pH measurement using a USB-6002

[AtlasCH]

Dear all,

I am trying to connect a Hamilton EasyFerm PLUS PHI S8 120 pH electrode to a USB-6002 DAQ device using a 1 m pH Cable S7 / Open End cable (also from Hamilton). The electrode is currently connected with the signal wire (+) to AI0 and the reference wire (-) to AIGND with the input being configured as RSE in LabVIEW. While I'm able to calibrate the electrode to give a proper pH measurement for the corresponding buffer solutions, the pH measured for one of my samples is far off the actual pH (pH 7.10 measured with this setup vs. pH 4.8 measured with a calibrated pH meter with a comparable electrode). Furthermore, very long settling times and signal drift can be observed.

Concerning the full setup, a laptop and a LabQ peristaltic pump are also connected to the DAQ device. Currently, the only signal conditioning done to the pH signal is averaging over 10s.

Would adding an op amp or unity gain buffer in line in between the pH electrode and DAQ device help? Or is using a electrode with a built-in pre-amplifier more recommended? I tried to orient myself according to this article:

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019MsvSAE&l=en-CH

[santo_13]

You need some signal conditioning in between the raw pH electrode and the DAQ.

Adding a unity gain amp (buffer) will help make the measurements better as the electrode output impedance is very high.

[AtlasCH]

Thanks for the quick response. Do you have any recommendations concerning components? I'm fairly inexperienced with electronics & circuits so any input will be very helpful.

[santo_13 · Accepted solution]

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@AtlasCH wrote:

Thanks for the quick response. Do you have any recommendations concerning components? I'm fairly inexperienced with electronics & circuits so any input will be very helpful.

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In that case, you can get something like this https://bc-robotics.com/shop/ph-sensor-amplifier/

https://www.amazon.com/Sensor-Electrode-Module-Arduino-Detection/dp/B0DYDBSQNV?gQT=1

If you buy this, you can directly connect it over USB and collect data in LabVIEW.

[AtlasCH]

The pH sensor amplifier/module seems like an optimal solution to my problem. I can get the full kit with the sensor to test the viability and then get a S7/BNC cable for my current electrode if it is a viable setup. It also only requires 5 V DC which can be supplied by the USB 6002 itself so the setup will still remain compact. I will definitely try this approach.

Thank you so much for the input, this helped me out a lot.

[AtlasCH]

Quick update: I got a pH sensor amplifier (nearly) identical to the one you linked and did some tests today. Calibration with pH 4.01 and pH 7.00 was quick and smooth and also the check at pH 6.0 was fine but once again, as soon as i dip the electrode into the sample solution (containing protein) the pH reading seems to spike and take forever to settle while also oscillating a significant amount:

I connected my pH electrode using a S7 to BNC cable to the amplifier and connected the AO and AOGND pins of the amplifier with the AI0 and AIGND channel of the USB 6002 respectively using jumper cables. For power supply, I connected the V+ and GND pins of the amplifier with the 5V and DGND channels of the USB 6002 respectively.

I redid the measurements after restarting the LabVIEW application and started with the sample and re-checked the calibration buffers in descending order of pH (7.00 -> 6.00 -> 4.01) before dipping the electrode back into the sample. The measurements seem to stabilize quickly at the correct pH level now but oscillations are still visible albeit less intense as before:

What could be the cause of this issue and is there a quick and easy way to fix it?

[AtlasCH]

I managed to identify the issue: I was using a second pH electrode connected to a pH meter to check the accuracy of my setup and it seems like the second electrode was the source of the interferance as the measurements afterwards were stable and accurate.