cheap conductor materials other than metal

[pereczes]

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[XOIIO]

Graphite, and I think quartz to some extent though not much.

Nether of those would suit a moisture probe though.

[poor_red_neck]

Inexpensive is relative... lol.

How about carbon fiber? You can find carbon fiber rods at hobby stores (Remote Control hobby, MAYBE Michael's or AC Moore). I fly R/C helicopters (the large, collective pitch type, not the "toys" at the mall) and our frames are made of carbon fiber, and QUITE conductive... especially when you don't sand down the edges and the 48V 45C 5000mah LiPo shorts across the frame... let's just say it handled a fairly substantial amount of current before puffing a few cells and burning the frame/wiring.

[Tac Eht Xilef]

The problem is that any metal you can have around corrodes sooner or later, rather sooner.

The question I am asking might make you laughing, it already makes me: is there any material solid/paint like etc that has a reasonable conductivity, water resistant etc that can be used to construct a soil moisture probe?

Graphite, as XOIIO said, or titanium. Certain grades of stainless steel, at a pinch. All are commonly used in both cheap and industrial/lab-grade soil moisture sensors, especially for simple conductive-type moisture sensing. (Remember, any DC on the probes will accelerate electrolytic corrosion).

Graphite tends to poison fairly quickly (it adsorbes ionic contaminants). I don't know of carbon fibre rod being used but I'd be suspicious of the same thing happening, given that the polymers binding it are at least nominally porous. Titanium welding rod is probably the most resistant easy-to-get material.

Epoxy/ceramic/glass/sapphire coated capacitive probes are typically used for long-term in-situ environmental monitoring, but aren't particularly cheap.

[Psi]

The problem is that any metal you can have around corrodes sooner or later, rather sooner.

Have you tried a good grade stainless steel?

[miguelvp]

You might want to see Dave's last mailbag

http://wemakethings.net/2013/06/07/chirp-plant-watering-alarm/

Dave's part of the video where he opens the device:



It's open hardware but you can always buy one here for $15:
https://www.tindie.com/products/miceuz/chirp-plant-watering-alarm/

Not what you want to do maybe, but it should give you some insight on what to use to sense the moisture.

BTW I think this one activates when dry but it should be easy to do the opposite.

Edit: and they provide the source for the firmware as well

[Psi]

electroplate the stainless/titanium with something you can solder to

[Phaedrus]

As much as it's used as a technowank gimmick, electroplated gold should actually be pretty good at resisting corrosion. The electroplating process involves plating your base metal with copper (unless it already is copper), then nickel, then gold. The nickel and gold layers are both extremely corrosion resistant, so as long as the coating is good quality and isn't subjected to too much mechanical wear you shouldn't see any corrosion. Plus you can solder to it, though your joint *will* be subject to corrosion.

[Phaedrus]

Don't try to electroplate gold at home. It requires noxious chemicals like boric acid and aqua regia. The only metals I'd recommend for home electroplating are copper and tin since these can be done with acetic acid and low-strength muriatic acid respectively.

You can probably find a company nearby who will do electroplating, but pricing is a bit of a crapshoot.

[Phaedrus]

Don't try to electroplate gold at home. It requires noxious chemicals like boric acid and aqua regia. The only metals I'd recommend for home electroplating are copper and tin since these can be done with acetic acid and low-strength muriatic acid respectively.

You can probably find a company nearby who will do electroplating, but pricing is a bit of a crapshoot.

not sure what you mean "noxius": dangerous, forbidden, toxic?

well, was thinking in doing it in the celler. Though there is not so good ventilation, but could put a ventilator. Do you think that would work? I will start anyway to "burn" pcb's etc. there.

Dangerous, toxic, fuming, corrosive, potentially explosive.

Correction, you don't need boric acid for the nickel plating, you need sulfuric acid. You also need aqua regia (mixture of hydrochloric and nitric acids) to electroplate gold.

Not saying that you can't do it at home; but it will shorten your life expectancy and probably give your life/medical insurance agent an aneurysm. There is both short term danger (acid spill, poisoning, fume inhalation, explosion, electric shock) and long term (lung damage, cancer).



Find someone to do the plating for you. If it's not too expensive, plating could be a good solution. Stainless steel is ok, but keep in mind it's not really stainless; given enough moisture and the presence of mild corrosives (such as those found in soil) stainless steel will still rust. Just, more slowly than carbon steel or copper.

[Phaedrus]

Zinc is a sacrificial metal, it corrodes easily. It's mainly just used as a coating over more durable materials on the basis that it will take five or ten or twenty years before the half inch of zinc rusts through and the metal underneath is attacked.

[Zero999]

Hi,

the same topic: soil moisture. the same paradigm: cheap

well, the problem is not only the price. The problem is that any metal you can have around corrodes sooner or later, rather sooner.

The question I am asking might make you laughing, it already makes me: is there any material solid/paint like etc that has a reasonable conductivity, water resistant etc that can be used to construct a soil moisture probe?

Poor design is the main thing which causes corrosion in moisture detectors. If you use AC to measure the conductivity, make sure there's no DC path of current between the electrodes and use the same type of metal for both electrodes, there will be a much lower risk of corrosion.

[tx8]

bronze?

I see those green covers of some churches.... What is that green thing?

The green patina is an oxidized layer on the outside so Bronce will look pretty and will not corrode further, i think.

[yashrk]

Try painting rod with galvanized paint on the metal 

[tx8]

bronze?

I see those green covers of some churches.... What is that green thing?

The green patina is an oxidized layer on the outside so Bronce will look pretty and will not corrode further, i think.

that is a good point. Though the oxid might disolve easier in moisture than in air. Question is how healthy is that for the earth. (Will eat the growing tomatos )

Yeah for your purpose its a bit iffy. The thing is for many purposes the blumat is very usefull and it doesnt need any electronics at all but it just in this moment came to my mind it would be super awesome to have a sensor you can put on top of the blumat that will measure the pressure and can then trigger a pump. I never thought of that altough i though of it alot in the past when i was doing some garden automation stuff. I think it would be a top product. Since there is alwasy is the issue with the corrosion and electronics and in this case there would be no contact of the water to the electronics itself....    Aesome maybe we should kickstart it....  O0

One issue in general though with a "pump and sensor" system is if there are serveral sensors on a large scale you need to disribute the water to the right place... so in the end its maybe useless  since the blumat normally regulates that. Hence one would need a system to disribute the water to right place as well.


PPS.: And if it is not a large scale the blumat would do the job just fine withoud any batteries or electronics and so on. I tried that drop blumat and its really good!

[Yago]

Don't try to electroplate gold at home. It requires noxious chemicals like boric acid and aqua regia. The only metals I'd recommend for home electroplating are copper and tin since these can be done with acetic acid and low-strength muriatic acid respectively.

You can probably find a company nearby who will do electroplating, but pricing is a bit of a crapshoot.

not sure what you mean "noxius": dangerous, forbidden, toxic?

well, was thinking in doing it in the celler. Though there is not so good ventilation, but could put a ventilator. Do you think that would work? I will start anyway to "burn" pcb's etc. there.

I had a goggle of "Aqua Regia" and the wiki would be more than enough to ensure it is never near my home!

[SeanB]

The most common water level detectors use AC to drive the probes, via a DC blocking capacitor even though the probes are simply connected to a transformer secondary via a resistor. The probes are typically 308 or 316 stainless steel for use in water other than salt water, because the stainless corrodes badly with chloride ions present. for salt water graphite is preferred, though they often also use marine bronze alloy, as it will not corrode much in sea water.

http://en.wikipedia.org/wiki/Aluminium_bronze

[tx8]

The tank "fitting" connectors are often made from brass. dont know if that can be usefull but its pretty cheap.

http://www.conexmetals.com/gif-1/brass_components_brass_turned_parts_brass_fittings_machined_components.jpg

[Tac Eht Xilef]

Poor design is the main thing which causes corrosion in moisture detectors. If you use AC to measure the conductivity, make sure there's no DC path of current between the electrodes and use the same type of metal for both electrodes, there will be a much lower risk of corrosion.

GRRRRRR... this is a good hint! But makes my life difficult!

Makes me wish I'd mentioned something about avoiding DC and electrolysis earlier. Oh, wait ...

But on the subject of connecting wires to probes: what's wrong with screwing or clamping? Use a screw terminal strip for small diameter probes - wire in one side, probe in the other - or an earth/hose clamp for larger probes. Dip it in plasti-dip to seal it up & minimise corrosion, and Bob's your auntie's live-in lover...

[SeanB]

Dip in plasti seal with moisture makes an electrochemical cell from the different oxygen concentrations if there is a slight separation , and welcome to crevice corrosion which will eat the area under the dip till the probe falls off.

[FJV]

For cheap graphite maybe use a pencil lead.

You even have pencils entirely made of pencil lead without wood.
Diameter about 5mm if I'm not mistaken

Buy the grade with the highest graphite content I guess

[Tac Eht Xilef]

Dip in plasti seal with moisture makes an electrochemical cell from the different oxygen concentrations if there is a slight separation , and welcome to crevice corrosion which will eat the area under the dip till the probe falls off.

Yes, but you will almost always get that to some degree anyway regardless of connection / sealing method.

FWIW, we've got probes built like this - titanium welding rod in nickel-plated brass terminal screw connectors, & sealed in hot plastic-dip - that have been in the field (long-term environmental monitoring of soil adjacent to a coastal estuary - so, very wet & saline) for nearly 4 years now without issue*. It's a matter of how far you need to go for the result you want...

(* edit: well, without much issue. I believe a couple have had to be replaced because some bogan drove over them...)

[Zero999]

GRRRRRR... this is a good hint! But makes my life difficult! AC will hinder the migration of ions! Good point. That would that be true also for titanium? What about gold coated titanium?

But that will make the circuitry much, much more complicated... any hints?

Not really. A simple way to do this is to build an oscillator and connect the output to a peak detector via a resistor with a capacitor going to the probe, so when it becomes wet, the AC signal is short circuited. A similar circuit to that discussed in the thread below should work. You may need to play with the component values and couple the power supply to the water via another capacitor.

https://www.eevblog.com/forum/beginners/easiest-touch-sensorswitch/msg429927/#msg429927

[FrankenPC]

I don't know if anyone mentioned this.  Get 303 stainless steel bolts.  Test them with a magnet.  Good 300 stainless is non-magnetic.  This stuff is incredibly difficult to corrode and reasonably priced. 

[SeanB]

Stainless steels are only non corroding because they build up, like aluminium, a protective oxide coating on the surface. Thus you really do need an AC excitation, as this will keep the coating intact.

With any coating if there is any sharp edge that has a hollow or chip that will create an electrochemical cell. This is the start of surface corrosion and then it advances under the coating. With a zinc coat the zinc corrodes preferably to the exposed steel as it is the anode, but with a massive surface area compared to the steel cathode. When the zinc is all eroded off the steel rust very fast.