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Topic: How to build a non-corrosive soil moisture sensor? (Read 4074 times) previous topic - next topic

sirch



So we had better start throwing out all our stainless steel cooking pots then!


Only if you're using the pan to electrolyze your food.


:D  :D  :D

Simpson_Jr


I'm only using milliamps and only for a few milliseconds at a time so hopefully I'm not killing anything, but it's a good point that I hadn't considered. Any suggestions for more suitable electrode material?


If you can find some thick "wood-less" pencils and scrape of the plastic, graphite conducts electricity and is inert...
Dunno what electricity does to the soil though...

Personally I also check the metal pins for just a few ms, but asked myself the same question about safety.

That's why I  included the second link in my previous message http://jeelabs.net/projects/cafe/wiki/Soil_moisture_sensor.

Encased in plastic its electrodes behave as variable capacitor, changed by the amount of h20 in the soil. 
There's no... contact between electrodes/soil.

PeterH


some thick "wood-less" pencils and scrape of the plastic, graphite conducts electricity and is inert.


It's very fragile though and hardly likely to survive being shoved into soil. The most promising suggestion I've found so far is a tinned copper wire.
I only provide help via the forum - please do not contact me for private consultancy.

elac

Using stainless steel should not leech a toxic amount of chromium into the soil.
Plants and animals (including humans) need chromium.
The size of your electrodes will be small a few grams. Even if the stainless steel you use has upwards of 20% chromium, with the amount of current you will be passing through it will take more than your lifetime to leach out all the chromium.
The minute amounts that leach out over time with use will be dissipated with rain and watering also the plants will use it.
The current in the soil has more of an adverse effect on plants than the leached chromium.
Roots don't like to grow where electricity is present.
But neither is enough to "harm" plants in this project.
It's all about the skills

Paul__B


Using stainless steel should not leech a toxic amount of chromium into the soil.


That is of course, why it is "stainless" in the first place!

MarkT


I have tried soil moisture sensors before, but not for an extended period of time (in my garden). You seem to be saying that a DC current will cause corrosion over a time, and effect the reading? But AC will not? Reversing the polarity reduceses the corrosion?


Nearly all metals will dissolve if used as an anode, gold and some of the platinum metals
are the least reactive.

Most metals will corrode with time on exposure to air water and acids, so inert electrodes
are going to last a lot longer.  Carbon is another inert option - two rods from old zinc-carbon
batteries would work.

Using low voltage AC substantially reduces electrolytic action and crucially cancels out
any stray electrochemical potential between the probes (which would be a major source
of error for low-voltage DC).  You can also take differential measurements with DC, turning
the current on and off and noting the change in voltage (this also cancels electrochemical
offsets).

The lower the total charge flow (lower current for shorter periods of time) the less you
have to worry about electrolytic action.
[ I won't respond to messages, use the forum please ]

Simpson_Jr

#21
Oct 25, 2013, 12:21 am Last Edit: Oct 25, 2013, 12:23 am by Simpson_Jr Reason: 1


some thick "wood-less" pencils and scrape of the plastic, graphite conducts electricity and is inert.


It's very fragile though and hardly likely to survive being shoved into soil. The most promising suggestion I've found so far is a tinned copper wire.


I don't think a  7-8 mm or 3/10 inch thick graphite rod will break very fast unless the ground is quite hard. If so there's still no problem if you  punch a hole first.


Carbon is another inert option - two rods from old zinc-carbon batteries would work.


Should indeed work, but most battery chemicals are harmful to both humans and... plants.  
I don't know how much influence it may have on a plant, different varieties will probably cope better,  but use battery rods  with non-food plants only.

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