Soil Moisture Sensing with Vegetronix Sensor

[TomWS]

If you want to bury your entire sensing nodes you will severely compromise the range of your radio signal.
I have a network of Vegetronix sensors with Jeenodes, but I keep the controller and battery above ground. With a 10 minute sampling interval, i get about 2 years from 3x AA alkaline cells.

What part of the country are you?  What are the temperature extremes?  Finally, how do you like the sensors?

Tom

[damonb]

BTW, as an aside, how many Vegetronix sensors are you using, and what's your opinion of it?  Do you like it?  Have you run across any downsides to it?

What part of the country are you?  What are the temperature extremes?  Finally, how do you like the sensors?

@WhiteHare, @TomWS: Sorry for the late reply, and a bit off-topic but anyway...
I have been using the Vegetronix sensors for about 3 years. The VH400 works well for domestic irrigation control. An earlier model I tried (VG400-LV) was very low power but not sensitive enough in the right part of the range. They have since discontinued it.
I am in Adelaide, South Australia - a temperate climate with about 550mm annual rainfall. Summer months average max is 29°C (but we get 4-5 peak days each year up to 45°C) and mostly low humidity. Winter daytime temps around 12°C to 16°C, overnight minimum typically 6°C to 8°C and occasionally down to 0°C.
I have six sensors in production at present, feeding moisture values via a JeeNode network to an instance of mcsSprinklers, which does all kinds of irrigation scheduling (interval, ET from weather data, as well as moisture sensing). It also tracks weather and water consumption - I have a Jeenode on my water meter.
The soil sensors are switched by a P-MOSFET so they are only on for 400ms once every 10 minutes. Likewise the MCU and radio sleep as much as possible (including the 400ms sampling time). Each node is powered by 3 AA alkaline cells (nominally 4.5V). My oldest one started 9 Sept 2012 and is now down to 3.86V and still going... that's just over 3 years.
I plan to roll out more sensors as time permits... I have 16 zones to control all up.
I have reduced water consumption by at least 30% since I set this all up.
I am starting to suspect the Vegetronix product has a limited service life. I have replaced several when they failed or lost accuracy/sensitivity after 2+ years in the ground.
I am not certain of the cause, but suspect the exposed PCB that forms the capacitive sensor eventually suffers from moisture attack. I cannot detect any visible damage in the ones I have pulled out, but there must be something there.
I am just about to start a trial with a new batch that I have treated with a coating of polyester resin. Ask me again in 2 years and hopefully I'll have some idea whether it has worked...
I am very interested in some of the DIY and open source designs for capacitive sensors that are popping up lately... unfortunately I lack the time to do the R&D. USD$38 for the Vegetronix starts to look very expensive if you want to get good coverage and they need replacing every 2 years.

[WhiteHare]

USD$38 for the Vegetronix starts to look very expensive if you want to get good coverage and they need replacing every 2 years.

Thank you!  This is exactly the kind of first-hand longitudinal feedback that I've been looking for but haven't been able to find anywhere else.

I have read anecdotes about moisture eventually intruding between PCB  layers on other designs, leading to failure.

I hope someone tries building the 400Mhz moisture sensor design (discussed in a different thread), because at least then the incremental cost should be quite low.

[TomWS]

The soil sensors are switched by a P-MOSFET so they are only on for 400ms once every 10 minutes. Likewise the MCU and radio sleep as much as possible (including the 400ms sampling time). Each node is powered by 3 AA alkaline cells (nominally 4.5V). My oldest one started 9 Sept 2012 and is now down to 3.86V and still going... that's just over 3 years.

Pretty good results!  Well done!  I've deployed a single Vegetronix VH-400 and found that they need at least 450mS from turn-on to reach a level sufficiently close to final to meet my accuracy 'wants'.  I've deployed this right near a Watermark sensor to compare the results, but, unfortunately we've had too much rain for the ground to measure anything other than 'damp'.

I am starting to suspect the Vegetronix product has a limited service life. I have replaced several when they failed or lost accuracy/sensitivity after 2+ years in the ground.
I am not certain of the cause, but suspect the exposed PCB that forms the capacitive sensor eventually suffers from moisture attack. I cannot detect any visible damage in the ones I have pulled out, but there must be something there.
I am just about to start a trial with a new batch that I have treated with a coating of polyester resin. Ask me again in 2 years and hopefully I'll have some idea whether it has worked...

The outdoors ground is very inhospitable to electronics, especially those in direct contact with the ground.  Its sort of why I'm still sticking with the Watermark sensors as they are designed to stay in contact with wet soil and don't corrode. The cable is rugged and has good resistance to water penetration.  As long as I can keep the electronics sufficiently removed from the ground moisture, I've had success (some units deployed in Texas for 6 years).  Still, as you say, the cost is prohibitive if you need a lot of them.  I try to pick representative zones so that I can extrapolate the readings to similar zones.

Thanks for the thorough update.

Tom