Graphite rods for moisture sensor?

I am not an electrical engineer but a chemist,

I was wondering if anyone has considered using hard graphite, such as used in motor brushes to make a resistance soil moisture sensor.

Standard graphite is too soft but hard graphite should be strong enough to use as long as the sensor is not bashed into the ground

Comments please

A chemist should have learned that DC current flowing between two electrodes in a conductive media will cause various ions to build up on the electrodes and eventually limit the flow of electricity.

Doesn't matter if the electrodes are carbon or platinum or copper or silver or gold.

Paul

These will eventually crumble in the moist electrical environment.

.

Although they are very popular, people soon learn that resistance sensors just don't work to measure soil moisture.

The capacitive sensors in principle don't suffer from corrosion, but also have the problem that the immediate sensor environment may not be representative.

Worth a try though. You can buy a cheap one here, try out the Chirp (which seems more promising) or experiment like this person did.

Let us know what works best!

DC current flowing between two electrodes in a conductive media will cause various ions to build up on the electrodes and eventually limit the flow of electricity.

That is why many soil moisture sensors use alternating polarities during operation.

Many people have made usable sensors using stainless steel "skewers" from the supermarket.

Here's another link: http://gardenbot.org/howTo/soilMoisture/#The_local_circuit_-_simple_voltage

Paul_KD7HB: A chemist should have learned that DC current flowing between two electrodes in a conductive media will cause various ions to build up on the electrodes and eventually limit the flow of electricity.

Doesn't matter if the electrodes are carbon or platinum or copper or silver or gold.

Paul

You mentioned DC, not the OP.

MarkT: You mentioned DC, not the OP.

I did! And the OP mentioned resistance measurements. Have seen non done with other than DC.

Paul

First any electrode in soil is not going to be permanent as soil is an "active" matrix and anything used will eventually fail.

Graphite electrodes are used all the time in generating hydrogen in caustic (0.4m NaOH) solutions, with a voltage higher than 1.23V, (usually around 2 volts) and 0.5 amps per square inch of electrode surface. these graphite electrodes last months in this caustic solution. So the thought is that using these in soil they should last significantly longer. Also graphite rods are cheap if you purchase them on eBay

As it is the breakdown of the graphite rod binders on the anode side my thought was that these electrodes would last longer and not emit any toxic elements from the decomposition of the anode. For DC resistance measurement only the anode would have to be graphite the cathode could be iron or some other conductive material

If used in a a potted container (with drainage) any buildup of accumulated degradation products should wash away with each watering of the container. (pending completed wetting of the soil in the container)

As it is autumn here and growing season is coming to an end I wanted to see if anyone ever experimented with graphite rods as probes.

I have ordered a few and I plan on testing their longevity on some indoor plants when they arrive.

Paul_KD7HB: I did! And the OP mentioned resistance measurements. Have seen non done with other than DC.

Have a look at the recent discussion about EC/TDS measurements. Those are resistance measurements done using AC.

In principle you can use the same method to measure soil moisture content.

dhmeiser: First any electrode in soil is not going to be permanent as soil is an "active" matrix and anything used will eventually fail.

Graphite electrodes are used all the time in generating hydrogen in caustic (0.4m NaOH) solutions, with a voltage higher than 1.23V, (usually around 2 volts) and 0.5 amps per square inch of electrode surface. these graphite electrodes last months in this caustic solution. So the thought is that using these in soil they should last significantly longer. Also graphite rods are cheap if you purchase them on eBay

As it is the breakdown of the graphite rod binders on the anode side my thought was that these electrodes would last longer and not emit any toxic elements from the decomposition of the anode. For DC resistance measurement only the anode would have to be graphite the cathode could be iron or some other conductive material

If used in a a potted container (with drainage) any buildup of accumulated degradation products should wash away with each watering of the container. (pending completed wetting of the soil in the container)

As it is autumn here and growing season is coming to an end I wanted to see if anyone ever experimented with graphite rods as probes.

I have ordered a few and I plan on testing their longevity on some indoor plants when they arrive.

No need to order graphite rods. You probably have some in your house right now. The "D" or "C" cells use a graphite rod in the center as the + electrode. Just a bit messy to get at.

Also go to a local welding supply and ask for carbon arc electrodes. They will have a copper coating, but that can be etched away leaving a nice hard carbon rod.

Paul

Paul_KD7HB: No need to order graphite rods. You probably have some in your house right now. The "D" or "C" cells use a graphite rod in the center as the + electrode. Just a bit messy to get at.

Also go to a local welding supply and ask for carbon arc electrodes. They will have a copper coating, but that can be etched away leaving a nice hard carbon rod.

Paul

Hey thanks for the idea, not certain about using the battery rods in an actual plant, but I can use them for experimentation.

I also went to the local hardware store and found some graphite motor brushes and am going to experiment with those as well.

I am collecting the data using CoolTerm to be able to examine it in excel