I'd like to use some moteino's for measuring soil moisture. I've been using conductive soil moisture sensors for over a year now, and the calibration drifts a lot. For instance, fertilizing throws it off. I want to upgrade to a sensor which isn't affected that way.
The two ways I'm considering are:
1. Bury two moteino's say, half a meter apart. Have one transmit and the other measure the RSSI. It's known that water will attenuate the RF signal. So, the more soil moisture, the lower the RSSI. I was previously thinking 2.4Ghz might be better, but it was proven to also work at 900Mhz: http://ir.uiowa.edu/cgi/viewcontent.cgi?article=2048&context=etd
2. Use Arduino's capsense library to measure soil capacitance. This would avoid extra hardware. Not sure how effective it would be though. Anyone tried it as a means of measuring soil moisture? I've read that some methods of measuring capacitance are skewed by soil conductivity, but that those taken by some method at 80Mhz or greater manages to avoid that problem (c.f. comments at: http://zerocharactersleft.blogspot.com/2011/11/pcb-as-capacitive-soil-moisture-sensor.html)
Before embarking on either one, I thought I'd give a shout out to see if anyone else may have already tried or is using a method which isn't affected by soil conductivity. Anyone?
Sorry for sounding like an advocate, but I'm wondering if you'd be better off with a professional solution such as this (http://www.davisnet.com/weather/products/weather_product.asp?pnum=06345). Its radio signal and frame format is well understood and can be received with Moteinos, if that's what you want, instead of using the factory receiver.
OTOH, I don't understand why you want to bury the motes, when you can just cable-hook them to the surface, on a pole, whatever. Something like this project (http://lowpowerlab.com/blog/2015/06/14/moteino-compost-sensor-network-adafruit-fona-gprs-relay/).
Quote from: kobuki on September 25, 2015, 04:09:00 PM
Sorry for sounding like an advocate, but I'm wondering if you'd be better off with a professional solution such as this (http://www.davisnet.com/weather/products/weather_product.asp?pnum=06345). Its radio signal and frame format is well understood and can be received with Moteinos, if that's what you want, instead of using the factory receiver.
OTOH, I don't understand why you want to bury the motes, when you can just cable-hook them to the surface, on a pole, whatever. Something like this project (http://lowpowerlab.com/blog/2015/06/14/moteino-compost-sensor-network-adafruit-fona-gprs-relay/).
Huh? I'm totally not seeing the connection between that and what I posted.
I'm sorry about that, but then let's overview it, step by step.
1. You're looking for an accurate soil moisture sensor solution, is that right? The Davis equipment I linked is a soil moisture sensor station with 4 soil and 4 temperature probes. (Disclaimer: I'm not associated with them in any way, I'm just a happy user.)
2. The awesome compost project I linked shows a pretty clever use of Moteinos in a similar environment, where sensors are "underground" (in the compost being processed), and transmitters/motes are above ground, on top of those poles.
Are you still not seeing the connection? I'll gladly help you understand it, should that be the case. Am I misunderstanding anything?
Quote from: kobuki on September 25, 2015, 04:25:06 PM
I'm sorry about that, but then let's overview it, step by step.
1. You're looking for an accurate soil moisture sensor solution, is that right? The Davis equipment I linked is a soil moisture sensor station with 4 soil and 4 temperature probes. (Disclaimer: I'm not associated with them in any way, I'm just a happy user.)
I can't say that I've used their product, but I've read about it. Davis uses Irrometer's granular matrix soil moisture sensors. Those come with some caveats:
1. temperature sensitivity
2. Limited lifespan of the sensor. It contains gypsum and possibly other dis-solvable minerals. Once they dissolve away, the buffering is gone, and they'll be vulnerable to changes in soil salinity, just like other EC sensors. They do manage to slow that down with their packaging, but it's still gonna happen.
3. Probably because of #2 directly above, Irrometer says they should be dug up each year and replanted in order to ensure good sensor-to-soil contact. I'd really rather not do that, as it's extra work.
4. Not cheap. About $50 a pop. I think for less than that I would get a vegetronix soil moisture sensor (about $35), which should last indefinitely and not require replanting. The sensor costs start to get in the way of how many I might deploy. So, if there were a way to leverage the arduino to reduce costs, perhaps by using the capsense library, then that would be great.
Quote from: kobuki on September 25, 2015, 04:25:06 PM
2. The awesome compost project I linked shows a pretty clever use of Moteinos in a similar environment, where sensors are "underground" (in the compost being processed), and transmitters/motes are above ground, on top of those poles.
As these are mostly going to be checking the moisture on a different sections of a lawn, having them underground is a bonus: nothing for a mower to hit or people to trip over. Also, the temperature swings won't be as big, which might help with battery life. I'm not too worried about receiving the signals. I have some buried 802.15.4 sensors that measure soil conductance for over a year, and I'm not having any trouble receiving their signals.
Thanks for the write-up. Now we know a lot more about your goals. That will probably help in finding a better solution. However from what you summed up, on the long term you might benefit from a probe that isn't very cheap but lasts long, even it it adds up to a bigger sum. It's a one-time investment at the beginning. I think an Arduino-based controller would lend itself just fine for any of them. BTW, how many probes do you need?
At least 10, but I'd prefer a much larger number, like 30, for redundancy in case of failure and also for better coverage within each of the 10 irrigation zones.
I need to build an irrigation system but haven't started yet. I've thought about it a little. Using RSSI measurements is definitely a cool approach, but doesn't work with coin cells unless you have a RTC (show stopper :-) ). Oh wait with the burst/listen mode you could actually do it with coin cells. However I think it's likely the method will be impacted by soil salinity just as much as the resistive method.
Capacitative sounds better - but you would need really high frequencies to drastically reduce the impact of changes in salinity. Which means extra chips, power consumption and effort.
What's the best we can do with a Moteino? The radio has a clock of 32mhz which can be routed to one of the pins. Feed that into a simple RC low pass filter (the C being the sensor). Feed the output into another low pass that only filters out the DC component and measure that with the ADC. 32mhz is probably good enough. And 2 resistors and 1 cap qualify as simple in my world.
Joe
Would this work? Take an 80Mhz crystal oscillator (http://www.ebay.com/itm/1pc-80-000MHz-80MHz-FULL-SIZE-CRYSTAL-OSCILLATORS-KCO-110S-DIP4-/181552356877?hash=item2a455d1e0d), connect it directly to the soil capacitance probe, and then use the arduino input pin to measure the maximum voltage developed across it? Presumably, the higher the capacitance, the lower the maximum voltage.
Alternately, this paper leverages an mcu to suggest a way using lower frequencies that doesn't look too bad: https://eesensors.com/media/wysiwyg/docs-pdfs/ESP06_moistP.pdf
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 wonder if you've asked yourself the most basic questions: what accuracy you need and what precision. Do you only need a threshold indication, or you somehow want to analyse the quality of your soil over time, etc. For a lawn you might be fine with a simple threshold and some temperature sensors as auxiliary metrics or temperature compensation. Testing for a threshold is quite simple if you use common sense, your own experience on the field and a good reference meter.
OTOH, just on the net there are countless DIY solutions of the same problem. One of my older project ideas is to automate pot flower watering in our home, since we have quite a few pots. I'll make gypsum sensors for that, since they're very simple and dirt cheap and fit my purposes well. I don't need precision there. All I need is a small transmitter to send soil moisture metrics to my base, so I'm holding off until Joe makes his Tinos available ;)
QuoteWould this work?
I can't find a datasheet for this part. I wouldn't add a component just to increase the frequency from 32 to 80 MHz. In any case there needs to be a resistor before the probe. Without that the probe capacitor would charge in no time.
And then at 32mhz measuring the maximum voltage with a 16mhz 328p is not really possible. That's why I suggested to add another resistor and capacitor behind the probe to measure the dc component of the signal.
QuoteI wonder if you've asked yourself the most basic questions
Well to be fair whitehare already provided the critical info: he wants to bury 30 of these with no intention of ever digging them up again. Sounds sensible to me. And it seems neither gypsum nor resistive techniques are applicable in that case.
I'd be curious to know which approach Tom has taken with his moisture sensors.
Of course a threshold is what you're after. But that threshold needs to work regardless of ongoing changes in the environment. Irrigation itself for example is known to change salt levels over time. If one could assume constant salinity over the lifetime of the sensor things would obviously be simpler. Just not sure if that assumption is reasonable.
Quote from: joelucid on September 26, 2015, 06:25:53 PM
Well to be fair whitehare already provided the critical info
Quote from: kobuki on September 26, 2015, 04:25:01 PM
I wonder if you've asked yourself the most basic questions: what accuracy you need and what precision. Do you only need a threshold indication...
No, he hasn't. The basic parameters of the probe to achieve his set goal is very important when choosing the appropriate one. But yes, I agree, the criteria you mentioned are just as important. He needs something that's rugged and running rock solid for many years,
underground. His project is huge. The sensor is only a tiny part of it, albeit one of the most important ones.
Reflecting to your latest edit: Vegetronix claims to be devoid of these problems with his probes. For a similar task, I'd prolly end up buying them.
Quote from: joelucid on September 26, 2015, 06:25:53 PM
If one could assume constant salinity over the lifetime of the sensor things would obviously be simpler. Just not sure if that assumption is reasonable.
I can say with certainty that it would
not a reasonable assumption. If it were, then simple resistive soil probes would work well enough. Unfortunately, most non-organic NPK fertilizers behave like salts and will skew the measurements enough that you have to re-calibrate
I honestly don't know whether organic fertilizers behave that way or not, but I'm assuming once they're broken down by soil micro-organisms they probably produce a similar effect.
In addition, as you point out, irrigation and/or rain might be adding salts or leeching them away.
Quote from: joelucid on September 26, 2015, 06:25:53 PM
QuoteI wonder if you've asked yourself the most basic questions
Well to be fair whitehare already provided the critical info: he wants to bury 30 of these with no intention of ever digging them up again. Sounds sensible to me. And it seems neither gypsum nor resistive techniques are applicable in that case.
I'd be curious to know which approach Tom has taken with his moisture sensors.
I currently use Watermark sensors which, as far as I've seen, haven't been discussed in this thread. These are not gypsum blocks, at least not entirely. According to the mfr, "The sensor consists of a pair of highly corrosion resistant electrodes that are imbedded within a granular matrix. A current is applied to the WATERMARK to obtain a resistance value. The WATERMARK Meter or Monitor correlates the resistance to centibars (cb) or kilopascals (kPa) of soil water tension.
The WATERMARK is designed to be a permanent sensor, placed in the soil to be monitored and "read" as often as necessary with a portable or stationary device. Internally installed gypsum provides some buffering for the effect of salinity levels normally found in irrigated agricultural crops and landscapes."
I have used these for years and have had them buried in the yard for years with the only maintenance required when they are mechanically disturbed (like a person or mole digging around it). I have cabled these as far as twenty feet from my telemetry equipment and couple a temperature sensor to the Watermark probe to handle temperature compensation in the telemetry equipment.
Are they accurate? Who knows? I've never calibrated them to a kPa device. However, I can look at the lawn and correlate a dry lawn with a 'dry' reading and vice versa. As long as they are repeatable, then I'm happy. Which brings us to:
Quote
Of course a threshold is what you're after. But that threshold needs to work regardless of ongoing changes in the environment.
Exactly so. Just how precise a reading do you need? You want to know if the root system is getting 'enough' water. To my mind, any precision beyond this is a pointless fallacy. How many kPa do you need? I suppose a farmer investing in hundreds of acres of a crop that has a specific target specified would care - I don't have that kind of information for any of the plants I'm trying to keep alive.
Quote
Irrigation itself for example is known to change salt levels over time. If one could assume constant salinity over the lifetime of the sensor things would obviously be simpler. Just not sure if that assumption is reasonable.
From what I've read, the main variant to salinity levels would be due to fertilizer and you can make adjustments during the brief period that those are permeating the soil. In theory I should have data that shows this behavior, but I don't recall specifically noticing the effect (maybe I didn't fertilize enough ;)
Finally, I've seen reference to 'Irrometers' as 'gypsum blocks'. Not so. An 'Irrometer' is a brand of tensiometer that actually measures the 'pull' of water into the soil. They are very accurate, expensive (by residential standards), and need to be removed before soil reaches freezing conditions. Watermark probes can tolerate frozen soil.
Tom
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.
Quote from: WhiteHare 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
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.
Quote
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
QuoteExactly 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.
Quote from: joelucid on September 27, 2015, 05:26:44 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.
Quote from: joelucid on September 27, 2015, 05:26:44 AM
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. :)
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.
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.
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.
Quote from: kobuki on September 26, 2015, 04:25:01 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.
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?
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.
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.
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.
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.
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 :)
Quote from: joelucid 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 :)
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
Quote from: WhiteHare on September 27, 2015, 08:13:05 PM
If you were to buy something that works "by magic", as vegetronix claims to, how would you test its sensitivity to salinity?
Their "magic" is based on transmission line techniques, which is an important part of radio technology. They claim:
Quote
Because our probe measures the dielectric constant of the soil using transmission line techniques, it is insensitive to water salinity, and will not corrode over time as does conductivity based probes. Our probes are small, rugged, and low power.
So either you believe their claims or make a series of repeatable tests. I think I'd buy a few, plant them, hook them up in my monitoring system any way I can at the moment, and then watch what's the situation in various moisture levels. I'd try to find the correlation point with the watering needs, or in other words, I'd try to tune the threshold for turning on the irrigation system. Is your land prone to salinity problems? If not, or minimally, you might want to skip this test.
@Joe and @Tom: Sounds promising! How far apart do you reckon the two antennas should be?
Also, not sure if this applies or not to an RFM69x radio, but would the Tx radio have to be carefully regulated to a constant repeatable voltage for it to work? I know that with cheap 433Mhz radios, Tx power is purely a function of supplied voltage.
OTOH, if there were an accurate formula for how Tx power varied depending on the supplied battery voltage, maybe that could be adjusted for.
Quote from: TomWS on September 28, 2015, 07:46:08 AM
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
Yup - even simpler.
QuoteSounds promising! How far apart do you reckon the two antennas should be?
It wouldn't be two antennas. Just feed the antenna signal into the probe and rectify and measure the resulting signal.
QuoteTx radio have to be carefully regulated to a constant repeatable voltage for it to work?
You'd want to measure the resulting signal relative to the unmodified signal. If the signal generated by the radio isn't constant across voltages you could short out the sensor with a mosfet and measure the unmodified signal that way.
Quoteshort out the sensor with a mosfet
Or given the RF nature here I don't know how one would do that - if a mosfet would work. Probably the same problem as the antenna switch. Tom, HELP ;)
Quote from: joelucid on September 28, 2015, 01:07:56 PM
Quoteshort out the sensor with a mosfet
Or given the RF nature here I don't know how one would do that - if a mosfet would work. Probably the same problem as the antenna switch. Tom, HELP ;)
I wouldn't expect a radio TX circuit output level to vary an amount that was significant enough to matter (of course this assumes that the load is fairly constant, as radio circuits tend to expect ;)
In this case, I wouldn't expect to need to make a 'reference' measurement, but I can experiment with that once I'm back on the air - I'm tied up for the next week.
I did find some nifty little antenna switches (http://www.rfmd.com/store/rf1200.html) The value of these is few components => small size and they aren't too expensive ~$0.85 IIRC. However, we can probably get away with a diode, transistor, and a couple of discrete components and accomplish what needs to be done.
Quote from: WhiteHare on September 28, 2015, 02:08:28 PM
This is probably an ignorant question, but in that case, since it's an RF signal feeding the capacitance probe (antenna wired directly to the probe through an antenna switch, yet to be defined), is it of any consequence that the source signal is no longer a square wave?
Nope.
Tom
QuoteIn this case, I wouldn't expect to need to make a 'reference' measurement
I hope you're right. The challenging scenario will be coin cells where voltage might differ between nodes by as much as 30%. Of course we can measure vcc. So if we do find a voltage impact we can just measure it once and then infer the right adjustment factor from our vcc measurement.
Have a safe trip and see you back in the air soon!
Quote from: joelucid on September 29, 2015, 08:43:44 AM
QuoteIn this case, I wouldn't expect to need to make a 'reference' measurement
I hope you're right. The challenging scenario will be coin cells where voltage might differ between nodes by as much as 30%. Of course we can measure vcc. So if we do find a voltage impact we can just measure it once and then infer the right adjustment factor from our vcc measurement.
Have a safe trip and see you back in the air soon!
It would actually be best if the measured voltage is proportional to VCC since the ADC is ratiometric, everything will track. However, if the result is constant (due to constant TX output level), then using the adjustment is relatively straightforward process since, presumably, you've already measured VCC to make sure you CAN measure and report...
Tom
(Traveling next two days, it's just everything is packed now :(
He's been travelling back to another residence and just got settled, he's been less active but should be back soon :)
Tom you are missed! :P
Have you looked at this from Rain Bird?
http://www.rainbird.com/landscape/products/accessories/smrty.htm (http://www.rainbird.com/landscape/products/accessories/smrty.htm)
Quote from: ssmall on November 12, 2015, 11:35:39 AM
Have you looked at this from Rain Bird?
http://www.rainbird.com/landscape/products/accessories/smrty.htm (http://www.rainbird.com/landscape/products/accessories/smrty.htm)
If cost is no obstacle, I imagine it's pretty good.
I woudl recommend that you look at this sensor:
https://www.tindie.com/products/miceuz/i2c-soil-moisture-sensor/?pt=ac_prod_search
It is i2c and works very well. Be sure to get a well shielded cable that is low capacitance. The guy that built it is open to feedback. in fact he modified the design for me on multiple occasions such as to add the 2 holes near the top to allow securing the cable by the shielding. and you can order it from Tindie or download the kicd files and build it yourself.
There is also a new Library for it if you don't want to do the i2c commands directly: https://github.com/Apollon77/I2CSoilMoistureSensor
FYI - he is the same guy that build "Chirp" on adafruit.
Quote from: jarrods on November 23, 2015, 02:21:14 AM
I woudl recommend that you look at this sensor:
https://www.tindie.com/products/miceuz/i2c-soil-moisture-sensor/?pt=ac_prod_search
It is i2c and works very well. Be sure to get a well shielded cable that is low capacitance. The guy that built it is open to feedback. in fact he modified the design for me on multiple occasions such as to add the 2 holes near the top to allow securing the cable by the shielding. and you can order it from Tindie or download the kicd files and build it yourself.
There is also a new Library for it if you don't want to do the i2c commands directly: https://github.com/Apollon77/I2CSoilMoistureSensor
FYI - he is the same guy that build "Chirp" on adafruit.
Thanks for the recommendation! I was wondering when he was going to make a sensor only version. Good price and it seems well made. Too bad it's I2C interface, however, I struggle with sending those signals 20 feet underground. How long and what type of cable have you used with it?
I've deployed a Vegetronix sensor, placing it side by side with one of my Watermarks to correlate them. Unfortunately, I've discovered that if you want to keep your landscaping well watered, all you have to do is set up a new sensor for testing. I deployed it about 4 weeks ago and it's been wet ever since! I think I'll get some of these to try.
Tom
Would be interesting to see how well these perform given the low frequency (I think he uses 1mhz?). At such low frequencies you should get large salinity effects on measurements again. One day I'll have to try the 433mhz idea. Next summer ...
Joe
I would have to look at his code again but he is running at 8mhz.
As for the i2c and wire. It has been hit and miss but I am going to be testing http://products.conwire.com/item/product-catalog/ow-cap-foil-braid-shield-ul-cm-ul-style-2919-cable/5922-cl it looks very promising. I got good results from cat6e but the airspace in the shielding allowed water in even after best efforts to seal it :(
This consolidated wire cable is only 12.5pf per foot and is uv resistant. The only bummer is that you have to order 1000 feet to get the 4 and 6 conductor.
BTW there is a version of his sensor in github that uses rs485 instead of i2cand he has a spreadsheet with the bom as well as the Gerber files. I didn't use it cause rs485 has a constant power draw and requires chips that are almost expensive as the processors.
My goal is to build my own (I have the PCB done and flowed still working on software) that will use a modified version of SDI-12 that will work at 3v. That way you could have of to 63 devices on a bus and up to 200 feet of cable. Once that is working i am going to figure out how to include LVDS (low voltage differential signaling) with my modified sdi12 then I should be able to get up to 1km of cable just like rs485.
This way from a single Node I could support 60 + sensors per bus spread across rows of trees and vines.
Quote from: jarrods on November 25, 2015, 02:13:02 AM
I would have to look at his code again but he is running at 8mhz.
As for the i2c and wire. It has been hit and miss but I am going to be testing http://products.conwire.com/item/product-catalog/ow-cap-foil-braid-shield-ul-cm-ul-style-2919-cable/5922-cl it looks very promising. I got good results from cat6e but the airspace in the shielding allowed water in even after best efforts to seal it :(
This consolidated wire cable is only 12.5pf per foot and is uv resistant. The only bummer is that you have to order 1000 feet to get the 4 and 6 conductor.
BTW there is a version of his sensor in github that uses rs485 instead of i2cand he has a spreadsheet with the bom as well as the Gerber files. I didn't use it cause rs485 has a constant power draw and requires chips that are almost expensive as the processors.
My goal is to build my own (I have the PCB done and flowed still working on software) that will use a modified version of SDI-12 that will work at 3v. That way you could have of to 63 devices on a bus and up to 200 feet of cable. Once that is working i am going to figure out how to include LVDS (low voltage differential signaling) with my modified sdi12 then I should be able to get up to 1km of cable just like rs485.
This way from a single Node I could support 60 + sensors per bus spread across rows of trees and vines.
It sounds good. Let us know when you complete your testing. Will you sell the probes or, at least, release the design?
BTW, you might look into outside burial cat6. It's messy as all getout (it's filled with silicone grease) but there is NO water problem with it. Very heavy outside jacket too to handle abuse from rock and such - this, of course, is a double edged sword, it is not very flexible. In my case I'd want to fully bury the entire probe (wire transmitter as well as sensor).
Tom
I might... But to be honest miceuz's hardware design is much better at this point. His latest rev is really good and my PCB skills are quite basic compared to yours and his :)
I have been looking at his board and just seeing if I can reprogram it to communicate the way I need. If i could do that i would release that is it would be comparable to comercial units out there. However this is a low priority at this point.
The only thing that is better with my design is that I am using the attiny841 which is an awesome chip. It functions much like it's bigger brother (atmega) and I want to use it to build a Moteino "Mini" much like your and Joes. The fuses and programming was a real pain to figure out but now that I have you can program it the same way you do a regular chip except for automatic upload via the arduino IDE. Don't quite have that working but I will eventually.
The atmega is overkill for just about everything I work on as I rarely need more the a few pins on each node. And since I am trying to deploy a LOT of nodes, building a cheaper smaller node is something I will have to eventually do.
If your interested I can write up getting that chip working abd some sample code to see if you guys want to use it to build a node with it ;)
http://www.atmel.com/Images/Atmel-8495-8-bit-AVR-Microcontrollers-ATtiny441-ATtiny841_Datasheet.pdf
What PCB design tool did you use to make your own prototype? We can probably work with that. I like the low pin count of that processor but I'm too wasteful of program/data memory to be able to use it. Like Kobuki-san, I want more memory! OTOH, Joe could probably install a RESTful client on it and have it posting to his Google cloud account... and, of course, running on harvested energy with no battery forever.
Tom
I read about homemade gypsum soil moisture sensor as part of the Vinduino (a
vineyard irritation system) project, which won a Hackaday prize -
https://hackaday.io/project/6444-vinduino-a-wine-growers-water-saving-project
More details about the soil moisture sensors -
http://vanderleevineyard.com/1/post/2012/11/gypsum-sensor-calibration.html
http://vanderleevineyard.com/1/post/2013/12/gypsum-sensor-casting.html
Quote from: KanyonKris on December 01, 2015, 07:49:37 PM
I read about homemade gypsum soil moisture sensor as part of the Vinduino (a
vineyard irritation system) project, which won a Hackaday prize -
https://hackaday.io/project/6444-vinduino-a-wine-growers-water-saving-project
More details about the soil moisture sensors -
http://vanderleevineyard.com/1/post/2012/11/gypsum-sensor-calibration.html
http://vanderleevineyard.com/1/post/2013/12/gypsum-sensor-casting.html
Cool, thanks! I really like the one referenced from that blog:
http://unpuntilloalambre.blogspot.com.es/2014/01/gypsum-block-for-soil-moisture-sensor.html
Now that is creative (and very simple). This could be inexpensive enough to be replaceable after a couple of years. In fact, the plastic parts could be reusable if you're careful taking the old one apart.
Tom
Tom,
I used Eagle to design the board.
As for the Gypsom block sensor, I strong recommend against that.
- Gypsom is Hrydroscopic, meaning it holds onto water and so it can show a much higher value then is really present in the soil. depends greatly on soil type
- Corrosion will cause it to loose resolution and eventually fall apart. In some cases I have seen them fall apart in 1 season with acidic soils
- Salinity affect the readings greatly. This means if salt forms (adding soil amendments to balance PH) then the reading will change
Honestly the best sensor for the DIY market at this point is the Capacitive. They are non-reactive and so if sealed properly can be in the soil for a very long time (5-10 years). Dont forget to take in consideration installation time. If you have to replace then every 1-2 years then that means having to dig new holes. Not sure what labor costs are near you but in California it is insane (15 USD an hour in some places). So it costs more to install the sensors then it does for the hardware. So I have to make sure the sensors will last more then 5 years for ROI and in some cases I will install an extra one in the hole in case there are issues with the others.
BTW - I started with this sensor: https://oshpark.com/projects/b9RIJ6S9 (https://oshpark.com/projects/b9RIJ6S9) it works pretty well as long as you keep the leads short and use low capacitance wire (CAT 5E for example).
Quote from: jarrods on December 02, 2015, 10:43:42 PM
<snip>They are non-reactive and so if sealed properly can be in the soil for a very long time (5-10 years). <snip>
Really? Which one is that? I've not seen a single electronic sensor last anywhere near that long.
I've had Watermarks in the ground for 5 years and, other than some tweaks to the sensitivity, they've worked reliably. However, Watermarks aren't Gypsum, but are a similar, more durable technology. Gypsum is supposed to block salinity, at least for the first few years of their life, but Salinity isn't an issue where I live in any case. I'm looking for something that really won't corrode and will last for 5 years or, at least, not cost $50 plus labor to replace.
Tom
PS: Don't mind my grumpiness. I'm PO'd at discovering its been my stupid Roku 3 that's been killing my WiFi performance... but that's another story. Stupid ^%$^%#@# POS!
PPS: Thanks for sharing the design, I appreciate it!
I think the point about salinity is more general than salinity in the water supply: gypsum block sensors work by measuring the resistance between the electrodes. The resistance is a function of not only how much water is present to support the passage of electrons, but also how many other ions are present in that water. Ions come from all kinds of dissolved salts, i.e. salinity from impure water, and components of fertilizer added deliberately. So the resistance vs moisture curve varies every time you fertilize. The gypsum block reduces but does not eliminate the effect of dissolved salts on the calibration.
Capacitive sensors avoid this problem.
Quote from: TomWS on December 02, 2015, 11:47:37 PM
Quote from: jarrods on December 02, 2015, 10:43:42 PM
<snip>They are non-reactive and so if sealed properly can be in the soil for a very long time (5-10 years). <snip>
Really? Which one is that? I've not seen a single electronic sensor last anywhere near that long.
Tom,
We should chat offline. This is a key part of my business as I have a way of sealing my sensors. They are still in testing but so far I have 1 year on early models. I am working on a install for January that will use the 3rd version of my setup and hopefully will last 5+ years (except for battery)
Quote from: TomWS on December 02, 2015, 11:47:37 PM
Tom
PS: Don't mind my grumpiness. I'm PO'd at discovering its been my stupid Roku 3 that's been killing my WiFi performance... but that's another story. Stupid ^%$^%#@# POS!
Would you mind sharing what you found in a separate thread? I have a Roku and have been fighting performance issues but never though the roku might be involved
-Jarrod
QuoteOOK 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.
Based on reading a couple of papers I'm starting to doubt that we can drive 430Mhz or even 900Mhz into the sensor and get good results. It would be optimal to use such a high frequency because it almost completely does away with salinity or soil composition effects. But at those frequencies the sensor is electrically very long and its inductance will play all kinds of tricks on us (like interference with returning signals etc).
Instead one could still use the 32mhz oscillator in the radio - with some salinity effects remaining.
This strikes me as a really smart way to measure soil moisture: https://www.google.com/patents/US7135871
ABSTRACT
The moisture content of soil may be determined using a swept-frequency microwave-based process and device. The process includes the steps of: producing a primary microwave signal with a varying frequency, splitting the primary signal to provide first and second microwave signals, which first signal is transmitted through an electric conductor in the soil where it will be delayed in proportion to the dielectric constant of the soil, while the second signal provides an internal reference signal, receiving a third signal which includes the first signal after it has passed through the electrical conductor, mixing the third signal together with the second signal, generating a mixed signal, filtering the mixed signal to remove upper side-band interference signals, generating a filtered-mixed signal, measuring the frequency of the filtered-mixed signal and calculating the moisture content of the soil.
I guess this could approximated by using the RFM69HW as signal generator, using gaussian FSK modulation to generate the frequency sweep. That will not create a constant speed sweep and therefore no constant frequency output signal. But one could measure the maximum output frequency using the 328p.
Still pretty complicated.
Quote from: joelucid on December 08, 2015, 07:31:33 AM
This strikes me as a really smart way to measure soil moisture: https://www.google.com/patents/US7135871
<snip>
Unless, of course, you're considering making a commercial or even open source project and not interested in paying a license fee...
:-X
Tom
QuoteUnless, of course, you're considering making a commercial or even open source project and not interested in paying a license fee...
You wouldn't believe how many patents exist in this space. Even chirp seems to be affected. But yeah, hopefully this can be done differently. All that's needed is a phase detection mechanism to measure the delay of the signal that traveled through the soil. Which works at 900 Mhz.
http://www.analog.com/media/cn/technical-documentation/data-sheets/AD8302.pdf
But it's $20. A no go.
Quote from: jarrods on November 25, 2015, 11:48:18 PM
I might... But to be honest miceuz's hardware design is much better at this point. His latest rev is really good and my PCB skills are quite basic compared to yours and his :)
I have been looking at his board and just seeing if I can reprogram it to communicate the way I need. If i could do that i would release that is it would be comparable to comercial units out there. However this is a low priority at this point.
The only thing that is better with my design is that I am using the attiny841 which is an awesome chip. It functions much like it's bigger brother (atmega) and I want to use it to build a Moteino "Mini" much like your and Joes. The fuses and programming was a real pain to figure out but now that I have you can program it the same way you do a regular chip except for automatic upload via the arduino IDE. Don't quite have that working but I will eventually.
The atmega is overkill for just about everything I work on as I rarely need more the a few pins on each node. And since I am trying to deploy a LOT of nodes, building a cheaper smaller node is something I will have to eventually do.
If your interested I can write up getting that chip working abd some sample code to see if you guys want to use it to build a node with it ;)
http://www.atmel.com/Images/Atmel-8495-8-bit-AVR-Microcontrollers-ATtiny441-ATtiny841_Datasheet.pdf
@Jarrods: Is cost of the attiny841 truly the main reason for your preferring it over the atmega328p? I don't know if you've shopped for atmega328p's recently, but these days you can buy an entire atmega328p Pro Mini for $1.50. To me, that's cheap. Just hook up Vin and Ground, and it's ready to rock: even the bootloader and fuses are already burned for you. If you want low power, you can get the 8Mhz 3.3v version and then simply remove the voltage regulator and one or more LED's.
On the other hand, from the datasheet you linked to, it looks as though the Attiny841 can run at 1mhz at very low power:
Active Mode: 0.2 mA at 1.8V and 1MHz
Idle Mode: 30 µA at 1.8V and 1MHz
For some applications, that might be interesting. Are you doing that?
[Edit: Silly me. I just now noticed that the atmega328p has comparable specs at 1Mhz:
Power Consumption at 1MHz, 1.8V, 25C
̶ Active Mode: 0.2mA
̶ Power-down Mode: 0.1µA
̶ Power-save Mode: 0.75µA (Including 32kHz RTC)
Being able to run the RTC at 0.75uA rather than the WDT might solve some issues (i.e. might it avoid the WDT's 8 second limit between wakeups?). ]
QuoteBeing able to run the RTC at 0.75uA rather than the WDT might solve some issues (i.e. might it avoid the WDT's 8 second limit between wakeups?).
I had somehow missed that you could drive the timer using a 32khz crystal independent of io clock. Even at 3.3V in power-save mode the power consumption is spec'd at < 1 uA (independent of io clock obviously). Just to compare that mode with the current low power setup (rfm69hw using noise wakeup timer):
Noise wakeup:
0.15 uA - 328p in power-down mode
1.2 uA - RFM69HW in idle mode
----
1.35 uA
32khz crystal:
0.95 uA - 328p in power-save mode with 32khz crystal
0.1 uA - RFM69HW in sleep mode
----
1.05 uA
That's a 22% saving with an accurate timer thrown in for the cost of one crystal. Something to consider. Thanks for the pointer.
Joe
Quote from: joelucid on December 09, 2015, 03:18:57 AM
QuoteBeing able to run the RTC at 0.75uA rather than the WDT might solve some issues (i.e. might it avoid the WDT's 8 second limit between wakeups?).
I had somehow missed that you could drive the timer using a 32khz crystal independent of io clock. Even at 3.3V in power-save mode the power consumption is spec'd at < 1 uA (independent of io clock obviously). Just to compare that mode with the current low power setup (rfm69hw using noise wakeup timer):
Noise wakeup:
0.15 uA - 328p in power-down mode
1.2 uA - RFM69HW in idle mode
----
1.35 uA
32khz crystal:
0.95 uA - 328p in power-save mode with 32khz crystal
0.1 uA - RFM69HW in sleep mode
----
1.05 uA
That's a 22% saving with an accurate timer thrown in for the cost of one crystal. Something to consider. Thanks for the pointer.
Joe
Alternatively:
Abracon AB18X5 running with a 32KHz Crystal 55nA typ, 330nA max and very accurate.
0.15 uA - 328p in power-down mode
0.1 uA - RFM69HW in sleep mode
0.055 uA - AB18X5 with alarm or timer set
------
0.305uA
Note that AB1815 has autocal mode that provides high accuracy but even lower power consumption by using Xtal for brief periods to calibrate its lower power RC oscillator. Power in this mode is: 22nA typ.
But then again... a decent ARM chip would bring the total to sub 200nA...
However, you lose the one advantage that ListenMode has: Wake Up on demand. I think this is unbeatable!!! (This coming from a guy who, a few months ago, thought ListenMode was the dumbest thing on the planet - next to the guy pushing it, that is... ;)
Finally, once you get into single digit uA regime, you're reaching diminishing returns on the other things that will kill the mote, corrosion being the worst. The best thing might be to use corrosion to your advantage and simply have an 'earth' battery. I have a clock on my office bookcase that my daughter gave me for Christmas last year - I filled it with water then and it's been running ever since...
Tom
For indoor potted plants, I'm starting to entertain the idea of measuring the darkness/lightness of a color changing soil probe like this: http://greenscapesinc.net/product/water-check/.
Walmart also carries them for nearly the same price. I don't a priori know how well it will work (and I couldn't find any reviews), but it's inexpensive and at least there would be no corrosion/electrolysis issue like there undoubtedly would be for a cheap etched PCB conductive probe. Also, capsense would seem to require some kind of custom PCB (e.g. such as that used by Chirp) to serve as a capacitive probe, which might not be exactly cheap either. Also, there has been writing about how water can intrude between the PCB layers and effectively ruin it by staying trapped there. That might be an issue faced by other types of PCB probes too, perhaps including the TDR type, unless it had a waterproof shell of some kind (plastidip, epoxy, etc.).
Anyhow, since this thread seems to have stalled, I'm starting to look into automating this other approach, as it should be comparatively straight forward to implement using a photocell in combination with a repeatable illumination source to light the color changing moisture strip in front of the photocell, and then taking an analog measurement of the intensity that gets reflected into the photocell. Well, that's the idea anyway. I'm hoping there are inexpensive reflective sensors (similar in form to an IR proximity sensor) already packaged to do this kind of thing, though it may take some hunting to find one. Or, as a fallback, I'm sure a homemade equivalent would also work.
[Edit: I just noticed that lowpowerlab appears to sell such a reflective optical sensor: https://lowpowerlab.com/shop/EE-SY310 . However, I notice from its datasheet it appears to be mostly infrared (as, at least so far, seemingly all of them are), which might be an ineffective wavelength for this application. A visible wavelength would seem more on-point, but I haven't yet found a sensor with that. ]
Hi WhiteHare,
QuoteA visible wavelength would seem more on-point, but I haven't yet found a sensor with that.
Would Adafruit #1334 work for you? https://www.adafruit.com/products/1334 (https://www.adafruit.com/products/1334)
John
Yes, I think it would work, although it's probably overkill. Interesting that it incorporates an IR filter.
I suspect a simple greyscale will suffice, but I can't say for sure until I try it. It's good to know about the Adafruit device though. Thank you. It looks to be of higher quality than some of the other color detectors I've seen.
I was going to try just a regular single color LED, but I just received a shipment of WS2812's from China. I think I'll try one of those instead. It should make it easier to find the best stimulus color without having to swap among different LED's.
Anyone have any thoughts on what would be a good photodetector to use in this situation? Something with a flat face (rather than curved) would be easier to mount onto the stick.
Hi WhiteHare,
When exposed to light, I've found green LEDs to generate the higher voltage than other colors such as red, yellow, blue or white. In addition, green LEDs are almost color blind.
Even though resistive "photo cells" provide a large resistance change with changes in light level, green LEDs seem much more repeatable. I would envision using two LEDs side by side, one to illuminate the target and the other to detect the change.
The focusing dome may be an advantage over a flat surface.
On the subject of the dual electrode soil moisture probes:
Are these being driven with alternating current?
I would think that less corrosion-prone metals would work reasonable well.
Are you open to trying some titanium or stainless-steel wire or strips?
I can send you some samples if you are interested.
John AE5HQ
Quote from: john4444 on June 10, 2016, 04:40:00 PM
Hi WhiteHare,
When exposed to light, I've found green LEDs to generate the higher voltage than other colors such as red, yellow, blue or white. In addition, green LEDs are almost color blind.
Even though resistive "photo cells" provide a large resistance change with changes in light level, green LEDs seem much more repeatable. I would envision using two LEDs side by side, one to illuminate the target and the other to detect the change.
The focusing dome may be an advantage over a flat surface.
On the subject of the dual electrode soil moisture probes:
Are these being driven with alternating current?
I would think that less corrosion-prone metals would work reasonable well.
Are you open to trying some titanium or stainless-steel wire or strips?
I can send you some samples if you are interested.
John AE5HQ
Thanks for the suggestion. I hadn't known that green LED's would be more repeatable, and repeatability is very definitely quite important. I'll give a couple green LED's a try: one as emitter and the other as receiver.
As for stainless steel, I've read about some people using stainless steel nails or screws as inexpensive probes. The wireless EC moisture sensors that I've been using over the last couple of years have special stainless steel probes built-in to them, and they don't seem to have rusted or corroded at all. In fact, they seem to work quite well for that purpose. The only problem with the sensors I have is that EC just isn't a good method for more than short term monitoring because they're always drifting out of calibration: first due to fertilizers entering the soil changing conductivity, and then again as plant roots uptake those nutrients. For that reason, I think EC is a dead-end unless you're willing to accept doing regular calibrations. That said, there is at least one faster way to calibrate than the way the manufacture recommended doing it, and that might yet make EC viable (still a hassle, but a lesser hassle).
Also, a probe needn't be limited to EC, and Titanium certainly sounds like an interesting material. It's the first time I've heard it mentioned in this context. What kind of wire/strips do you have that are made out of it?
@WhiteHare,
The company I work for goes through hundreds of pounds of titanium per year.
It is mostly 25-mm OD tubing but also 1.5-mm and 3-mm titanium-clad copper-core solid wire.
As you might imagine sometimes things get damaged or fabricated incorrectly.
Because these items are not sellable for their intended purpose they are usually scrapped since it has little salvage value.
Titanium is an interesting metal; light weight, high strength, hard, not very ductile, corrosion resistant,
cannot be lead-soldered or brazed. We use hydraulic tools to connect copper-wire using compression connectors.
However, an electrical connection can be made using a ring-terminal under a nut/bolt.
PM me if you would like a few feet.
John AE5HQ
Getting back to the post topic...
I was thinking that a capacitive soil moisture sensor could be made by:
placing two electrodes into a cloth bag full of gypsum or plaster.
One electrode (titanium) would be in direct contact with the gyp.
The other electrode (any conductive material) would have an insulating coating.
The capacitor would be from the gyp to the insulated electrode.
I have not done this experiment but I would expect the gyp would create a reasonably
stable environment in which the moisture content would follow the surrounding soil.
It also seems this arrangement would be fairly immune to the salts / minerals in the soil.
I would not mind giving it a try, but I'm not sure how to "calibrate" it.
John AE5HQ
Maybe too old.. Maybe not ;)
Somewhere some time ago , i was under the impression that a piezo disc can be used to measure weight, feed power in one side & measure voltage on the other.
Dry weight , Wet weight, Over watered weight. Please do correct me if i am wrong ::)
Hi Raggedy Anne,
Welcome to the forum.
I'm not sure that a piezo would be very suitable for measuring weight.
You would get a pulse of voltage when the disc is flexed due the the change in weight.
But, that pulse only lasts while the disc is flexed.
Or, apply a (fairly high) voltage and the disc flexes
but only stays flexed as long as the voltage remains.
A 'load-cell', on the other hand, may be what you were thinking of.
Yes its true, that it produces voltage. Surely there is an increase of resistance or decrease of capacitance when the disc is under pressure before applying voltage because you are changing the structure or electrical characteristics of the crystal.
Try this instructable
http://www.instructables.com/id/How-to-Make-a-Ridiculously-Cheap-Analog-Pressure-S/ (http://www.instructables.com/id/How-to-Make-a-Ridiculously-Cheap-Analog-Pressure-S/)
After further contemplation of that instructable here are my thoughts;
Follow the instructable , cover in liquid latex instead.
Glue between two acrylic/wood/steel sheets that are spring loaded (for the dry weight) in each corner.
Voila!
How do you envision a pressure sensor will help you infer soil moisture content? Is this for a potted plant (where over the near term, the pressure (weight) of the potted plant would increase do the weight of the water)? Over the longer term it may be somewhat problematic and may require recalibrations, as the plant grows (increases in weight) and/or as leaves die and need to be pruned.
In Australia we have a government agency called the CSIRO that produces scientific publications on such things as exponential plant growth of species.
Plants grow all the time , they grow under particular conditions related to nutrient, sun, heat etc.
So the first step would be knowing your expected growth rate/biomass.
Biomass over a period of time, time being a constant.
The second thing that is important is the rate in which moisture evaporates from soil in a volume.
There is a median temperature particular to the region based on the season.
Indicative to biomass over time / evaporation over time
You could estimate when the calibration will be & what the offset will be.
Remembering that a little water is better than no water.
This however an article somewhere on the internet that discusses making a probe with plaster of paris.
Not sure how well, or even if, this approach to measuring soil moisture would work, but it does have the virtue of being simple (provided that your temp-rh mote is small enough to fit inside the waterproof shell):
https://www.aliexpress.com/item/2pcs-Waterproof-Temperature-and-humidity-sensor-shell-SHT10-SHT21-SHT15-shT11-sht20-SHT75-sensor-protective-sleeve/32725766858.html?spm=2114.10010108.1000013.6.PVGRb3&traffic_analysisId=recommend_2088_3_81019_new2&scm=1007.13339.81019.0&pvid=b9edd7ff-9069-4c39-be95-b2d3ca8bdf17&tpp=1
https://www.aliexpress.com/item/2pcs-YS1560J-Temperature-and-humidity-sensor-shell-Waterproof-temperature-sensor-protective-cover-SHT10-SHT15-SHT20/32725840346.html?spm=2114.10010108.1000013.25.PVGRb3&traffic_analysisId=recommend_2088_13_81019_new2&scm=1007.13339.81019.0&pvid=b9edd7ff-9069-4c39-be95-b2d3ca8bdf17&tpp=1
https://www.aliexpress.com/item/TM16-115-temperature-and-humidity-sensor-protective-cover-waterproof-SHT10-SHT11-SHT20-SHT21-SHT15-SHT75-sensor/32578099865.html?spm=2114.10010108.1000013.10.gZI6yC&traffic_analysisId=recommend_2088_5_82199_new&scm=1007.13339.82199.0&pvid=3cc09c58-99e7-4a67-834a-3313c87ab573&tpp=1
On the face of it, it seems plausible. Not sure what the failure modes are, or how they might be avoided if there are any. Plainly, you don't want to use components which might rust or otherwise corrode from humidity. Probably my biggest worry would be the possibility of condensate forming on the humidity sensor and skewing results until it evaporated. However, a sensor heater, such as some sensors (e.g. si7021) already have, might remedy that if it were to occur.
Has anyone either tried one of the shells above or anything similar? For instance, perhaps wrapping a mote in Tyvek and sealing the seam with Dupont housewrap tape would work just as well.
Upon closer inspection, apparently the shells are only large enough to contain a sensor, not a moteino. It doesn't completely kill the idea, but it would mean additional work.