Measurement method of soil moisture sensor

Hardware
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Hello there :),

first of all: i really really like the idea of the project and farming vegetables without stress in general,
so on some tests and projects in the past with different kinds of soil sensors and measurement methods, my conclusion was that measuring with the resistance method (measuring the resistance of the ground by transferring voltage through the ground), which you are using on the first look, has 3 mayor disadvantages:

1st: the probes will corrode from the moisture over time, because the probes have to be metallic and thus the readings from the sensor won’t be stable over time

2nd: electrolysis will poison the ground over time. But this won’t be that much of a problem, because the sensor is not embedded in the ground permanently

3rd: The values are not independent from the temperature

The best solution, which eliminated all 3 problems was to switch to the capacitively method of measurement. This means a little bit more effort at the beginning, but the outcome is so worth it! Stable readings over a long time, day and night and no more poisoning of the soil. For ideas try this blog: http://sgerber.de/2014/08/kapazitiver-feuchtigkeitssensor-fuer-blumenerde (unfortunately in german).

If i find some time after my exams and enough people are interested, i will try to look further into it. So what do you think?

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Great insight @DonWasabi! At this time I think we need to stick with the current design because its what we’ve been working with so far and our ship date is nearing. But this capacitive approach does look promising!

I’ve read a lot about the first problem you mentioned but figured that as long as the probe is only in the ground for a few minutes each day, it won’t be too much of an issue. Its really only a problem when it is constantly wet/submerged in the soil.

For the third problem, it could be possible to also record temperature at the time of taking the soil measurement, and then returning a more accurate value. I’m hoping that temperature doesn’t play too large of an effect for now. (10% difference in readings between night/day or summer/winter) would probably be okay for most people.

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http://www.dfrobot.com/wiki/index.php?title=Capacitive_Soil_Moisture_Sensor_SKU:SEN0193
roryaronson personally I believe you have gone completely with the wrong sensor.

This is a person building own.


Capactive sensors give broader reading than two probe in ground.

http://www.mait.com.au/products/aquaflex-soil-moisture-sensors-soil-moisture-probes/
Note here capacitive does not have to come in contact with soil at all to get a reading.

http://www.coppersulfatecrystals.com/killroots.html
Putting copper in and out of the ground is not something you want to be doing. Higher grade sensors resistive sensors like I guess you are using are gold plated but that plate does wear off then you come back to copper contamination reacts with sulphates in soil to make copper sulphate that plants don’t particularly like. The issue you have is soil dry is like very light sand paper. So probe only been in the ground for a few min day is only fix one problem the fact its being progressively sanded off into the soil is another. The gold plated resistive base sensors are designed to be inserted and left not put in and pulled out over and over again.

Commerical farm usage is capacitive sensors were possible. 1 they last 2 you don’t end up with contamination. Putting fixed pipes in and running capacitive sensors inside that is done in fact to reduce wear and read to a lot deeper depths.

Something as simple a lettuce is 6 to 21 inches of root depth depending on soil compaction. Good garden you expecting at least 17 inchs. Current design you only measuring the top bit as well.

10% difference in readings between night/day or summer/winter<< There is something that can cause greater than this.


Above is a kids experiment but shows key properly of Capillary Action. Hotter the temperature of the soil the more the water will rise up though the soil and the slower water with sink into soil and as the soil cools the slower the water will rise though the soil more the water will sink into the soil until you get to freezing. So yes water in soil does move up and down between day and night and the rate water sinks in changes between summer and winter

Taking surface readings can lead you up the garden path thinking you need to water when the plant has tones of water. Why the water has sunk in deeper than where you sensor is reading but not past were the plants roots are getting at it. Also it can trick you the other way were the surface is wet and the subsoil is dry so the water disappears very quickly even that you appeared to have had quite a good reading.

Here is what is really going to ruin you day is taking a soil surface temperature will not define how fast water is sinking into the lower layers. You will need subsurface readings. Reason why I use wicking garden beds is it make the water layer come up from under the plants. This creates a supported stack of water. Watering from above it can basically can have no moisture under so it sinks into the ground away from the plants. A supported stack of water does not move up or down as much when the temperature of soil changes. So the overall design of the garden bed has huge effects on how water will behave in it.

Commercial farms don’t go to all the effort of taking reading down through the complete soil profile for no good reason if you don’t you are only getting information that will make you make mistakes. Yes using capacitive sensors to map the complete profile on farms you can see in the numbers the water moving up and down through the day and the amount of movement linked to temperature not surface temperature on it own either…

There is a lot of science to taking care of plants that if you are not aware of can make you make huge mistakes. Yes something that sound simple as a soil moisture reading its possible to mess method up completely because its not as simple as it sounds. Taking the surface temp of bed is not going to tell you where the sub surface temp is 3 inchs in. Temperature probe will not fix this error if it would commercial farms would be using it.

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I have looked ar soil moisture sensing and it truely appears that the only reliable method is via capacitance techniques.
The probes can be gold plated or better still can be insulated and inert.

The resistance method is incredibly unreliable with very little calibration of errors possible.

The capacitive technique also allows depth testing as the problem area can be lumped at the tip of the probe and the capacitance read as the probe is driven in the soil. You can now profile the soil. Also you can rotate the problem to get closer to the root zone. Remember that you want the roots deep to get moisture and nutrients. You then can keep the surface evaporation rate lower. The cost of water is now a significant cost of agriculture.
Also you can incorporate a small temperature sensor to aid in getting better calibration and again soil temperature profile.
The capacitance sensors available commercially are good but expensive. An integrated sensor with the Arduino doing drive signals and ADC keeps the cost down. Tricky part is designing the analogue circuits. But there are still a few of us geeks out there that can do this.

Oh yes the capacitance measurement can be fast. The temperature sensor will be the slower response,

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I have looked at the articles linked to for capacitive sensors. These use simple “RC” oscillator circuits to estimate capacitance. One is a “555” timer IC and the other is using a digital IC in analogue mode. These are not accurate oscillators and require calibration and suffer from temperature and raging drift.
The most accurate way is to use and accurate clock source with its amplitude well controlled. This feeds on tine of the sensor. The other tine is fed to a detection circuit that provides “detection” of the signal to a DC value that is fed to the micro’s ADC port. The probe is checked each time it is out of the soil. This method means that it only takes a few clock cycles to measure capacitance.

To give higher dynamic range the Counter Timer/ PWM can be used to provide a variable frequency. The detection circuit remains the same.

Adding the soil temperature sensor enables the calibration over temperature. It does change but with capacitance sensing this behaviour is quantified and stable.

This sensor does not have to be low power so the design is simpler.