How to build a non-corrosive soil moisture sensor?

[marshalladam]

Ive found a lot on how to build a moisture sensor, but not much on people using commercially available options such as http://www.dfrobot.com/index.php?route=product/product&product_id=599#.UhflOL-N_H0 that boasts a gold plating that protects it from the normal corrosion of a DC moisture sensor. I was wondering if anyone has had reliable, long term use of one of these, or if an AC set up might be more suitable. Thank you.

[LarryD]

Would you give more information on what your application is?

[jack wp]

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?

If that's the case, could we use two output pins to provide ground, and +5v to the voltage divider. Set them in the forward direction for 1/100 of a second, take our reading from the center tap. Switch the two outputs to provide a reverse voltage for a calculated time, then wait for 100 seconds, and do it again ? Now the probe is not seeing DC, it is seeing AC.  Just a thought.

[PeterH]

I use a pair of stainless steel cooking skewers driven into the soil about 10mm apart. My measuring technique leaves the two probes unpowered except when I'm actively taking a reading, and powers them briefly to take the reading, them powers them with the reverse polarity for the same duration - the goal of the reverse polarity wasn't to do with corrosion but to stop electrolysis from moving ions around in the soil which I thought might affect the accuracy long-term.

[jack wp]

Interesting Peter.
How long have you been using that?
I really want to test the moisture down deeper than the top inch. I think maybe 6 inches down. Can I put on some heat shrink for the top 5 inches you recon? What do you think?

[holmes4]

You only need to take reading at most every few mins so something cheap like skewers will work for years before corrosion is an issue. I'd use varnish not heat shrink for the parts below ground.

Mark

[Simpson_Jr]

I've tried sensing soil-moisture using just two pins. By only applying a DC-voltage for just 50 mS when I read a sensor every 600 secs, I don't notice... any corrosion.

But you can also try capacitive sensing and use/make electrical insulated sensors.
Can't tell you much about the first link, just discovered it.
The second one looks promising (imo), but may require google translate since most of the info is in German.

http://tuxgraphics.org/electronics/200908/eth-flower-watering.shtml

http://jeelabs.net/projects/cafe/wiki/Soil_moisture_sensor

[PeterH]

Can I put on some heat shrink for the top 5 inches you recon? What do you think?

That's exactly what I did.

[marshalladam]

How would you go about only turning it on for certain intervals? A MOFSET?

[jack wp]

No, just take the reading when you want. There is no power turn on, just read. You will need a pullup resistor, of about 10k, that can run all the time with very little current drain. If you want it to be even lower than almost nothing, you can hook it to a I/O pin, and just make it HIGH when you are ready to take the reading. If you are running off a small battery, that would be worth while doing.

[PeterH]

I have both lines connected to digital I/O lines, so they can be powered up in either direction as required.

[seanspotatobusiness]

I use a pair of stainless steel cooking skewers driven into the soil about 10mm apart. My measuring technique leaves the two probes unpowered except when I'm actively taking a reading, and powers them briefly to take the reading, them powers them with the reverse polarity for the same duration - the goal of the reverse polarity wasn't to do with corrosion but to stop electrolysis from moving ions around in the soil which I thought might affect the accuracy long-term.

I'm not a chemist but I'm pretty sure that using stainless steel as an electrode will release chromium and in some forms this is toxic. Since the currents are going to be really small, perhaps it's not as big a problem as it would be if you were using it in electrolysis or plating but I'd advise against stainless steel electrodes until someone of appropriate education could give an all clear. My own interest is in a system to remind me to water my herbs so the possibility of chromium release is a nopety nope for me.

[PeterH]

I'm not a chemist but I'm pretty sure that using stainless steel as an electrode will release chromium and in some forms this is toxic.

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?

[Paul__B]

I'm not a chemist but I'm pretty sure that using stainless steel as an electrode will release chromium and in some forms this is toxic. Since the currents are going to be really small, perhaps it's not as big a problem as it would be if you were using it in electrolysis or plating but I'd advise against stainless steel electrodes until someone of appropriate education could give an all clear. My own interest is in a system to remind me to water my herbs so the possibility of chromium release is a nopety nope for me.

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

[PeterH]

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.