcdiar...@schneider-electric.com]
Sent: 06 January 2017 17:20
To: EMC-PSTC@LISTSERV.IEEE.ORG
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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