Thermistor temperature not correctly calculated

7 posts / 0 new
Last post
dima-nxa
dima-nxa's picture
Thermistor temperature not correctly calculated

Hello,

I use the thermistor "B57861S0103F045" (https://en.tdk-electronics.tdk.com/inf/50/db/ntc_13/NTC_Mini_sensors_S861.pdf ) to measure the temperature connected via Excitation circuit # 1 - 10 µA Current Source.

To calculate the Steinhart-Hart coefficients, I use a script from the site: https://www.thinksrs.com/downloads/programs/therm%20calc/ntccalibrator/ntccalculator.html.
I substituted the values from the datasheet into the script and got the coefficients A, B, C.

Next I wrote a Lua-script to read the thermistor data, the temperature values are very different from the calculated ones (see attachment). What am I doing wrong?
 

File Attachment: 
LabJack Support
LabJack Support's picture
I suspect the problem is that

I suspect the problem is that you have computed the 3-term coefficients but we use the full 4-term model.

https://daycounter.com/Calculators/Steinhart-Hart-Thermistor-Calculator....

 

File Attachment: 
LabJack Support
LabJack Support's picture
Actually, it looks like the

Actually, it looks like the difference is that model uses "R" where our model uses R/Rt.  So 2 things:

1.  Your C is the ^3 term so should be passed as D.  C should be set to 0.

2.  Set R25 to 1.

 

... but that does not seem to work for me either as shown in attached.

File Attachment: 
dima-nxa
dima-nxa's picture
Then I just do not understand

Then I just do not understand how to calculate these coefficients on a datasheet. Everywhere it is proposed to simply enter 3 points of temperature and resistance from the datasheet and the coefficients are calculated. Here, for example, on the site:

https://edwardmallon.files.wordpress.com/2017/04/ntc-steinhart_and_hart_calculator.xls

offer to do it too. The result is the same as my coefficients. 

As I understand it, when using 4 coefficients it should just slightly increase the calculations exactly as compared to the method using 3 coefficients, but there should not be any fundamental differences in the coefficients.

File Attachment: 
LabJack Support
LabJack Support's picture
It does work.  I was just

It does work.  I was just missing a 0 when I entered D.  See attached.

 

File Attachment: 
LabJack Support
LabJack Support's picture
I made a tab for your part on

I made a tab for your part on our thermistor calculator spreadsheet.  See that last set of calculations labeled Steinhart-Hart with "R/Rt" with "R" constants.

https://docs.google.com/spreadsheets/d/1hWFXV-46-L376f4q8T55wZmOqX9fUcbb...

 

dima-nxa
dima-nxa's picture
Thank you for help.

Thank you for help. Everything works perfectly!

File Attachment: