Aquiring Analog Input from Extech 401025 Light Meter

[shpeigleton]

I'm using an Extech 401025 Light Meter to measure lux from various headlamps. My goal is to build a Labview program to log lux over time using the analog output from the meter. The analog output has a max output of 200mV with a 0.1mV resolution (0.1mV = 1 lux so I'm just scaling the values to give me lux). My issue is my NI USB-6216 DAQ or my program is giving me values all over the place. I took the meter apart so I could measure the output terminals with a multimeter and the voltage is dead on. I'm new to Labview so my code is a bit of a mess but when probing the data wire before filtering and averaging, the values are still wrong. I've also tried a different DAQ unit thinking there could be something wrong with the one I was using but I'm still getting the problem. Any help would be much appreciated! Attached are images of my code and setup. Thanks.

[Kyle97330]

You don't show which terminals on your DAQ you're connecting to the meter.  However I can see that you have connected a green wire to one of the terminals, which is usually a ground wire. 

However, you've chosen "Differential" on the front panel, which usually means that you want to connect two different AI terminals to measure the difference between them.  In most cases the secondary AI terminal should be 8 higher than the first, i.e. AI9 if you are using AI1 for the input.

Is that how you have connected to the DAQ?  If not, what connections did you use?

[shpeigleton]

The green wire is negative from the meter (green is just what I had laying around). Attached is a better photo of what connections I'm using.

[santo_13 · Accepted solution]

You need to connect AIGND as well. 

It would vary based on how your Extech meter outputs

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000YIfVCAW&l=en-US

[Kyle97330]

If you short AI1 to AI9 with just a plain wire do you read 0V or do you read more crazy values?

If you close LabVIEW completely and open up NI-MAX, and from there find your device and use the Test Panels with the same channel configuration, do you get the same results there or something else?

[shpeigleton]

That was it. Makes total sense after I read into that article, even for my mechanical engineering brain. I'm getting a fair amount of variability in my value still but it hovers around nominal so that is more of a filtering/averaging thing. Thanks for your help!