NI-9211 Measurement Accuracy

[MarkMoss]

Figures 5 through 9 of the operating instructions for the NI-9211 4-Channel Thermocouple Input Module contains several charts that give typical and maximum measurement error under various conditions and for various types of thermocouples.  These charts cover a very wide range of temperatures.  For example, the Type K chart covers -200 °C to +1400 °C with a division only every 200°C.  This makes it difficult to user these charts to determine the maximum measurement error at a specific temperature.
 
Is there a forumla or table of values I can use to determine the maximum measurement error for a smaller range of temperatures?  Specifically, I'm interested in determining the maximum measurement error for measurements between -40 °C and 125 °C with a cold-junction temperature of 23 °C ± 5 °C for measurements type J, K, and T thermocouples.  (Not including errors due to the accuracy of the thermocouple itself.)
 
Thanks,
 
Mark Moss
Electrical Validation Engineer
GHSP
 
 

[J_Thomas]

Hi MarkMoss,

First of all, let me apologize for the delay in posting
back.  Things got really busy these past
few weeks with NIWeek 2008. 

 
The error shown in the figures for the 9211 was derived
using a very tightly controlled process in R&D that takes into account both
voltage errors and CJC sensor errors.

 
You could get a rough estimate using the Gain, offset and
CJC temperatures.  The math would be a
bit challenging, but doable.  Basically
you could calculate the voltages for the various temperatures for the specified
thermocouples. You would then calculate the gain and error in terms of voltage.
You then will need to convert this value to a change in temperature in that
range and add the CJC error. This will give you a rough estimate, but will not
include the thermocouple error. The graphs in the manual include this
thermocouple error.

 
The NIST defines a voltage vs. temperature curve for each
thermocouple type at their website. This information can be found at this link:
<a href="http://srdata.nist.gov/its90/main/" target="_blank">http://srdata.nist.gov/its90/main/</a>.

&nbsp;
Another way to get an estimate would be to get as many data
points on the graphs in the manual as you can reasonably estimate and plot them
into an Excel Spreadsheet.&nbsp; Plot them on
a graph and then do a curve fit between the data points, essentially recreating
the figures.&nbsp; For most of the
thermocouples the error profile is a pretty smooth curve so this method would
not introduce to much additional uncertainty into the measurement. Any error
estimation is subject to uncertainty, and so even an extremely precise table of
data points for the error may not represent the error in your actual test
system any better than the approximations you can get from these figures.