Author Topic: using capsense to measure soil moisture?  (Read 27335 times)

WhiteHare

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Re: using capsense to measure soil moisture?
« Reply #15 on: September 26, 2015, 08:46:18 PM »
I thought Irrometer was the name of the company that made the Watermark sensor that you're using:  http://www.irrometer.com/pdf/sensors/403%20Sensor%20%20Web5.pdf

You're right, though, they also make tensiometers.  Ultimately it's the water that's available to your plants, which is what a tensiometer measures, that matters. not just how much is in the soil.

TomWS

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Re: using capsense to measure soil moisture?
« Reply #16 on: September 26, 2015, 09:05:01 PM »
I thought Irrometer was the name of the company that made the Watermark sensor that you're using:  http://www.irrometer.com/pdf/sensors/403%20Sensor%20%20Web5.pdf
Irrometer is also the name of their tensiometer.  From the mfr: "The IRROMETER Tensiometer is the standard in accurate soil moisture measurement, offering growers an inexpensive and reliable means of measuring soil moisture for irrigation scheduling. An IRROMETER will measure the actual soil water tension, which indicates the effort required by root systems to extract water from the soil. Because the IRROMETER is a true measurement of soil water potential, the instrument is not affected by salinity and does not require site calibration. " and shouldn't be used to describe a granular matrix sensor.
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You're right, though, they also make tensiometers.  Ultimately it's the water that's available to your plants, which is what a tensiometer measures, that matters. not just how much is in the soil.
The question remains, how are you going to measure it and, more importantly, what are you going to do with the information?

Tom

joelucid

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Re: using capsense to measure soil moisture?
« Reply #17 on: September 27, 2015, 05:26:44 AM »
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Exactly so.  Just how precise a reading do you need?

I think practically speaking the threshold needs to be accurate enough that the plants grow well. If I don't get to my weekend house for a couple of months I would need to have the threshold stable enough so the plants don't die.

Have you ever had to recalibrate your threshold?

It would be a shame if all this could be done well with a commercially available sensor. I was really looking forward to abusing the rfm69w crystal to drive capacitative measurements  :)

This could be so simple yet so cool: combine a Tino with a sensor similar to http://zerocharactersleft.blogspot.de/2011/11/pcb-as-capacitive-soil-moisture-sensor.html on one tiny pcb. House the Tino part with the coin cell in a tiny enclosure and just stick these little sticks into the ground.
« Last Edit: September 27, 2015, 05:39:51 AM by joelucid »

WhiteHare

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Re: using capsense to measure soil moisture?
« Reply #18 on: September 27, 2015, 05:57:27 AM »
Have you ever had to recalibrate your threshold?
That's what I end up doing.  However, it requires monitoring and intervention by me, such that it's no longer a set-and-forget system.


This could be so simple yet so cool: combine a Tino with a sensor similar to http://zerocharactersleft.blogspot.de/2011/11/pcb-as-capacitive-soil-moisture-sensor.html on one tiny pcb. House the Tino part with the coin cell in a tiny enclosure and just stick these little sticks into the ground.

It would indeed be nice, especially since you're already making PCB's anyway.  You could standardize on that one design, and in situations where you don't need soil moisture sensing, you could just snap off the part of the board devoted to that.  :)
« Last Edit: September 27, 2015, 06:05:08 AM by WhiteHare »

joelucid

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Re: using capsense to measure soil moisture?
« Reply #19 on: September 27, 2015, 07:55:54 AM »
Hey - someone built pretty much exactly what I described: http://wemakethings.net/2012/09/26/capacitance_measurement/

Instead of the second low pass he uses a diode plus cap which should work just as well. Since he doesn't have the Swiss Army Knife he can only do 1mhz which is poor. Anyway he sells these sensors for 10 bucks on tindie - might be an alternative.

WhiteHare

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Re: using capsense to measure soil moisture?
« Reply #20 on: September 27, 2015, 01:44:58 PM »
Good find.  I didn't see a $10 version on Tindie, but in reading the comments on the $13 version, the June 24, 2015 comment says the a new version is "using 2 diodes to capture pos and neg wave part" but "with completely different R&C values".   Not sure yet what that's about.
« Last Edit: September 27, 2015, 03:15:41 PM by WhiteHare »

kobuki

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Re: using capsense to measure soil moisture?
« Reply #21 on: September 27, 2015, 01:47:12 PM »
Just a quick heads-up on the Tindie product: one of the users commented that the rod corroded in a very bad way and had to have it replaced. With a better coating it might be a nice probe anyway.

WhiteHare

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Re: using capsense to measure soil moisture?
« Reply #22 on: September 27, 2015, 06:34:55 PM »
I have a hunch that the invested workman hours, the design/refinement cycles, the creation and testing of multiple prototypes for endless days and nights isn't worth the price you'd actually pay in total for an established, tested and working product. You might not even save a cent. And at the end the result will be "good enough" for you only, and if all is well, it's going to return the investment only for you since it's not a marketable product. Of course it's fun and you learn a lot, but for example I think the Vegetronix is pretty good for the price. It doesn't need maintenance, it's unaffected by almost all factors the other sensors are plagued with. It's not cheap if you need 30 of them, though. But it's also a lot of work to interface to it, create your motes, make it ultra low power, base station software, adequate undergound shelter, yada yada.

I'm not sure why, but I seem wired to I resist that notion, even though in truth you're probably right.  Perhaps it's just a faint manifestation of the old saw that all progress depends on unreasonable men.


kobuki

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Re: using capsense to measure soil moisture?
« Reply #23 on: September 27, 2015, 06:41:50 PM »
Courageous, eager, forging the future. I get it. ;) But strictly practically speaking, if you buy a complete solution or one that's easy to lego together, the outcome might be better than a fully self-made solution, because of the factory-made, possibly durable components. Even then it's still a lot of work, but you learn a lot either way. If it's for fun, you should do it. If it's for profit, you don't have a choice anyway.

On to the subject: have you decided what kind of probe you're going to use? Capsense or otherwise?

WhiteHare

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Re: using capsense to measure soil moisture?
« Reply #24 on: September 27, 2015, 07:02:19 PM »
No, I don't think I can answer the question a priori.  I think the discrimination test may boil down to a simple experiment though: two identical glasses of water, except one distilled and one brine. Compare the probe readings on the two solutions.  Hopefully, within some as yet undefined tolerance, they are the same value.  If they measure  wildly different, then it's not a good circuit.

The same test could be applied to to pre-made sensors, so if I had those sensors already, I would try it.  For sure, I would try it before buying 30 of them.  ;)

The fringing approach (above) seems to lend itself more to a DIY solution, as it doesn't require making your own PCB or buying somebody else's PCB.  Besides, as you point out, PCB's can have an issue with moisture penetration into the board itself, effectively ruining it, if extra steps aren't taken to thoroughly moisture-proof it.  Perhaps Joe's or Chirp's method would work with a couple conductive rings, instead of PCB's. 

To your point, most of the people (at least the ones whose comments I've read) who own either the Vegetronix or the Watermark seem to like them.  I've already enumerated the most common objections to the watermark.  I'm not sure enough people actually own the Vegetronix to have heard whatever negative feedback might exist.  It has no forum, and comments are scattered.  In addition, Decagon makes moisture sensors that utilize TDR, which is supposedly even more  immune to salinity effects than capacitive moisture sensors, but it also costs about $100 a pop.  However, having one might be a worthwhile yardstick for calibrating expectations.  They sell the scientific community, so they have good documentation.  In contrast, Vegetronix claims to work "by magic."  That said, it can work by vodoo as far as I'm concerned if it's 1/3 the price, provided that it really works.  I'm honestly surprised there aren't better choices, given that this is a near universal problem to be solved.  I probably will get a tensiometer, as those are the gold standard, though it measures permitivity, not  volumetric water content like the others.

I have doubt that capsense will cope adequately with salinity, if only because it probably would already be in widespread use for that purpose if it were effective.
« Last Edit: September 27, 2015, 08:11:43 PM by WhiteHare »

kobuki

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Re: using capsense to measure soil moisture?
« Reply #25 on: September 27, 2015, 07:07:11 PM »
Forgive my ignorance, but what is the exact theory behind this experiment? Will it be able to accurately model the einvironment in the soil? I think this artificial test made with a single reference point might not produce useful results. But I'm not an expert at soil moisture sensing, this method might have a firm background.

WhiteHare

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Re: using capsense to measure soil moisture?
« Reply #26 on: September 27, 2015, 08:13:05 PM »
Admittedly,  I'm just muddling my way through.  If you have a better, yet pragmatic idea for testing, I'm all ears (so to speak).  :)

If you were to buy something that works "by magic", as vegetronix claims to, how would you test its sensitivity to salinity?  You could pack soil around it and add water, and then try to do it again identically, but with the same volume of salt water, and compare the results.  That would be more realistic.  I doubt the answer would be different, even if the measurements weren't the same, but maybe.  It's harder to create exactly identical conditions though, but maybe that could be adjusted for by averaging over several different trials.
« Last Edit: September 27, 2015, 08:35:24 PM by WhiteHare »

joelucid

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Re: using capsense to measure soil moisture?
« Reply #27 on: September 28, 2015, 03:00:55 AM »
Check out this paper for a graphic of the drastic dependency of signal attenuation in soil on soil conductivity as I had expected: http://www.cse.lehigh.edu/~cheng/papers/IEEE_Globecom_URPM_final.pdf

Unfortunately this just doesn't work.


http://www.usu.edu/soilphysics/scott/2008_unitauconf_jones_etal_mobilewatercontent.pdf has a nice graph showing you need at least 100mhz for capacitive measurement to reduce  the impact of the maxwell-Wagner effect.

So 32mhz sampling is also out of the question. Now the 433mhz signal of the radio itself would be ideal for measurement. Extra credit would accrue for a abusing the rfm69w in yet another way.

joelucid

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Re: using capsense to measure soil moisture?
« Reply #28 on: September 28, 2015, 03:24:36 AM »
Ok, concrete idea:

Take a Moteino and an additional rfm69w. Mount the rfm69w on top of the first one, connecting slave select to a different Moteino gpio line. Add a capacitive sensor between the antennas. Have the first radio emit a ook signal and measure rssi on the second. Modulo unexpected RF phenomena this might provide a highly stable moisture sensor which might well be significantly superior to commercial ones available at much higher prices. And it could fit on a Tino  :)

TomWS

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Re: using capsense to measure soil moisture?
« Reply #29 on: September 28, 2015, 07:46:08 AM »
Ok, concrete idea:

Take a Moteino and an additional rfm69w. Mount the rfm69w on top of the first one, connecting slave select to a different Moteino gpio line. Add a capacitive sensor between the antennas. Have the first radio emit a ook signal and measure rssi on the second. Modulo unexpected RF phenomena this might provide a highly stable moisture sensor which might well be significantly superior to commercial ones available at much higher prices. And it could fit on a Tino  :)
Not bad, but since you don't really need to decode the received signal - you just need amplitude, why not simply have an rf amplitude demodulator (AKA capacitor and diode, similar to Chirp's) feeding a DC signal proportional to signal strength into an Analog sense pin, OOK TX a low level 'whatever' MHz radio you've got, with TX driven into the ground probe, similar to the latest PCB probe design of the Chirp and you should have a fairly reliable, albeit desperately needing calibration, moisture probe.

In this case, the marginally tricky part is the RF switch.  Don't care too much about the probe side, but you want good isolation on the antenna side.  This should be relatively easy to do.

Tom


« Last Edit: September 28, 2015, 07:57:30 AM by TomWS »