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Is there a moisture sensor to put in the ground?


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#1 blubblub

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Posted 30 June 2011 - 02:05 AM

Hi, I'd like to take readings of the ground's moisture content. What type of sensor would work best for this application? Does the sensor need to be buried or is there a better way to go about this? Thanks in advance! -blubblub

#2 JonnyBoats

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Posted 30 June 2011 - 01:03 PM

Here is a post you may find helpful: http://www.faludi.co...sensor-circuit/

Basically the way it works is that the resistance of wet soil is lower than that of dry soil. You could either make your own probe or hack one of the cheap probes from a garden supply shop.

#3 blubblub

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Posted 01 July 2011 - 04:03 AM

JonnyBoats, Thanks for the insight. So I guess reading the moisture content of something physical is slightly more of a challenge than originally thought. And that's because i can't find a sensor that will do that for me... But really isn't it the same idea as a pentiometer? One prong receiving electricity, another for it to exit, and one in the middle to read what's moving across the other two?

#4 JonnyBoats

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Posted 02 July 2011 - 01:40 AM

JonnyBoats,
Thanks for the insight. So I guess reading the moisture content of something physical is slightly more of a challenge than originally thought. And that's because i can't find a sensor that will do that for me... But really isn't it the same idea as a pentiometer? One prong receiving electricity, another for it to exit, and one in the middle to read what's moving across the other two?


Very similar. It you would like to try a simple experiment you could stick two pieces of metal with wires attached into a pot of dirt. Connect one to the ground on the Netwuino and the second to one of the analog input pins. This second wire should also be connected to a resistor (try 1 MegOhm, value not critical) and the other end of the resistor to +3.3v on the Netduino. This is what is known as a pull-up resistor.

If the soil is bone dry then you should read a voltage of 3.3v (give or take) on the input pin, this is because the input pin is connected to v+ via the resistors and the resistance between the two pieces of metal is essentially infinite.

Now pour some tap water into the pot. The resistance between the two pieces of metal will now become some finite value and the voltage on the input pin will decrease.

If you were to immerse the two pieces of metal into a glass of salt water the resistance should become very low and the voltage on the input pin become close to zero.

The only real problem at this point is that the resistance also depends on the distance between the two pieces of metal in the dirt. If you wiggle the wires anc change the distance the reading will change as well. A "real" probe will be made so that the two pieces are held at a small fixed distance and usually combined into what looks like a single probe. In fact one connection is simply above the other, just like on a headphone plug that you would plug into the audio jack on your PC.

You can now buy a commercial probe if you like, and as with everything else the price will vary depending on who makes it and the required degree of accuracy.

#5 Matt Taylor

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Posted 03 July 2011 - 03:11 PM

In fact one connection is simply above the other, just like on a headphone plug that you would plug into the audio jack on your PC.


Could you get a 1/4 inch audio jack and just stick it into the soil? :D

last night I was trying to work out a probe that reads the water level in my hydroponic window box. because the feed water is slightly saline, it conducts, so this approach will work great for it.

In my case, I want to read 'full' or 'empty', so I will just have two wires that are stripped at the ends, just above the low level.

I had thought about using resistance wire, with the water level forming a potentiomiter. that would allow a measure of depth.

thanks for the info!

#6 JonnyBoats

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Posted 04 July 2011 - 02:46 AM

Could you get a 1/4 inch audio jack and just stick it into the soil? :D

last night I was trying to work out a probe that reads the water level in my hydroponic window box. because the feed water is slightly saline, it conducts, so this approach will work great for it.

In my case, I want to read 'full' or 'empty', so I will just have two wires that are stripped at the ends, just above the low level.

I had thought about using resistance wire, with the water level forming a potentiomiter. that would allow a measure of depth.

thanks for the info!


One thing that is not clear from your question is if you are just trying to experiment or learn or if you are looking for a robust solution.

The reason I bring this up is because measuring full and empty with two wires that are stripped is cheap and easy but does not have good long term reliability. The first issue is that when a DC current passes through the probes electrolysis occurs and the wires degrade over time. One can use better probes such as platnum electrodes and use an AC rather than DC current etc.

On the other hand one can purchase for about $5 a MEMS sensor such as is used in washing machines to detect water level which outputs an analog voltage proportional to the water level. They are extremely reliable and will last practically for ever. I have used the ones from Freescale (http://www.freescale...et/MPXV4006.pdf ) for just this purpose. Take a look at the MPxx4006 family.

#7 Matt Taylor

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Posted 04 July 2011 - 12:02 PM

One thing that is not clear from your question is if you are just trying to experiment or learn or if you are looking for a robust solution.


Currently I'm experimenting on a small scale using a window-box sized system. I'm in the process of building a greenhouse, and hope to automate it as much as possible, which is why I'm experimenting on the cheep now :D
For the full thing, I'll be building/buying sensors design for the long-term.

Thanks for the links! I actually have a pair or scrapped washing machines and a dishwasher, that I've been getting parts from as I need them. I'll see if I can find the sensor.

#8 JonnyBoats

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Posted 04 July 2011 - 04:49 PM

Currently I'm experimenting on a small scale using a window-box sized system. I'm in the process of building a greenhouse, and hope to automate it as much as possible, which is why I'm experimenting on the cheep now :D
For the full thing, I'll be building/buying sensors design for the long-term.

Thanks for the links! I actually have a pair or scrapped washing machines and a dishwasher, that I've been getting parts from as I need them. I'll see if I can find the sensor.


You might enjoy the video in this blog post: Plantduino Greenhouse Cares Of Your Plats While You’re Away In particular note how they use two ordinary nails to sense moisture in the ground.

All the details, parts list, schematic etc can be found at http://revoltlab.com/.

#9 Matt Taylor

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Posted 05 July 2011 - 12:38 AM

All the details, parts list, schematic etc can be found at http://revoltlab.com/.


That's cool :)

I wonder how standard nails would hold up in soil though. I guess that if you only power them intermittently, there would not be as much of an issue with the slow corrosion and build-up on the anode and cathode.

In a liquid, I would guess that this corrosion would affect the system faster, as the material that is being removed is able to fall away. Also, they are cheap and easy to replace.

How would galvanised nails affect the soil is my other worry. I tried using galvanised screws in an electrolytic hydrogen cell once for a science demo, and by the color the water went, I'm thinking that there was some pretty nasty by-products coming off. I know that the power levels are way lower in this application, but I'd still be a bit dubious.

I definitely would not want to do that to my fish tanks :D (although making this for a tank with life in it is a whole different kettle of fish)

#10 Fred

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Posted 05 July 2011 - 09:51 AM

There seem to be plenty of examples of how to make a moisture sensor. This embeds the metal electrodes in plaster of Paris. That should be more consistent with regards to fluctuating moisture levels and distance. It should also prevent contamination of the soil.

I'm a keen chili plant grower. My gardening skills don't extend much further than that. I did consider something like this for when I'm on holiday, but decided that the mess of hoses for the water supply would be too much for my wife. (I grow indoors in a conservatory that's practically a greenhouse but is also our dining room.)




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