As stated above i'm using two soil sensors with arduino to tell me when i should water my plants.
The sensors are basically just two electrodes each, next to eachother with one going to +5v and the other going to an analog in pin.
The problem i'm having is that as current flows, naturally the anode (or was it the cathode? i tend to confuse these terms) corrodes rather fast.
Currently i'm using stainless steel tea spoons for the sake of being readily available and i was wondering if that would be the right material to use to prevent corrosion or if something else would be better suited.
Check out this soil sensor using 316 stainless probes, there's a sketch at the bottom of the page and instructions, maybe you can get some ideas from there.
I did not know that every metal would corrode, what about wolfram electrodes (just being curious)? or something gold plated?
While i do not want to use alternating current because my goal is to keep these sensors as simple as possible, you got me thinking and maybe i'll do 'pulsed' readings in the future. Like apply +5v only every 10 minutes for 10 seconds to get a reading and turn them back off rather than constantly sending electricity through the probe.
The tea spoons are building up a little grey-blackish residue on their surface, is this the result of stainless steel corroding?
I did not know that every metal would corrode, what about wolfram electrodes (just being curious)? or something gold plated?
"Wolfram" is just tungsten, isn't it?
Any metal... it is called electrolytic corrosion, or galvanic corrosion. In contact with a damp environment, it can happen between two dissimilar metals as they generate their own current, or any metals with a voltage impressed upon them (anodic corrision), or even just different parts of the same piece of metal due to slight differences in pH, salinity, minerals, etc. (cathodic corrosion).
It is worse in something with a gold plate, because as soon as you get a pinhole in the gold, it starts corroding the metal out from under the gold, because gold is (I seem to recall) the most cathodic (is that a word?) of all the metals.
Add to that corrosion by alkaline or acidic soil.
Stainless steel isn't really immune to oxidization. It actually builds up an oxide coat in seconds of exposure to oxygen, but when formed in air, it is a very hard, stable coating.
A couple of ideas-
How about an inanimate carbon rod? Seriously, two carbon rods. Cheap, just take apart a couple of carbon-zinc AA cells. Not alkaline.
Insulated electrodes with a large surface area. Use a frequency high enough to make the capacitive reactance a small contributor to the impedance between the plates. You are now measuring the resistance of the soil between the plates.
Essentially what you are doing is measuring the ESR of the capacitor formed by the two insulated electrodes. As the water load of the soil goes down, the resistance should go up.
Comments on Amazon for those tin plated soil moisture detectors are generally just "got it out of the box, works" without any followup. However, one person did post that his was corroded and no longer working after a few months.
And the gold plated variety says right in the description, service life 1 year.
