Hello all,

I'm planing to design a control system using microcontroller, this control system will control the sand humidity (water ratio of the sand) in my farm. The problem I have now is how to measure the sand humidity of the farm sand.

Please if any body knows how to measere the humidity in the sand help me.

 

Push probes into the sand and measure the resistance between them, the higher the humidity the lower the resistance.

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IS this an accurate method taking in consideration the tempreture effect? I think the resistance will change with the change of the water contents.

 

Yes it will, but isn't that what humidity is?.

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It will also depend on the disolved ionic compounds in the water, so I wouldn't say this is the most acurate method. I don't know how you can just measure moisture without the disolved compounds effecting the measurement.

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If you meant that different amount of salts in the water will change resistance - yes, that is correct. Since it's a farm, I assume that you will be applying fertilizers (organic or not) and thus changing the chemical contents of the water. that will change the resistance.

someting that confuses me is your use of the term humidity which is most often used to mean water vapor in air. Since the water in sand is mostly liquid, I think you are asking how to determine the amount of water present in sand. If the chemical content of the water remains constant, then the resistance approach is probably going to work fine. make sure you use non-reactive probes (stainless steel, probably).

If you are trying to decide when to irrigate (i.e. when it gets dry), then the resistance approch will work for most cases even with varying amounts of dissolved salts.

However, if you are trying to accurately measure the amount of water in the sand with varying salt concentration, the resistance approach won't be enough.

 

Is this a pile of sand, a railcar full of sand, sand moving on a conveyor belt, etc? Uniformity might be an issue - dry at the top, wet at the bottom. Does that matter?

The most accurate simple method would be to take a sample, weigh it, dry it then weigh it again - assuming that the only volatile is water. You might correlate that with conductivity measurements to see if you can employ the probe method continuously - with the wet/dry measures to serve as anchor points.

If you put the sand in a sealed container and let everything stabilize you might measure the relative humidity of an airspace at the top - and correlate that with more precise batch measures. Measuring RH over a moving conveyor belt might tell something in the right circumstance.

As already suggested, the sand itself may or may not be uniform in water hold ing capacity. It may not be uniformly dense, etc. If you know some of these things it will help you zero in on a method.

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You never really gave any details on how you wish to take the measurement. Ground moisture will vary by depth and time of day, season and so forth. To get a precise measurement of water content, you will need to dig up a sample, weigh it, remove the water (heat), and weigh the dry soil. The difference will be the amount of water removed. I don't remember the conversion numbers if you are looking for a volume figure, but shouldn't be hard to look up.

For continuious monitoring, the resistance method is most likely what you'll be using. Carbon rods will not corrode, you shouldn't have anything in your soil that will have any effect. If you do, might want to contact the EPA, and avoid consuming your vegetables (or whatever type of plant you grow)...

Regardless of the method of measurement, the results will vary by the minute, its natural.

Might take a look at some county extention websites, usually have a lot of agricultural information. Some universities have websites with research information. You will probably find more useful information there, than in this forum. Once you find the what and how to measure, we can help with the electronics.

 

Passing a direct current through an ionic solution will give you electrolysis, not an accurate measure of the resistance. To see this, push a pair of ohm-meter probes into the sand, and turn on your meter. You will get a reading at first, then the reading will climb to infinite. You would need to use an alternating current to get accurate results. If you wish to go this route, what you will be building is an Electro-Conductivity meter.

What would be easier, and work well, I think, is to remove the outer coating of a resistor, exposing the carbon core. You can then pass a DC current through this. The resistance will vary with changes in moisture. You can scale this with an op amp, and send it to an ADC. I don't think this will give a linear output, so the easiest way to calibrate your device would be to borrow an accurate moisture meter, to scale it against.