soil moisture probes

#1
Hi All,

i have the need to make a pair of soil moisture probes and use with a pic16f88(18x) and it's internal adc , it's to log moisture levels over time and also control a solenoid for irrigation
has anyone else done it? and what did you use for probes in the soil?
 
#4
Some great suggestions there. The one system that doesn't work very well long term is to put two wires in the ground. The metal corrodes and decomposes, unless you use something like gold or platinum wire and even then it has to be pure. There are lots of commercial soil moisture probes that measure soil resistance but they are designed to be stuck in the soil, take the reading then remove. I like the suggestions in the links above - they make a lot of sense. Indeed, I might have a use for these probes soon so thankyou to the people who posted these :)
 
#6
i like the capacitance idea it's not a bad one, i was thinking of using a pair of copper probes and simply calibrate acordingly as they corrode,

the pressure idea i might use in another setup but for this one i need to control the moisture level fairly well
 
#7
AC or DC into probes won't change the corrosion, because at the currents you will be using for a picaxe ADC (microamps) electrolysis isn't really the problem. It is the corrosion due to wet metal with all the ions and soil organisms etc. And oxides conduct differently to metals. The cheapest way might be to just go for volume (lots of metal), ie two pieces of angle iron hammered into the ground. A metal fence post will corrode away to nothing over 30 years or so, but it ought to work over a few years.
 
#13
What about a carbon rod eg. a carbon welding/cutting electrode or a centre electrode from a primary cell battery, would say a tunsten TIG/GTAW electrode be OK?
 
#15
Yeah an arc "cutting/heating" rod they are carbon with a copper coating which could probbably be eched off with ferric chloride, just leave a bit of the copper to solder onto.
Else use a tungsten TIG welding electrode, they are approximately 175mm long & cost about $2-$3 each depending on diameter.
Otherwise maybe even a pure nickel electrode used for welding cast iron and break off the flux.
BORROW a Stainless Steel knife out of the kitchen, but they are hard to drill a hole in, and even harder to solder onto.
 
#16
Sometime ago i reserchead also about soil misture adc to uC.
Got a simple amp. schematic.
Thick copper or aluminum monofilament wire is a good cheap choice and u can reduce the galvanazation/oxidation in the probes by only power them a small time interval only when the adc routine is executed, leaving them "offline" the rest of the time.
 

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#19
ok i've found somthing that seems to work very well

in my house cleaning adventures i found a box of rivet tailings so i've wired up 2 of them between gnd and adc2 and from adc2 to v+ i've got a 10k resistor

when the soil is dry i get adc readings around 150 to 200
the dryer it goes the lower the reading and after i watered it the readings peaked at 350

which is pretty much what i want
the aluminium tailing only have to last 2 months
 
#20
That Gypsum Block article was interesting.

DPG has shown that making a basic resistance meter is a piece of cake but I can't help wondering if some enterprising person can make a cheap/reliable non-contact method.

Simple resistance (AC&DC) methods have been around for years so this is nothing new and, in fact, have been discussed on this Forum aeons ago along with the potential problems. Come on guys... get inventing something new/reliable/low-cost.
 
#21
ok i've found somthing that seems to work very well

in my house cleaning adventures i found a box of rivet tailings so i've wired up 2 of them between gnd and adc2 and from adc2 to v+ i've got a 10k resistor

when the soil is dry i get adc readings around 150 to 200
the dryer it goes the lower the reading and after i watered it the readings peaked at 350

which is pretty much what i want
the aluminium tailing only have to last 2 months
The trouble with the resistance approach seems to be that any change to the chemical composition of the soil will mean recalibration. If you apply fertiliser, for instance, the conductivity of the ground will change.
 
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