Fluctuating Voltage Signals with NI-9205

[Tomwell17]

Hello, 

I have been having issues with differential voltage measurements when running my data acquisition cDAQs. I have a mix of thermocouple modules (NI-9213), voltage modules (NI-9205), and a control output module (NI-9403) running on a cDAQ-9189. This issue has appeared in 3 separate cDAQs with 3 different voltage modules, so it must be something that I am doing (or not doing). I use the DAQmx read function in LabVIEW, Analog 1D DBL NChan 1Samp, every one minute to collect the data from the sensors. Details about the sensors follow. 

The issue is that the voltage measurements come out very discrete and choppy. I have attached three different data sources for you to see the behavior, I don't exactly know how to describe it. The air quality sensors output two wires and is measured as a differential voltage. They are powered by an external power supply, which is plugged into the wall outlet of the home, that definitely has enough current to power the devices. The indoor pyranometers use a current loop so the differential voltage measurement comes from the voltage drop across a resistor. Lastly, the outdoor pyranometer is also a differential voltage sensor that power externally just like the indoor sensors. I am not too familiar with preventing ground loops or how to figure out if that is the cause. I have connected the grounds to the COM port and not, neither solves the issue. The external power supplies are plugged into the walls, just like the DAQs. I cannot seem to figure out why all of my voltage sensors are doing this. Any help is greatly appreciated!

Air Quality sensors: 

Air Quality sensor voltage fluctuations.

Indoor Pyranometer: 

Indoor Pyranometer voltage fluctuations.

Outdoor Pyranometer:

Outdoor Pyranometer voltage fluctuations.

[Tomwell17]

I did find one solution to the indoor air quality sensors. I realized that the drops in voltage happened when the air conditioner was off in the home and it would come back up when it turned on. Weird. I think there were too many electronics plugged into a power strip (laptop, desktop, 2 cDAQ, power supply) and when I moved the cDAQ chassis power to another outlet in the house, this behavior stopped. However, now these sensors show the fluctuation about every hour as well. 

I am wondering if this is a grounding issue. For example, is this because I do not have an Earth ground connection to the chassis? Do I have the correct wiring with the COM port of the voltage modules (NI-9205)? 

I also have thermocouple modules (NI-9213) on the cDAQ and there is no sign of this signal fluctuation every hour. I am aware that the thermocouples are passive sensors and do not require a power supply. This is why it is making me think there is some power issue with the voltage measurements. 

Any advice would be appreciated. Thanks!