Increasing sensitivity of a probe?

Discussion in 'The Projects Forum' started by Kstar., Jun 24, 2013.

  1. Kstar.

    Thread Starter New Member

    Jun 24, 2013
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    Hello AAC forum members, :)

    My electronic skills are cro-magnon in comparison to most, so I do apoligize if my questions seem a little blind.

    I am currently trying to adapt a soil moisture probe to science application:

    http://www.vegetronix.com/Products/VG400/

    Which also comes with a relay board:

    http://www.vegetronix.com/Products/VG-RELAY-DC/

    The moisture that I am using on this sensor does not have a medium though (no soil), it is being sprayed with atomised mist.

    I have connected this probe to a multimeter, but when the probe receives a shot of mist, the voltage only raises between 20-40mV.

    With a medium and water saturation, the range would usually indicate between 3V (saturated) and 0V (completely dry)

    Based on the parameters of the probe and relay board, how can I amplify this to give me a larger reading?

    explanations/diagrams/cheap circuit to buy welcome :)

    Thankyou in advance.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    You're attempting to use a soil moisture meter to measure....what exactly?

    Are you trying to detect the presence of the mist? That would be easier, I think, with an optical sensor, like an IR LED separated from an IR detector across a gap in which you are hoping to see mist.
     
  3. Kstar.

    Thread Starter New Member

    Jun 24, 2013
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    Im trying to measure the moisture level in a Basil plant root system.
    My theory, is that probe sits down into the Basil root system.
    When mist is projected into the chamber, it causes a saturation of say 100%
    As the roots them uptake the moisture, the moisture level drops, giving a lower reading.
    When the reading reaches it's lower threshold, it fires another burst and the saturation goes to 100% again, repeating cycle.

    Thanks Wayneh.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Here is another example of when it is best to state up front what you are hoping to accomplish.

    Forget about trying to increase the sensitivity of the probe.

    Cover the sensor with some kind of water absorbent material. I have used micropore tape in the past.

    Or replace the sensor with a relative humidity sensor.
     
  5. Kstar.

    Thread Starter New Member

    Jun 24, 2013
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    Thanks MrChips,

    I do apoligize, I should have elaborated a little more in my OP :rolleyes:
    I thought I may have given enough information, my bad.

    Yes, my original thought was to give the sensor some sort of medium.
    I thought of Micropore tape, but I thought that perhaps the adhesive would block or give off false readings.

    My second thought was to use a very porous foam like Oasis propagation foam and cut a thin slither that acted like a sheath on the probe giving a very fine medium for atomised air, but still better contact.

    But, I then again thought tha if the signal sensitivity could be boosted, then a medium would not be necessary.

    Failing the probe, a Absoloute Humidity sensor from a microwave might be my next idea?

    Thanks guys.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    You are missing the fact that the VG400 is not just a simple voltage output sensor. There may be some internal processing that we are not aware of. I think you have to rethink the whole idea of what it is that you want to measure, mist, moisture content, humidity etc. and select the appropriate sensor. You may be required to do some signal processing in order to get the type of action, i.e. trigger a solenoid.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    You need more thought around what exactly you mean by that.

    There's humidity in the air around the roots. If that's what you want to watch, use a humidity meter or a wet-bulb thermometer. Easy, but I suspect this is not really what you're after.

    Then there's "free" water - actual liquid water - that adheres to roots and such. I can't think of a clever way to really monitor that, but I suspect this IS what you want to monitor; no free water means it's time for a spritz. Maybe if you spritzed a surface - a mirror, say - at the same time the roots are spritzed, you could use the water on the surface as a proxy for water on the roots. I think you could measure the presence of water on a surface fairly easily. Your soil meter might work if you put some "soil" between the probes.

    Then there's absorbed moisture, the moisture absorbed into the roots. That's probably what the plant cares about. Other than weight, which would change daily, I'm not sure how you'd monitor this either.
     
  8. Kstar.

    Thread Starter New Member

    Jun 24, 2013
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    The advice given to me to use an OP amp amplifier... was from Vegetronix.
    Unfortunately, the couldnt help me any further on the matter.

    I want to maintain a set moisture level in the chamber.
    Just like maintaining a set moisture level in soil.
    But because this is atomized air, I need the reading to be as sensitive and responsive as possible.
    I can monitor this with my multimeter.
    Once I get the sensitivity I want, then I can move to the processor, then the relay control... one step at a time.

    If you think there would be a better sensor type for doing this, I would love your reccomendations =)

    Cheers!
     
  9. Kstar.

    Thread Starter New Member

    Jun 24, 2013
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    0
    Yes, this is what Im thinking wayneh ;)
    It's the 'free water' that attaches to the roots that I want to monitor.
    The water droplet size is between 30-50 micron, and is absorbed very quickly by the plant.
    At the current time, trial and error in on/off cycling is used to achieve the 'not to wet, not to dry' environment, but this changes with the plant life cycle, time of day/night etc.

    If I can get a sensor to sense the tiny little moisture droplets that attach to the very fine root hairs (get a reading), then I should be able to get a reading when they absorb into the plant, thus the sensitivity.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    No, no, no. "Moisture" in air only refers to water in the gas phase. The presence of liquid droplets in the air suggests that the moisture level is 100% of saturation and in near equilibrium with the liquid water cloud.

    It is true that an op-amp can amplify a small signal but that won't help you if you are measuring the wrong thing. If you want to proceed with your current sensor to see if it works, go ahead and find an op-amp. I recommend LM358 because it's very common and easy to find, and will sense all the way down to the negative power rail. In other words you can use it with a single supply like a battery and still look for low voltages like you are seeing.

    Another approach would be the LM393 quad comparator. It's a specialized op-amp that will give you an on-or-off output. This may be more handy for you to use for switching on the sprayer. You would place a small voltage, say 300mV, on one input and your sensor voltage on the other. If the sensor voltage exceeds the reference, the output will switch from one state to the other.

    You'll want to use hysteresis, so that there is a dead band with clean switching on both ends, like your furnace. It doesn't sit there switching on and off rapidly with tiny fluctuations.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    OK, so we need a sensor that acts as a proxy for a wet root. Maybe your sensor will work. See my comments about using a comparator - I think that's the way to go. FWIW, you can get the part at any Radio Shack.
     
  12. Kstar.

    Thread Starter New Member

    Jun 24, 2013
    6
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    Thanks Wayneh, you are of great help =)
    I will look into this again tomorrow (it's 3am here) and get back

    Cheers!
     
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