Underground wireless sensors

[joelucid]

I'm starting to think more about the 433mhz capacitive moisture sensor we've been discussing. Two of my requirements are that (a) the sensor should be wireless and (b) nothing should stick out of the ground. Turns out these requirements open up a whole new field of investigation:

Depending on the properties of the soil (e.g the water content) the wavelengths of rf radiation shorten significantly, so that ultra wideband antennas need to be used to maintain a wavelength compatible antenna under changing conditions. Our traditional single frequency/wavelength tuned antennas won't work well at all. This paper has a good overview: http://soilscape.usc.edu/~agnelors/Communication%20with%20Aboveground%20Devices%20in%20Wireless%20Underground%20Sensor%20Networks%20An%20Empirical%20Study.pdf.

The good news though: I think this is doable with an rfm69hw, a good antenna design and low burial depth of 10-15cm.

After reading a good deal about the topic I wanted to check whether anybody here had already tried to build underground antennas and if yes what worked for you.

Joe

[joelucid]

This looks like a nice solution: http://soilscout.com/solution/elements/

Here are the technical specs: https://drive.google.com/file/d/0B4_CUFDRpzgXN3pRclRvVXByaEE/view

Definitely a nice form factor. 20 years battery life with 3000mAh lithium batteries. With a rfm328hw we won't be able to go down to 4m into the soil - but for home applications that's not required either.

They seem to use the antenna design from the paper I posted before - but at 915 Mhz the antenna is much smaller. Might be worth to switch to that frequency to keep the sensors of manageable size.

Joe

[joelucid]

You can actually see their printed elliptical monopole antenna on this picture: https://www.flickr.com/photos/chivacongelado/20148150749/

[WhiteHare]

Hi Joe,

I see where it gives information about the frequency of the radio communications, but I don't see where it gives any detail about the moisture sensor other than to say that it is taking both resistive and capacitive measurements.  The resistive measurements appear to be from the exposed copper traces on the two outside prongs.  I don't see where it gives any information or details about how it is doing the capacitive measurement.  Also, I'm not seeing any efficacy data to validate that the measurements it's transmitting are accurate.  I don't even see any accuracy claims being made.  Did I overlook something?

[joelucid]

You're absolutely right. For me their product is interesting not because of the sensor (I already know how I want to build that) but as validation of the viability of wireless underground sensors.

[WhiteHare]

Ah.  I'm already past that concern, as I've had some 802.15.4 based moisture sensors fully buried for more than 18 months now. They're still ticking strong, giving updates about every 10 minutes, and with plenty of battery life left.   Zero problems so far, except that when the soil gets totally saturated with water I do notice that the rate of the packet loss does go up then.  In practice, that hasn't been a problem, as the frequent sample rate has compensated for that.  In their case the electronics are fully encased in resin, and there's just an o-ring and some screws making the watertight seal on the battery compartment.  There's also a side benefit, which is that the operating temperature is much more moderate than it would be above ground, which I imagine probably helps battery longevity and as well as the aging of other components.

If your battery life is long enough, you'll probably want to simply encase literally everything in some kind of totally waterproof resin, as the only likely enemy is corrosion over time.  If my current sensors ever do fail unexpectedly, I'm guessing it would probably be at the o-ring. 

My only reason for wanting better sensors is that the ones I already have measure soil conductance, so they're mainly good for relative measurements rather than absolute measurements: they're always drifting (sometimes a lot) since the last calibration because of inevitable changes in soil ion concentration.

[WhiteHare]

By the way, the preferred way to install your probe is to jam it into undisturbed soil, either vertically from above or sideways by digging a shaft first.   In contrast, the moisture scout directions say to dig a hole, lay their sensor at the bottom (orientating the directional antenna toward their above ground receiver) and then backfill.  I'd say that's a likely methodology error, as they're likely not getting the same level of soil compaction after backfill as already settled soil. 

How well a probe holds up after a good jamming into undisturbed soil needs to be considered.  If the probe is just a pcb that has maybe been thinly coated in a waterproofing agent, part of that coating might well get scratched through when jamming it into the soil.

[joelucid]

Ah.  I'm already past that concern, as I've had some 802.15.4 based moisture sensors fully buried for more than 18 months now.

Very interesting! Is this a dyi mote or commercially available? If the later do you have a pointer?

[WhiteHare]

They were made by Toro:  http://www.sprinklerwarehouse.com/PSS-SEN-p/PSS-SEN.htm?gclid=Cj0KEQiA-ZSzBRDp3ITHm5KO_JYBEiQA1JjHHIHYXSUeQ0trQmXNHmSkZ2l4ys59BEWWnJ09eU4yqEQaAkeh8P8HAQ

I'm not sure whether they're even being manufactured anymore.  At the time I purchased them, they were being liquidated throughout the country and were available (together with the base receiver) for as little as $20, delivered.  In any case, I wouldn't recommend them.  The base receiver is junk as far as usefulness, so I'm intercepting the wireless signal to get the raw data.  They're designed to be pushed vertically down into the soil, but I installed mine at different depths beneath the surface, which hasn't been a problem at shallow depths (my deepest is about 8 inches deep).  At the time I purchased them I didn't know how they measured soil moisture, but through experimentation I determined that they measure moisture by soil conductance.  As far as measuring soil conductance goes, they're pretty good, as they use stainless steel probes rather than something which would corrode.  The problem is simply that measuring soil conductance as a proxy for soil moisture isn't a good method for a sensor that's installed long-term.

[joelucid]

They're designed to be pushed vertically down into the soil, but I installed mine at different depths beneath the surface, which hasn't been a problem at shallow depths (my deepest is about 8 inches deep).

Interesting - just imagine how well it could work then with a broadband antenna which would have about 350% of the signal strength.