I am trying to build resistive pulse counting system for research purpose. I noticed that a small variation in flow rate change the voltage drop across sense resistor. I did not understand the reason of this behavior. I attached a small video for demo.
Any explanation is appreciated,
thanks.
In this experiment you intend to measure the conductance/resistance of saline water and its variation when a particle is passing through the channel.
A correct measurement of conductance is made in alternative current, not direct current, to avoid electrolysis. You have a significant electrolysis effect, because you use a 9V DC source (too high, try below 1 V). Observe gas bubbles near electrodes (source of noise). The flow rate effect is explained by the electrolyte refreshing near the electrode surface (higher flow rate = higher electrolysis rate).
Try to modify a common conductimeter for this experiment.
I have seen your older post here:
https://www.researchgate.net/post/Getting_pulse_on_oscilloscope_by_resistive_pulse_sensing_method_I_am_trying_to_measure_the_pulses_directly_but_oscilloscope_measurement_is_so_noisy
Cornel Radu This experiment is not related to the post. Here I want to count micro particles.
The setup consists of two electrodes (Ag) and these two electrodes are separated by 800 micron hole. I put 100 micron polyproline (PP) bead in electrolyte solution (keeping these beads in suspension in a saline solution is another headache). I attached a sense resistor in a series with one of electrodes. The movement of PP beads through the hole displaced the electrolyte solution and decrease the resistance in my case, which changes the voltage in the sense resistor.
I am using 1V DC input for electrodes. The noise is because, I attached two DMMs and long ground leads for oscilloscope, removing these removes the noise. This video is to just give a visual view of the voltage drop. Noise is not my concern. For saline flow I am using a syringe pump and for keeping the saline solution in proper suspension I am using magnetic stirrer. I just noticed that the flow rate changes the voltage across the sense resistor which I COULD not understand.
I thought that the current change might heat the channel (hole).
Yes, a higher flow rate may better dissipate the heat in the small hole. The measured conductance depends on temperature.
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