Bill,
Big oops.
Measuring the resistance to determine the temperature is not productive
*unless* the resistance dominates the resistance measurement.
Picture three equal valued resistances in a row. The middle one gets very
hot (more than 100C rise) and increases over 40%, the two on the edges are
heat sinked and barely increase in temperature. The resulting change in
resistance is 13% which implies the temperature in there has only gone up
around 33C.
Measuring the resistance doesn't tell you much. At least with transformers
the dominant resistance is pretty much the bulk resistance.
- Robert -
-----Original Message-----
From: Bill Ellingford <bill.ellingf...@motion-media.com>
To: 'Robert Macy' <m...@global.california.com>; emc-p...@majordomo.ieee.org
<emc-p...@majordomo.ieee.org>
List-Post: emc-pstc@listserv.ieee.org
Date: Monday, May 13, 2002 1:56 PM
Subject: RE: Constant for Change of Resistance formula.
>
>Hi Robert / group
>OK, Not the best choice of website to demo the answer. The differing
>figures are because the formula has been transposed to give Temp from
change
>of R from the original formula which gives R from change of T. To do this,
>another constant (The 234.5 constant) is required. This is the implied
>point of zero resistance for copper on the Celsius scale. The formula we
>use is:
>
> Rfinal - Rorig
> ---------------- x (234.5 + Tamb start) -(Tamb finsh - Tamb start)
> Rorig
>
>The Tamb start and finish are the changes (if any) in Room ambient. If the
>room remains at 20c then 234.5 + 20 is the multiplier.
>
>-----Original Message-----
>From: Robert Macy [mailto:m...@california.com]
>Sent: 13 May 2002 14:54
>To: Bill Ellingford; emc-p...@majordomo.ieee.org
>Subject: Re: Constant for Change of Resistance formula.
>
>
>Bill,
>
>Thanks for the site.
>
>Went there and found the same formula and constant I use.
>
>For copper, Temp Coeff = 3.9 x 10-3
>
>Then I clicked on table of coeff and there was a very long list of
>materials, but the temp coeff of copper there was 6.8 x 10-3 ???!!!
>
>Any ideas for this disparity?
>
> - Robert -
>
> Robert A. Macy, PE m...@california.com
> 408 286 3985 fx 408 297 9121
> AJM International Electronics Consultants
> 619 North First St, San Jose, CA 95112
>
>
>-----Original Message-----
>From: Bill Ellingford <bill.ellingf...@motion-media.com>
>To: 'Colgan, Chris' <chris.col...@tagmclaren.com>;
>emc-p...@majordomo.ieee.org <emc-p...@majordomo.ieee.org>
>Date: Monday, May 13, 2002 5:38 AM
>Subject: RE: Constant for Change of Resistance formula.
>
>
>
>Hi Folks
>Further to the answer given, here is a little more data.
>The constant used is for the change of resistance with temperature. metals
>and alloys (conductors) all exhibit a different constant. This can be used
>for calculating temperature rise or resistance change. i.e. find the temp
>rise from a start and finish test measurement on a winding (for example) at
>the begining and end of a on load heat run or, find R for a given temp:
>using a table or the formula, resistance at various temperatures can be
>pre-determined from a measurement made at one particular temperature.
>
>A website with the formulae can be found at
>http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html
>
>Where you have a transition from one metal type to another, you must
measure
>each metal part individually. If you have only two metals in contact, you
>may be able to apply a combination of the temp coefficient methods and
>transposition of the measurement of change of junction voltage formulae
i.e.
>Thermocouple laws.
>
>Hope this adds some value: Bill Ellingford
>
>-----Original Message-----
>From: Colgan, Chris [mailto:chris.col...@tagmclaren.com]
>Sent: 13 May 2002 10:28
>To: emc-p...@majordomo.ieee.org
>Subject: RE: Constant for Change of Resistance formula.
>
>
>
>Ned is referring to the constant used in the "temperature rise calculated
by
>change in resistance formula" ie
>
><<...>>
>
>Where dt is the temperature rise, R1 is start resistance, R2 is end
>resistance, T1 is start ambient and T2 is end ambient. 234.5 is the
formula
>constant for copper.
>
>This formula is used extensively when heat testing transformers and coils.
>
>I'm afraid I don't know the constant for brass but I believe the figure may
>be related to the "inferred absolute zero" of a material. Try asking a
>metallurgist?
>
>Regards
>
>Chris Colgan
>Compliance Engineer
>TAG McLaren Audio Ltd
>The Summit, Latham Road
>Huntingdon, Cambs, PE29 6ZU
>*Tel: +44 (0)1480 415 627
>*Fax: +44 (0)1480 52159
>* Mailto:chris.col...@tagmclaren.com
>* http://www.tagmclaren.com
>
>
>
>
>> -----Original Message-----
>> From: Robert Wilson [SMTP:robert_wil...@tirsys.com]
>> Sent: Friday, May 10, 2002 7:00 PM
>> To: emc-p...@majordomo.ieee.org; Ned Devine
>> Subject: RE: Constant for Change of Resistance formula.
>>
>> What are the units? 234.5 ...what?? Looking at what the units are, will
>> basically tell you exactly what the property is related to.
>>
>>
>>
>> Nonetheless, you cannot possibly directly determine what the temperature
>> change of something as physically and geometrically complex as a
>> connector, merely by factoring in what its resistance change is. Among
>> other things, the solution is extremely non-linear and iterative.
Changing
>> resistance will generate more heat, which will increase temperature,
which
>> will generate even more heat ....and on and on! Add this to the fact the
>> resistance coefficient with temperature is itself non-linear, and you can
>> see how this complicates things further. The final temperature that the
>> "system" stabilizes at, is reached when the logarithmically increasing
>> (i.e. also very non-linear) heat transfer to the environment caused by
>> increasing temperature, balances increased heat being generated.
>>
>>
>>
>> To reach a solution, you need to iterate your calculations, where the
>> results of one calculation are plugged as variables into the next
>> iteration. Typically a thermal analysis program will require several
>> hundred iteration before a converged solution results.
>>
>>
>>
>> Bob Wilson
>> TIR Systems Ltd.
>> Vancouver.
>>
>> -----Original Message-----
>> From: Ned Devine [mailto:ndev...@entela.com]
>> Sent: May 10, 2002 8:29 AM
>> To: emc-p...@majordomo.ieee.org
>> Subject: Constant for Change of Resistance formula.
>>
>>
>>
>> Hi,
>>
>>
>>
>> Does any one know how the constant for CoR formula was determined? I know
>> the K is 234.5 for copper and 226 for aluminum, but what property is this
>> related to?
>>
>>
>>
>> I am trying to determine the change in temperature of a connector, based
>> on the change of resistance. The connector contacts are made of brass.
>>
>>
>>
>> Thanks
>>
>>
>>
>> Ned
>>
>>
>>
>>
>>
>> Ned Devine
>> Program Manager
>> Entela, Inc.
>> 3033 Madison Ave. SE
>> Grand Rapids, MI 49548
>> 1 616 248 9671 Phone
>> 1 616 574 9752 Fax
>> ndev...@entela.com e-mail
>>
>> Entela, Inc. A Certified Woman Owned Business
>> www.entela.com
>>
>>
>>
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