Temperature Measurement with Thermistor and PCI 6211

[schreineradam]

I am very new to this, so please forgive me if I sound too novice. I am trying to measure the temperature of a system using a thermistor and a PCI 6211 DAQ. I am using LABView 7.1. As far as I can tell, this board can only read resistances from 100 ohms to 1000 ohms. I'm not sure what steps need to be taken to preform the task. Any help would be appreciated.

[Brian F.]

Hello.

Thank you for posting to the NI Discussion Forums. 

To give you a bit a background on making temperature measurements, please reference the following resource:

Measuring Temperature with an RTD or Thermistor
http://zone.ni.com/devzone/cda/tut/p/id/3643

This tutorial introduces and explains the concepts and techniques of measuring temperature with an RTD or Thermistor.

For information on how to program in LabVIEW a temperature acquisition task, take a look at the shipping examples. In LabVIEW, click on Help>>Find Examples. Now browse to Hardware Input and Output>>DAQmx>>Analog Measurement>>Temperature. There you will find several examples on this subject.

I would recommend using DAQmx to take a differential measurement with the USB-6211 device.  For more information on differential measurements, please refer to:

What Is a Differential Measurement System?
http://digital.ni.com/public.nsf/allkb/C8156250190A1386862568B000753174

Let me know if this helps get you started or if you have any other questions.  We would be happy to help. 

Have a great day!

Brian F
Applications Engineer
National Instruments

[schreineradam]

Thank you for your help. I have read the information you have provided. I have not been to my lab, so I have not been able to check out the LabVIEW examples. I do, however, have a few questions on what I have read. I understand how the thermistor works and how to measure it, etc. I'm not sure what extra equipment (if any) would be needed to do this with my 6211. I read in the first article you provided about the SCXI Signal Conditioning System. I understand that the 6211 is capable of analog input and output, but am not sure if it is precise enough to have even relatively accurate readings in this implementation. Again, thanks for your help.

[Brian F.]

Hello. 

The USB-6211 does provide 16 bits of resolution so it is a board capable of taking very "accurate" readings.  However, since we are dealing with readings in the low mV, if there is any noise present in the system, it could alter your readings by a relatively large amount.  The best solution for this is to provide some type of filtering to remove that noise.  You could choose to do this yourself by building a simple filter or you could use our SCXI solution which has onboard filters that could remove this noise automatically.  Also, if you are using LabVIEW, you could provide filters in software which would clean up your signal.  It really depends on the accuracy that your application requires as well as the noise present in your setup.  I would recommend trying the measurement with the 6211 and seeing if this meets your needs or is too noisy.  However, SCXI, an acronym for "Signal Conditioning eXtensions for Instrumentation", would probably be the best solution possible.

Good luck!

Brian F
Applications Engineer

National Instruments

[schreineradam]

I am having trouble setting up the VI for this application. As I understand, the card I have does not have an internal excitation source, nor is it capable of current output. I am trying to implement this without any 'external' source. I have not been able to find any example VI's that use a voltage excitation source, and am having some trouble making my own. Any help would be appreciated. Thanks.

[Brian F.]

Hello. 

A very helpful example for taking temperature measurements that require excitation is "Acq Thermistor Samples-SW Timed.vi".  This example seems to do exactly what you want except for the voltage excitation.  Currently, it is configured for current excitation.  However, this can be changed easily by changing the "DAQmx create channel" 's polymorphic instance to AI Thermistor Vex from AI Thermistor Iex.  Click on the text below the actual VI block and navigate to Analog Input>>Temperature>>Thermistor>>Voltage Excitation.  After that, it should just be a matter of changing a few controls around and you should be up and running. 

Good luck!

Brian F
Applications Engineer
National Instruments