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Measuring the dynamic electrical resistance response of plant leaves

  1. Jul 31, 2017 #1
    My hope is to explore [the existence of] dynamic correlation of plant tissue (leaves to start with) electrical resistance to stimuli. I've heretofore conducted experimentation reasonably sure enough of the techniques and materials I'll need that I've never posted for such advice in a public forum. So this is my first post to do so.:nb)

    I foresee a signal-to-noise challenge with the resistance measurements that I'd just as soon enlist help with. Has anyone had a problem getting good readings in light of this? Or is my concern too much ado about nothing?

    Any advice from experience as to the proper:
    - equipment?
    - materials?
    - technique?

    Do you have links to previous work of this type? How did they make electrical contact with the plant leaves? Do you just use the same contact pads as for an EEG or EKG? (Thank you, berkeman)

    Thank you!!
     
  2. jcsd
  3. Aug 1, 2017 #2

    mfb

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    As plants don't have proper contacts, contact resistance will be an issue. You can avoid this with 4-wire measurements, where nearly no current flows through the voltage measurement contacts (=> their resistance does not matter).

    Organic matter changes over time, both if the plant is still living and if the tissue is disconnected, in addition running electric current through the plant will influence it as well. You'll need a test sequence that can filter out this effect, e. g. before/after measurements with [whatever you want to test] in between. More measurements in longer sequences are better.

    The resistance can change during your measurement - you can explore this by measuring the resistance as function of measurement time, and then see how to deal with this.
    DC vs. AC is something else you can explore.
     
  4. Aug 1, 2017 #3

    berkeman

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    Good first post in the technical forums. :smile:

    BTW, I think you mentioned a DC measurement paradigm previously in your intro thread, and IMO you should be using an AC measurement. For contact with human tissue, if DC is used it causes an issue with ions at the interface that alters the impedance. Using AC excitation in your impedance measurement avoids this complication, and you can do a sharp bandpass filter around the excitation frequency to help increase your rejection of noise sources. It should be a smooth sine wave excitation, not a chopped AC excitation that I think you mentioned previously.
     
  5. Aug 1, 2017 #4
    All well and good. Keep in mind that the advantage of the 4-wire concept is realized with the higher voltage power supplies to provide low-noise constant-current source, ergo you can be sure I will monitor for the correlations of interest with what I've got now first: USB/Arduino 5 volt.
     
  6. Aug 1, 2017 #5
    I understand the need to alternate the polarity, but sine wave? For resistance one just obtains a reading and removes the excitation current. I am planning on keeping average excitation current at a minimum. Means apply it, let circuit stabilize, take the reading ASAP, remove the current immediately. Apply opposite offsetting polarity pulse, no reading taken. Do this as infrequently as you can get away with. Where does the sine wave become important in this scheme?

    Have to go and move my multi-DHT-using-PCInterrupts project along now.
     
  7. Aug 1, 2017 #6

    berkeman

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    When it stabilizes, you will have the wrong answer. The goal is to use a clean sine wave to keep the interface from polarizing (although with plants, who knows if this is an issue). The sine wave also lets you put a strong bandpass (BP) filter around the measurement frequency to help your SNR.

    Some papers for background:

    https://www.mn.uio.no/fysikk/english/research/projects/bioimpedance/publications/papers/SG_83a.pdf

    https://www.researchgate.net/publication/19986112_Skin_impedance_from_1_Hz_to_1_MHz

    https://www.mn.uio.no/fysikk/english/research/projects/bioimpedance/publications/papers/depth.pdf
     
  8. Aug 1, 2017 #7
    Ah, you've been thinking impedance while I've been thinking resistance. I'll do a double check in a bit to see if I need to be looking at impedance. If so, I am totally not prepared to go that route, and this project comes to a standstill very quickly.

    Thank you.
     
  9. Aug 1, 2017 #8

    jim mcnamara

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    Impedance = AC, resistance = DC.
     
  10. Aug 1, 2017 #9
    As near as I can discern, I should believe the resistance reading that my technique will measure.

    But at least I can now see where all these people have been misunderstanding me. I never said a word about impedance, but somehow impedance was in their experiences. The light goes on. Thank you for those links.

    RESISTANCE
     
  11. Aug 1, 2017 #10

    berkeman

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    Yeah, the galvanic issues mainly apply when measuring the impedance/resistance of tissue, and may not apply to leaves. I'd guess that the issue would not apply to wood, for example.

    The gold standard would be to compare the AC impedance with the DC resistance. But it sounds like you don't have that capability yet, so maybe just stick with a DVM resistance measurement for your first experiments with plants.

    Have you been able to find prior experiments of this kind in the literature?
     
  12. Aug 1, 2017 #11
    I don't mean to insult you or be mean to you, but with that go-around about resistance, I confess that I cannot trust your level of electronics expertise in this discussion. That said, please stay with me and learn, but I won't allow myself down another bunny trail following you relative to electronics. Please, no insult intended, we all lack knowledge in various areas of discipline and are bettering ourselves here in this forum. I do appreciate your honest effort.

    I confess I did get jaded from COMPLETE know-nothings-who-think-they-know-everything-bullies in other forums, so I ask your forgiveness if this offends you.

    I'm wrapped up in a different project, my car is needing me to replace the steering box and I've never done that, and my lawn needed mowing, so I've had to get behind in this research. Got a travel weekend planned with a questionable car, no end in sight....
     
  13. Aug 2, 2017 #12

    berkeman

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    Kind of a strange reply, but whatever. I have designed and built medical grade impedance measurement devices for my consulting clients (MDs), so I kind of know what I'm talking about when it comes to human medical devices and electrodes and impedance measurements.

    Plants, not so much. If it were me doing these experiments, I would start with the gold standard level of a measurement circuit, and then back off from there if I could convince myself that a simpler DC measurement setup and electrodes gave the same results for plants.
     
  14. Aug 3, 2017 #13
    Hoping to prevent further rabbit trails, I should restate two important details of my OP:
    1) I am speaking of RESISTANCE, not impedance,
    2) I ask for advice "from experience".

    I've already been jaded from the replies got in other forums; replies that were either totally speculative, or based on an assumed interchangeability of resistance and impedance concepts without even thinking to say so, or meant to challenge and detract from my goals. I hope you'll understand my past rush to defensiveness based on these experiences and please forgive me of it. Thank you!!!!!
     
    Last edited: Aug 3, 2017
  15. Aug 3, 2017 #14
    Regarding electrode contact, I am convincing myself that every doubling of the electrode contact area not only will improve SNR by at least quadruple, but will double the signal strength per se as well. I'm sure hoping next week will see me begin construction. Fully aware that measuring anything ALWAYS introduces undesirable effect on the measurement being make, I plan on prioritizing the electrode contact interface bio-compatibility - avoiding salt and metals likely to induce reaction themselves.
     
  16. Aug 3, 2017 #15

    mfb

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    Resistance is a special case of impedance for 0 Hz.

    You are unlikely to find someone with experience in exactly the measurement you plan to do in an internet forum. With the small number of people who did something similar, an expert on medical impedance measurements (berkeman) and someone who measured sheet resistances in (dead) carbon-based materials (myself) are surprisingly close to what you plan to do, I think.

    Back to topic: DC measurements might be unreliable and change if you measure the resistance repeatedly. AC measurements might avoid this problem. Test DC measurements, sure, but a backup plan if they don't work is a good idea.
     
  17. Aug 3, 2017 #16
    Agreed, and thank you for being civil to a fumbler such as me. I'm sure we'll never find me going the impedance route, just the same. That'd take resources that I don't have in so many ways I can't begin to name them.

    And by the way, if I find the correlation that others have, I'll be sharing it in the Quantum Physics forum, assuming there is one here. I'm getting pretty excited about it, hence all the comments. In a week or two, Lord willing.
     
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