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Labjack Interference Problem

interference signal problem

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#1 Heckler511

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Posted 25 February 2014 - 06:20 PM

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,

Mike 

 

 

Attached Thumbnails

  • data.jpg
  • lj_bench.jpg
  • lj_moat.jpg


#2 LabJack Support

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Posted 26 February 2014 - 09:59 AM

Do you see the issue if you just have the thermistor (and USB) connected and nothing else?

 

What kind of thermistor is it and how is it connected?

 

What are you using for range & resolution?



#3 Heckler511

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Posted 26 February 2014 - 03:25 PM

Do you see the issue if you just have the thermistor (and USB) connected and nothing else?

 

If I unplug the experimental unit from the wall main power, I don't see the issue.  However I do see the issue if the unit is just plugged in, even when nothing is powered on.  I also don't see the issue if I move the labjack from the experimental unit and put it on the cart (no matter what is connected / running).  

 

I'm running the labjack with a laptop that's sitting on a cart, next to the unit (although the unit has it's own control computer which shows the same results).  The only thing connected to the laptop is the labjack - and the thermistor is the only think connected to the labjack.

 

What kind of thermistor is it and how is it connected?

 

I have an Omega thermistor (http://www.omega.com...HSTH-44000.html5000 ohms resistance at 25 deg C) attached to the AIN0 and the GND of the Labjack.  I also have a jumper from the 200uA current to the AIN0.  I'm getting around 1.75 volts at AIN0 so I think there's about 8750 ohms resistance in the thermistor (running now at 12 deg C).

 

I've seen the same issue (even similar noise pattern) with another sensor I've tried to use.  Namely, a pH probe called a Durafet III pH probe with cap adapter cable.  

 

What are you using for range & resolution?

 

My range is set to 0-10 volts and all the other settings (resolution, settling) are default.  Perhaps I should play with those settings.  To be perfectly honest, I'm still trying to figure out what the resolution and settling values mean.  



#4 LabJack Support

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Posted 26 February 2014 - 03:58 PM

If you do a test with a fixed resistor of about the same value (~8.5k) do you see the same behavior?  Ideal would be a low tempco resistor like the Y1453 series mentioned in Section 2.5 of the U6 User's Guide:

 

http://labjack.com/s...users-guide/2.5

 

If you do a test with AIN0 jumpered to GND do you see similar behavior?

 

How about a test where you instead connect DAC0 (set to 1.75V) to AIN0?  Or a 1.5V battery to AIN0/GND if that is convenient.



#5 Heckler511

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Posted 28 February 2014 - 11:43 AM

Thanks Labjack - I'll implement these test asap and get back to you soon.





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