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PXI-4351 Connection questions

Hi

According to the manual for the 435X:

 

The NI 435x accessories—the TC-2190, TBX-68T, and CB-68T for theNI 435x for PXI, PCI, and USB—are designed to be used with

thermocouples. Consult the accessory installation guide for instructions on

how to connect the thermocouples.

 

However, I cannot find any information which tells me exactly why I need to use the TC-2190 (or other accessories) with this unit.  Could I not connect a thermocouple directly to the connector on the front of the 435X?  If not, why would that not work, what does the accessory do for me?

 

Thanks in advance!

Reece L. Bain, Jr.
Electronics Engineer, Stf.
Lockheed Martin Aeronautics Co.
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Hi Reece, Thank you for using our discussion forums.

The thermal couple accessories feature isothermal construction to minimize the temperature gradients across the thermocouple junctions and a high-accuracy thermistor cold junction temperature sensor.

You could attach a thermal couple directly to the 435X device and take temperature measurements without an accessory, however you would not be compensating for the parasitic cold junctions voltages that are present where the thermocouples are attached to the terminal block.  Not compensating for this will reduce your overall accuracy of the  temperature measurements.  Have a look at the “Thermocouple Measurement and Signal Conditioning” section of the following Developer Zone Tutorial.

Taking Thermocouple Temperature Measurements  

It explains why this type of signal conditioning is needed for accurate measurements. You could also dedicate a thermocouple on your own as the cold junction compensation temperature sensor, but this requires additional considerations which are taken care of if you use the accessories recommended with the 435X.

Chris 
Message Edited by Chris_K_ on 10-02-2008 08:48 AM
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Thank, you, Chris.

First, that's exactly what I needed to know and could not find anywhere.  I have read the tutorial that you referenced, but it still did not give me an answer about directly connecting thermocouples to the 4351.

 

The problem that I have is that in my test equipment I will be sending the thermocouple through at least 2 sets of contacts before it gets to the unit being monitored anyway.  So in my case, I think that the CB68T is going to be useless in that situation anyway as long as the 4351 will accept thermocouples direct.  I will need to worry about CJC since I'm going thru these contacts before the monitoring point, but I'm thinking that the CB68T is not going to be useful.

 

thanks for your time!

Reece L. Bain, Jr.
Electronics Engineer, Stf.
Lockheed Martin Aeronautics Co.
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