Want to view 4 channels of mV (thermocouples) simultaneously on V/time graph.

[RandyMyers]

We wish to buy 2 either PCI or USB DAQ boards from you (preferably cheapest priced #PCI-6010 or equivalent USB) and then run a Labview file to view 4 thermocouples outputs on a V vs Time graph and after about 10 minutes save the data for use on an excel spreadsheet.

1. What is the best/cheapest board to buy that would hook up to 4 independant inputs to suit our needs?

2. Could you please point to an easy .vi file that we can download to use with our already licensed Labview 7.1?

We would like to purchase 2 of these DAQ boards and would not like to spend much time if there is a program file that would get us up and running.

Thanks In Advance!

[Chris_K_]

Hi Randy 

Thank you for using our discussion forums.

PCI 6010 is one of our B-series cards, and it is good for basic DAQ applications, however a much better solution for measuring thermocouples is the USB-9211A.  The 6010 is 16 bit with no cold junction compensation (cjc) sensor, which is needed for accurate temperature measurements.  The 9211A is a 24-bit CompactDAQ module in a USB sleeve with 4 input channels designed for thermocouples.   It is limited with a slow sampling rate but it has a built in cjc sensor.  Since thermocouples do not change very fast the 9211A is well suited for this type of measurements.

The 9211A also uses the DAQmx driver, and when installed, the driver installs a large number of example programs.  From LabVIEW go to Help >> Find examples to open the example finder. From the Example Finder browse to Hardware Input and Output>>DAQmx>>Analog Measurements>>Temperature.  Here there are a lot of VIs that can be used and easily modified to perform thermocouple measurements  

Chris_K

Message Edited by Chris_K_ on 05-16-2008 10:43 AM

[SeanJ]

Hi Chris,

I'm trying to use the 6010 to do some basic measurements. 

In the above you state there is no cjc sensor. When I run the DAQ assistant and specified the 6010 it prompted for a cjc constant.  Is it possible to feedback a sample set and thereby adjust the cjc constant.

Do you know what kind of issues I could expect to see if the cjc value is set at a constant 25C?

Rgds,

Sean

[R Sandoval]

Hi Sean,

Technically, you cannot change this constant once you set it and acquire.  When setting the CJC Source to a constant, you are taking relative temperature and your values are less accurate in comparison to using an external channel source.  The best practice when using the constant source is to allow the device to settle for about 15 minutes, take the environment temperature, and use that temperature as your CJC constant value.

Roman S

National Instruments

Applications Engineering

[maidai]

Hello,

I am also trying to read the mV of thermocouples not the degree nor in C,F,K,R.

I am using 9211 and I have tried the LabView SignalExpress 2009 also searched on the NI Example Finder on the Temperature section in DAQmx, but unfortunately I still cannot find the right software.

Do you have any suggestion regarding this matter ?

Thank you very much in advance.

Regards,

[R Sandoval]

Hello there,

Have you tried to manually program SignalExpress to do analog voltage acquisition?  Under the Project window right click.  Select Acquire Signals -> DAQmx Acquire -> Analog Input -> Voltage.  A window will prompt requesting the device and analog input channel.  Select the NI-9211 and the desired analog input channel.  Select Run.  SignalExpress should then display the voltages in real-time.

[maidai]

Dear Roman,

Thank you very much for your soon response 

Yes, I have tried the Voltage acquisition in SignalExpress with 4 pcs of type K thermocouple, the average temperature measurement of them for ambient is -69 nV.

Meanwhile the ambient temperature measured with Themocouple acquisition in SignalExpress showed 25 degC equivalen to 1000 uV.

So, do you have advices to correct this error ?

Thank you very much for your help 

Best Regards,

[maidai]

Dear Roman,

I have additional questions, how can we measure the CJC Source (Cold-Junction Compensation) in uV or degC with NI 9211 or NI 9219 ? How can we specify the virtual Channel on CJC? Or can we add a Zero Refference with melting Ice point on a channel ?

Thank you very much in advance.

[R Sandoval]

Hello there,

Since you are measuring thermocouple voltages, you should correct this offset error using the CJC value.  You should be able to read the CJC values using the property node of DAQmx Channel.  Please refer to the following link:

Reading the CJC Values with the Thermocouple Measurements

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019N2WSAU&l=en-US 

Similar to the NI 9213, the NI 9211E and NI 9219E circuitry uses a thermistor to measure the cold-junction temperature. The following links provide more information about calculation of temperatures using the CJC values:

NI 9213 Thermocouple CJC voltage reading

https://forums.ni.com/t5/Multifunction-DAQ/NI-9213-Thermocouple-CJC-voltage-reading/m-p/1212299

Isothermal Errors and Cold-Junction Compensation (CJC) Data Scaling for the NI 9211E and NI 9219E

http://digital.ni.com/public.nsf/allkb/E6DFDA7F1AA16C1C86257599006D4A0A 

[maidai]

Hi Ramon,

Thank you very much for your assistance, I'll try them. ^^

Best Regards,