Thermocouple Data Acquisition System

[grifter]

Dear Guys,

    I'm a new user with labview and currently using labview for a  project of mine. Im trying to acquire real time temperature using thermocouples and LabVIEW 5.1. The signal conditioning board that i used is PCLD-789D daughter board, a suitable signal conditioner for thermocouple measurement. This board is a multiplexer and an amplifier with cjc of 24.4mV/DegC ( 0.0V at 0.0DegC) while the card that i used is DAQCard-700 which only support single-channel or single scan acquisition in multiplexed mode. The question are:

1) How do i design  my labview 5.1 software? (whereby im not sure on whether to  create the application using SCXI programming or pluq-in device programming and i already design an aplication using simple method using single channel anaolog input vi - an example of the vi is really appreciated)

2) Since the DAQCard-700 only support single scan acquisition, is integration between the board and the card possible?

3) How do i compensate the cold junction in labview 5.1?

    Feedback, example and help is really appreciated. I give my thanks in advance. Thank you very much

[Travis G.]

Hello Grifter,

The first thing you will need to verify is that you have the correct version of the Traditional NI-DAQ driver for your DAQCard-700.  For LabVIEW 5.1 and that device, the latest version of the Traditional NI-DAQ driver you can use is 6.9.3.  You can check what version of the driver you are using by looking at the software category in Measurement & Automation Explorer (MAX). 

The signal conditioning VIs that are part of the Traditional NI-DAQ functions are meant to work with National Instruments SCXI or SCC signal conditioning products, and I'm not sure that they will work properly with a 3rd party signal conditioning system.  You should still be able to use the analog input functions to take measurements from your DAQCard-700.  The best place to start with programming with Traditional NI-DAQ in LabVIEW is the example programs in the NI Example Finder (Help >> Find Examples).

From looking at the specifications I found online for the PCLD-789D, it basically multiplexes 16 inputs into one signal.  Theoretically, you should be able to read all 16 inputs through one analog input channel of your DAQCard-700.  The problem is that you are mixing products from different manufacturers, so there won't be an easy out-of-the-box solution for coordinating the sampling of the DAQCard-700 and the multiplexing of the PCLD-789D.  The product page for the PCLD-789D states that it is for use with a PC-LabCard, so to use that signal conditioning device with a National Instruments data acquisition card will require that you basically write your own driver for the device.

This is a pretty advanced task, and since you said you were new to LabVIEW, this probably won't be a good solution.  What I would recommend is that you use some National Instruments signal conditioning hardware, which is garunteed to seemlessly integrate with our DAQ devices.  This way, you will be able to use the NI-DAQ driver, and its application programming interface (API), to easily program a data acquisition task in LabVIEW.

Sorry, I know this probably isn't the answer you were looking for, but let us know if you have any further questions or need any help in speccing out any National Instruments signal condition hardware.

Regards,
Travis G.
Applications Engineering

[grifter]

Dear Travis,

Thank you so much for your reply. Unfortunately for the mean time I must use the PLCD board for my project due to time constraint. I do have a question regarding cold junction compensation. If the board already compensate for cold junction, how do I adapt the CJC in the vi? since for a multiplexed mode, DAQCard-700 only support single-channel or single scan acquisition, hence im not able to assign more than 1 channel to my vi( im using AI sample channel vi) . And since for a multiplexed module, the default analog input channel is 0 hence I cabled the board analog output (channel 0) to the channel 0 of the DAQCard.

That’s all for now. Thank you very much for your time

[Travis G.]

Hello Grifter,

If data is multiplexed already by your PLCD board, it will all be coming into a single channel of your DAQCard-700.  You can easily scan that single channel of your DAQCard-700 using the VIs in the Traditional NI-DAQ > Analog Input palette.  The problem is that you will need to coordinate the timing of how the PLCD board is multiplexing between the channels, and when the DAQCard-700 is reading from the PLCD board.  If you were using a National Instruments SCXI signal conditioning system to multiplex the signals, this coordination of multiplexing and sampling would be taken care of behind the scenes using the Signal Conditioning VIs in the Traditional NI-DAQ driver.  The Traditional NI-DAQ driver actually uses some of the spare digital lines on the DAQ card to coordinate timing between the DAQ device and an SCXI multiplexer.  In your case, you will need to basically write your own program to coordinate multiplexing and sampling, which is not an easy task.  It will require a detailed knowledge of how the PLCD board operates, and from my best guess, access to some type of digital triggering or handshaking lines on that board.  If time is truly your constraint with this project, it will save you many hours of development time to either purchase a National Instruments signal conditioning system that is guaranteed to operate seamlessly with your DAQCard-700, or purchase a third party data acquisition device built specifically to operate and communicate with your PLCD board.  Of course with the second option, you loose the ability to easily use the LabVIEW graphical development environment to develop your application.  Sorry for being the bearer of bad news, but realistically what you are trying to do is more difficult than you think.

Regards,
Travis G.
Applications Engineering