Hello Once Again Labjack,
I have another question for you guys - one that I hope will be easy enough to solve, as it's really been stumping me.
I'm using the Labjack to read in voltages from several different sensors. I've posted before about my Omega thermistor, but I'm also using the Labjack to convert the signal from a Honeywell Durafet III pH probe. These signals are then used as feedback in process control loops. That is to say, if the temperature gets too hot, I can have the control computer turn on a chiller, or if the pH gets too high, I can bubble CO2 gas into seawater (which lowers pH). We are interested in conducting research on the effects of ocean acidification on marine life. In the pictures below you'll see an apparatus consisting of some gray PVC pipe and a gray box, as well as the control computer and several other components which are not pictured. The apparatus is wired to a standard 120 VAC, 15 amp circuit, to power the computer and the various appliances which make the whole thing work (chiller, heaters, relays, solenoid valves). Everything uses 120 VAC right out of the wall, although I do have a DC power supply to drive some sensors, and of course the Labjack, as well as my relay card, plug straight into the computer via USB cables. The entire apparatus is made out of PVC and plastics, save for a few metal screws installed inside of waterproof electrical enclosures.
While using both my thermistor and my pH probe, I've noticed a similar problem in the data that I can only think has to do with interference (or possibly grounding?). Please have a look at the attached 10 minute datasets I've recorded using the LJLogUD.exe application. The first dataset shows my thermistor voltage while the Labjack is sitting next to my apparatus on a plastic cart, and the second dataset shows the thermistor voltage when I placed the Labjack directly on the apparatus itself. It's not hard to see that placing the Labjack on the apparatus results in much noisier data. Total distance of the Labjack move is just over 2 feet.
I'm not sure what is causing this noise but I don't think it has to do with any of the appliances running. It doesn't seem to be cycling with the chiller's compressor, for example. Even with everything turned off, I get the interference. The only way that I've found so far to get the same dataset with both Labjack positions is to unplug my apparatus from the wall. I still have a couple of tests I'd like to try, but I thought I'd reach out for any suggestions you guys might have.
The data were taken at 500ms intervals, and that sort of sine wave pattern you see there is consistent with all my trials.
Thank you very much,