Glen Walpert wrote:
On Sun, 12 Dec 2010 00:17:16 -0800, Robert Baer wrote:

Start with the tables available from the Minco.com site.
Pick and print a table for Platinum:PE TCR=3.85e-3, R0=100,temp range
20C to 220C inclusive.
Minco indicates A=0.0039083, B=-5.775e-07, C=-4.183e-12. Put the data
in a spreadsheet and do an XY error curve, tabular data
VS equation.
Note that i had to fiddle with B and C to get a reasonable fit, and
one is challenged to get two digits of significance for C.
See attached pic of error giraffe.

Interesting giraffe. Were you bragging about the goodness of fit or
complaining about it?

I see that your ±.00005 ohm deviation is well within the tolerance for
even IEC 751 Class A 100 ohm Pt RTDs, at ±(.06+.0008Tˆ’2E-7(T^2)) ohms -
less than 0.1% of the allowable tolerance.

It looks like you either have measurement data in your tables, with
measurement noise (rather than tables reconstructed from equations LMS
fitted to measurement data), or perhaps just round-off error.

I have used the equations (or as many terms of it as were useful for my
range), and then done a two point calibration against two temperature
standards near the ends of my measurement range with the actual RTDs to
be used, for final equation adjustment on a per-RTD basis, when setting
up temperature monitoring for test purposes where better than 1 C
accuracy was desired. The accuracies expected without individual sensor
calibration make the bounce in your giraffe seem pretty much irrelevant.

Regards,
Glen
1) That deviation is *after* the fix in the standard equation. In fact,
the fit is better if C=0.
2) The "noise" 1s due to roundoff error in the presented table data and
is to be expected; if it was not there or a lot smaller, the nsomething
would definitely be wrong with the data.
3) Take that C term, -4.183e-12 and multiply it by 200C^3...and all of a
sudden get about -3.35e-5 which looks terrible on a graph WRT to my
"corrected" graph, as that gets multiplied by R0=100 ohms, for a value
of 3.3mohms.
When one starts without an expensive standard, and wants to make
reasonable accurate temperature measurements, then a RTD is the way to
go, and use the tables as the reference "standard".
So, how do you make a correction without a standard??
Do you derive the lead resistance from the tables?