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I'm working on a project to measure soil dryness in multiple outdoor potted plants, and I'm looking for a moisture sensor that can withstand the elements. I'm found several stores selling sensors like this, but I can't imagine that thing lasting more than a couple weeks outside before sun and rain destroy all the exposed contacts.

Is anyone aware of other cheap but robust moisture sensors? How would I weather proof the sensor?
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You can make it waterproof by using kit you also use in the bathroom or with sugru - http://sugru.com/about -
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Rob Tillaart

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The last thing you did is where you should start looking.
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Use stainless steel maybe?
 
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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

Mayne Island, BC, Canada
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Industrial sensors use a conformal coating. http://en.wikipedia.org/wiki/Conformal_coating  MGChemicals sell this (among others). It is a thin epoxy type of coating which I use for exterior sensors in a marine environment. It is extremely durable and moderately flexible.
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If you seal the sensor , so that it is waterproof,   then the soil moisture and humidity won't be able
to get into it.   
I suspect that a completely waterproof solution is not what you actually want, for your stated application.
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If you seal the sensor , so that it is waterproof,   then the soil moisture and humidity won't be able
to get into it.   
I suspect that a completely waterproof solution is not what you actually want, for your stated application.

True. I was referring to the above-ground part of the sensor, especially the plug. I'm not too worried about the below-ground parts, as long as the leads are gold-plated or similarly protected against corrosion.
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Dubuque, Iowa, USA
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I keep a two part epoxy (as used with fiberglass) for problems like this. The epoxy can be thinned down with denatured alcohol if necessary and in this case would probably result in a coating similar to what arbutus is describing. Don't expect to remove the plug afterward though smiley-wink

Shelf life is very long (years). Tons of uses as either a glue or coating. You might find it at a auto store -- just remember it's epoxy and not polyester resin which is generally just a huge PITA.
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Just my 2 cents... Use capacitance to measure moisture, this allows you to completely seal the unit and you don't have to use expensive metals for the tester. Also, use a decent connector and dielectric grease in the connector. The grease is the kind they use to keep the elements out of spark-plugs and buried telephone line junctions and such.
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Just my 2 cents... Use capacitance to measure moisture, this allows you to completely seal the unit and you don't have to use expensive metals for the tester. Also, use a decent connector and dielectric grease in the connector. The grease is the kind they use to keep the elements out of spark-plugs and buried telephone line junctions and such.

Does anyone sell these at a reasonable price in a form I can easily connect to an Arduino's ADC port? Googling doesn't find much, and the few I can find, like this, don't have any price listed...which usually means they're insanely expensive. I have a budget of a couple hundred dollars, and it looks like a single "good" capacitive soil moisture sensor may well cost much more than that.
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