Sensing Moisture - Resistive vs Capacitive vs Other? - Understanding The Benefits and Issues

Discussion in 'General Electronics Chat' started by David Kelly, Jan 6, 2016.

  1. David Kelly

    Thread Starter New Member

    Jan 6, 2016
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    This is my first post, so please bare with me. I do not have much knowledge of how electronics work, but I have been reading multiple posts on this forum regarding moisture sensing, and on other sites.

    What I am trying to achieve is a stronger knowledge of how Resistive and Capacitive moisture sensing works. And also if there are other (better or worse) ways of doing it. I'd like to move this knowledge onto a project, but right now I'm just "fact finding". I look forward to whatever information you can provide.

    For arguments sake lets say I am trying to find the moisture level in soil since that is a very popular use for sensing moisture. Lets also assume that there is nothing else in the soil (minerals, etc) that will affect the outcome (that's an issue for a later time if I get there). Just soil and water.

    Here's what I know (or think I know):

    Resistive: Two probes are placed in the ground. Electricity is passed between the two probes. The more moist the soil, the less resistance, and the more electricity that is transferred. This is then read by the sensor.

    Capacitive: This is one I really get lost on. I've looked at examples, and read posts and projects on them, but I don't really understand how it works. Somehow the moisture will once again allow electricity to flow faster, or more freely?

    Others: Are there are options? Would a humidity sensor work? I've read that it wouldn't, but maybe it would?

    Hopefully this is a good start. I'll of course answer any questions or provide more info if needed.

    Thanks in advance for your help.

    Dave.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    "Moisture" is vague and a lot of things impact values you are sensing. For example you use soil because it is popular in discussions but if you are trying to translate that to tissue hydration, that will be completely different because bodily fluids have much higher dissolved salt content than surface water and, therefore, much more conductive than surface water. If you are looking at moisture in a baby diaper, the super-absorbent polymer, although solid, has ionic character and will conduct as a slurry/gel better than tap water.

    When you talk about capacitive sensing, there are multiple ways that circuit can be designed and then you need to discuss frequency, forming ionized cavities within the moist (or wet) pathways.

    I think it would be best to share more details or. If it is confidential, approach an expert with a secrecy agreement because asking here without details could be more mis-information
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
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    Soil contains metals, salts, air and just about anything else. This is why soil resistance and capacitance can vary greatly within a short distance, with or without a change in moisture.

    A picture from space might be more accurate than the sensors, unless you calibrate with core samples.

    However, I understand the microwave soil sensors can directly measure moisture density in the soil.

    I have no idea of the cost.
     
  4. David Kelly

    Thread Starter New Member

    Jan 6, 2016
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    First of all, thanks very much for the replies so far. Much appreciated.

    I'm really just trying to understand the theory behind moisture sensing, and how it works in the simplest sense. I realize that the results will vary greatly if I'm checking soil, or skin or anything else. But I'm assuming that the process would be the same. For instance I could take a resistive soil sensor and use it on skin (ouch) or a clothes, or anything else. I would get a result, but it wouldn't necessarily be correct and it would be very different from material to material. Right?

    But having said that, that is one issue I will keep in mind of course. But that's for later.

    Do these issues happen with Capacitive sensors as well? I'm assuming yes, but not 100% sure.

    And, I'm still not sure how Capacitive works. For instance in this device/link http://zerocharactersleft.blogspot.ca/2011/11/pcb-as-capacitive-soil-moisture-sensor.html how is this working? Is it a case of the electricity going along the board in the traces laid out, and when a "dielectric" (I learned that, lol) is introduced the electricity starts taking a short cut? I'm really not sure.

    Also, thanks for the mention of the microwave soil sensors. I've found some papers on it, probably not practical from what I've quickly skimmed, but I'll read up on it anyway.

    Thanks again!
     
  5. RichardO

    Well-Known Member

    May 4, 2013
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    The capacitive humidity sensors I played with a long time ago used a thin plastic sheet that changes thickness with changes in humidity. The surfaces of the plastic were metalized to make a very small value capacitor. When the thickness changes, the capacitance changes (basic physics). The capacitance change can be measured by using the sensor as the frequency determining element in an oscillator and measuring the oscillator's frequency.
     
  6. David Kelly

    Thread Starter New Member

    Jan 6, 2016
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    Thanks Richard. I haven't heard of anything like that yet. I'll have to look into that as well.

    So I've gotten a better understanding of resistive and capacitive sensors (I think). My continuation of this topic is:

    What one would provide more accurate results? Assuming that both sensors were put into identical soil, and water was added, and removed, which sensor would provide better results?

    Thanks!

    Dave.
     
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