Bill/group
Let's check all these numbers.....
Room temp of 20C? 68F (brrrrr). More like 25C, 77F.
Most people assume room temp is 297K (or is that 298?) which is more like
23.82C, ~75F (seems more reasonable), putting that into the linear equation
gives
234.5 + 23.82 = 0.00387
error less than 0.7% compared to
0.0039
However the formula is very clear that this is a short range linearity
When you extrapolate to 100C, or 75C rise above ambient, the linear formula
predicts 1.295 times the room temp resistance, but the actual value climbs
more like:
(1+0.0039)^75 = 1.339 (for 1 degree steps)
error is now more than 3%
My conclusion is to just measure it.
Run current through the contacts, measure the resistance, then stick the
whole thing in a temp chamber and increase temp 'til you read the same
resistance.
Or, lower the temp from ambient, until you read the SAME resistance. Then
that temperature difference is your rise above ambient.
I know, I know. The first method does not take into account thermal
gradients that cool parts of the conductors and would suggest less rise than
actually occurs - which could be a large error, say measure 25C rise, but
actually have 35C rise in the hotspots.
The second method is moving into the nonlinear range, but for a temp change
of 25C is only 0.4% error. Therefore, to more closely approximate the
actual conditions, I'd recommend you cool the connector and reproduce the
resistance measurement made at room temp.
Any idea why their chart (at that website) showed temp coefficient of 0.0068
for copper?
- Robert -
Robert A. Macy, PE [email protected]
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
619 North First St, San Jose, CA 95112
-----Original Message-----
From: Bill Ellingford <[email protected]>
To: 'Robert Macy' <[email protected]>; [email protected]
<[email protected]>
List-Post: [email protected]
Date: Monday, May 13, 2002 8:15 AM
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:[email protected]]
>Sent: 13 May 2002 14:54
>To: Bill Ellingford; [email protected]
>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 [email protected]
> 408 286 3985 fx 408 297 9121
> AJM International Electronics Consultants
> 619 North First St, San Jose, CA 95112
>
>
>-----Original Message-----
>From: Bill Ellingford <[email protected]>
>To: 'Colgan, Chris' <[email protected]>;
>[email protected] <[email protected]>
>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
>
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