AIR400K Scaling Issues


#1

Hi guys!

Would like to get some input from everyone. I have a SNAP-AIR400K-8 which connects to Resistance output NTC Thermistors with a range of 510.06Ohms - 96,974Ohms. I’ve got the AIR400K-8 module configured to 0-100k Ohms and configured the scaling in actual vs the scaled. 510.06Ohms (120degC) and 96,974Ohms (-20degC).

Problem is I’m not getting the proper equivalent. I’m getting the proper resistance readings (via multitester) and the resistance value is spot on to our reference (old temperature measurement equipment). But when it goes through the AIR400K-8, it reads as 90degC, when it should be 9degC. What’s worse is that if its’ 11degC, it reads as 114degC on PAC Control.

Any idea what this might be? Do I just have it configured wrong?




#2

Your thermistor doesn’t have a linear curve, so you won’t be able to use the built in scaling on that edit analog point window. Hopefully you can find the Steinhart-Hart coefficients (might be in the data sheet) - then you plug them into the Steinhart-Hart coefficients for the AIR400K in PAC Manager (See document 1065 - analog input modules data sheet and search for Steinhart - around page 16 or 17). There are some built in ones as well - yours looks close to a 10K type 2, but it is slightly different so if you can find the coefficients, it should be more accurate. Don’t forget to save to flash when you are done.

Edit:

Since you are using chilled water and want accurate values at lower temperatures <40C - I plugged three of the lower temperature values from your picture into the calculator here: http://www.thinksrs.com/downloads/programs/Therm%20Calc/NTCCalibrator/NTCcalculator.htm

I came up with the following coefficients that you can enter in PAC Manager for your points. These will have significant error at high temperatures, but are very accurate below 40C.

A 1.130667E-03
B 2.338823E-04
C 8.856813E-08
K = 0