Channels available for themocouples in differential mode on SCB-68 block CJC enabled

[Fair Hope]

My understanding is that only 7 channels can be used for thermocouples on the SCB-68 block when CJC is enabled, pair 0,8 are used by the CJC.  All channels but 0, 8, 14 and 15 are in use and I need to read two thermocouples.  Using single ended mode I need to connect as RSE, correct? 

I seem to be getting a lot of noise (full scale fluctuation!) when the equipment is running.  I don't have much problem (±5C) when the equipment is off (the computer then has to be plugged into a different outlet instead of the outlet on the main power panel for the equipment.  Don't know yet if this is significant).  I've looked at the tutorial on noise reduction, but am not sure how to apply it in this case.  Differential mode would help, but can't afford a system with more channels right now.  Thermocouple wires aren't running close to power.  They are within 12" of a motor.  Other inputs on the board are 0-20 mA, 4-20 mA, 0-5V and 0-10V.  Is there anything obvious to any of you?

[Fair Hope]

I have shown that it is not the outlet the computer is plugged into.  Noise is the same as long as the control panel (housing two motor controls and the terminal block to which signals from the process are connected and then taken out of the panel in a cable to the SCB-68 block) is powered, with out without other equipment turned on.  The thermocouples are the only inputs not going through this control panel. They go directly from the process to the SCB-68 block. 

All signals are a little noisy, but a variation of a few mV on a 0-5V signal doesn't cause a problem.  Most signals are running 0.2V at their lowest.

[Chris_D]

Hey Fair Hope,

You are correct that when using CJC, AI0 and AI8 are not available for use. You can read some more information about this below.

How Do I Use the Cold Junction Compensation on the SCB-68?
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019MnvSAE&l=en-US

Connecting the thermocouple as differential should show some improvements in the noise. What DAQ device are you using? I am assuming you are connecting the thermocouples to channels 14 and 15 in RSE. It seems like your only real options are to either remove some of the connections that are in use or upgrade your device to more channels. You could perform a test by disconnecting some of the other connections in use and connect the thermocouple as differential to see if it does in fact improve the noise.

I have also linked a couple of articles that discuss improving noise.

Reducing Thermocouple Noise to Improve Measurements
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P8wxSAC&l=en-US

Field Wiring and Noise Considerations for Analog Signals
https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

Message Edited by Chris_D on 04-01-2008 12:54 PM