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Setting Cold Junction Compensation Value While Using Virtual Bench Logger with NI-435x and Temperature Accessory

Hardware: Modular Instruments>>High-Precision DAQ (Dataloggers)>>PXI-4351, Modular Instruments>>High-Precision DAQ (Dataloggers)>>PC-4350

Problem:
While using NI 435x card along with the temperature accessory such as CB-68T, TBX-68T etc. and Virtual Bench - Logger, how should I set the Cold Junction Compensation (CJC) Setting value correctly? Under Edit >> Settings, there is an input called "Cold Junction Constant" for "Thermocouple" transducer. How do I set the correct value for it?

The help says "VirtualBench-Logger automatically configures the CJC settings if you are using a 4350/4351 device and one of its temperature accessories. If you are not using a temperature accessory with your 4350/4351 device, you must specify a cold junction constant value."

I am already using a temperature accessory. If I do not put any value or 0 in the "Cold Junction Constant" input box, I get erroneous reading on my channel to which my thermocouple is connected. How can I specify the CJC value correctly? In the help, I have seen something called CJC Configuration dialog. But I do not see it while using the Virtual Bench-Logger with the NI 435x card and the temperature accessory. How should I set the CJC sensor and how do I know the correct CJC value?

Solution:
First, make sure you are explicitly setting your temperature accessory to be an accessory for your NI 435x card in Measurement and Automation Explorer (MAX). In MAX, you can do this by right-clicking on your board under Devices and Interfaces and selecting Properties. From there, select the Accessory tab and choose your accessory from the drop down list.

In addition, the "CJC Configuration" dialog will not appear on the Settings panel while using NI 435x and the temperature accessory. The correct value of CJC needs to be entered in the "Cold Junction Constant" input box. Now the question is, how to input the correct value. Please follow the following steps:


  1. Finding the correct CJC Value

    2. Let us take the case of CB-68T. On CB-68T, channel 0 is dedicated for Cold Junction Compensation. Hence, in Virtual Bench Logger, under Edit >> Settings,

    • Select the "Start Channel" as well as "End Channel" as ch0.

    • Select Transducer as "Thermistor" since the CJC sensor on CB-68T is thermistor.

    • Click on "Thermistor Configuration" and select "435X CJC Sensor". The coefficients are automatically selected.

    • Click on "Start" in the Virtual Bench-Logger environment and note the measurement from ch0. This will be the CJC value. Note down this value.


  2. Providing the CJC Input for the Channel where Actual Thermocouple Measurement is Done

    • Select the "Start Channel" as well as "End Channel" as the appropriate channel to which thermocouple is connected.

    • Select the transducer as "Thermocouple" since the transducer connected is thermocouple.

    • In the Cold Junction Constant, provide the value of CJC that you got in Step 1.

    • Set the other settings appropriately.

    • Click on "Start" in the Virtual Bench-Logger environment to observe the results of the measurement channel. The results should be perfect.



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Report Date: 03/19/2003
Last Updated: 11/17/2004
Document ID: 2VI4H2O

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