Thermistor wiring in cb-68lp with PCI 6034E

[Gorman]

I have a CB-68LP pinout board with a PCI 6034E card on a computer with XP and LabVIEW 7.0.  I have been using type K thermocouples to collect temperatures in a lab and have had no problems thanks to previous help.  I am now going to be moving my temperature collection out doors where I will need to run ~1000 ft of wire to reach my destination.  I want to switch from thermocouples to thermistors to save money on wiring.  I have done a little research on thermistors and there are still unanswered questions.  I know thermocouples connect differentially but I could not find information about how thermistors connect?  Since it is just measuring a change in resistance my guess was that it is single ended but I want to make sure.  That is the main question, if anyone has suggestions on current excitation values or possible problems that I might expect that would be great also.  Thanks

[Raajit_Lall]

Gorman,

With NI-DAQmx and Measurement Automation Explorer there is actually a feature called connection diagram which will let you select your connection block and tell you exactly which pins you need to connect your sensor to, whether it is a thermistor or a thermocouple.  You can actually set up a thermistor measurement in Measurement and Automation Explorer for current excitation and voltage excitation seperately.

For example, I created a current excitation thermistor task in Measurement and Automation Explorer running into ai0 (channel 0) and I clicked on the connection diagram tab at the bottom.  Here is the diagram that I got.



As you can see, you just need to connect to pins 68 and 34. 

There is also an example program that you can download, which is in LabVIEW.  Here it is.

Hope all this helps.

Regards,

Message Edited by Raajit L on 10-12-2007 06:05 PM

[Gorman]

I hardly use MAX so I didn't think to go there for information.  From the diagram you have in  your message I can see that it is connected differentially.  However, I thought it would be good for me to learn where to find this information for further reference and I couldn't get it.  So perhaps you could give a little more guidence or figure out which step I missed.  Here is what I did.  I opened MAX, popped up on Data Neighborhood under My System.  From there I created a new task and arbitrarily chose current excitation thermistor with it reading in on Ai0.  It created information that I would see if I created the same task using DAQ Assistant.  However this is where my journey ends because no where do I see a tab for a connection diagram.  Maybe I am in the wrong window or am creating the task from the wrong place.  This is where I thought I should go to create the task.  Thank you for your help.

Bill

[AnalogKid2DigitalMan]

For your wiring, suggest you use individually twisted pair shielded cable to minimize noise pickup. A 1000' run is a big antenna!

[johnsold]

NASA published a circuit several years ago which combined several sensors in series. They were excited by a current source and one wire was returned from each junction. By subtracting the voltages at adjacent points, the voltage across each sensor can be determined. N sensors require N+1 wires. Big savings on long runs. For optimum performance on a long run using Kelvin connections on the excitation (N+3) wires would eliminate the voltage drop in the wires.

As was previously mentioned, shielded twisted pairs are going to be important.

Lynn

[Raajit_Lall]

Gorman,

Looks like you did get all the steps right.  The connection diagram tab is located at the bottom of the window.  This screenshot should help.



Regards,

Message Edited by Raajit L on 10-18-2007 04:33 PM

[Gorman]

Is it possible that older versions of MAX do not have this feature ?  I have version 3.0.1.3005.  I recreated the task and included a snapshot of what I see.  (Sorry, no software to make the image more presentable)  Maybe the feature is locked somewhere or my MAX version is too old.  Thanks again to everyone for all of your help.

[Raajit_Lall]

It does look like the version of Measurement and Automation Explorer (MAX) that you have does not have the connection diagram feature with it.  This might have been added in a more recent version of Measurement and Automation Explorer.  If you want, you could upgrade your version of DAQmx and there fore the version of MAX as well.  However since you have LabVIEW version 7.0, you would have to use DAQmx 8.0, which I am assuming, you are already using.  All later versions of DAQmx will work only with LabVIEW version 7.1 and above.

Regards,