cheap conductor materials other than metal

[Whales]

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

Wikipedia: why I can now casually bring up the words aqua regia and aqua vitae in conversation.  Also how I plan to earn my millions: throw aqua regia at people wearing exotic jewellery and collect what remains of them in a puddle on the floor for further purification.

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

I've used pencil graphite for low-value (<100ohm, depending on the make) variable resistors before.  The difficulty is soldering to them: unless you want to wrap a wire around it tightly or use an alligator clip, the only practical joining method is conductive glue.

Stainless steel does seem to be the most available solution, however be warned how much the alloys vary.  Somethings labelled as 'stainless' are poor in quality and will rust through like iron, just slower, whilst others will shine forever.  Ditto for surface corrosion layers: different sources of stainless steel will behave in different ways -- make sure you try another source if you at first get failure.

[fcb]

I'm not sure why you are looking at alternatives to stainless steel.  You'll end up with it in the end.

316 grade is used in marine, medical implants, industrial chemical handling etc.. (there are different grades of 316 BTW). 316 costs 5-10% more than more basic grades, but it's all pretty cheap.

As for how to seal it in the casing, I would use o-rings to seal it - lots of different grades again (VITON, rubber, butyl, nitrile, etc...) cheap and reliable.

To make the electrical connection, you could thread the ends and use a ring terminal, crimps are another good way, even use a piece of choc-strip. Even battery springs will work well and are cheap (god love Keystone Electronics).

[tx8]

googled for this blumat... was not aware about it... Was always wondering what all these things are. Interesting... though I wonder if changing amount of water is easy... will read about it. Thanks!

There is a little turnable cap on top. Once you know how to make the initaial set up u dont need to change anything till harvest. It serves the plant always the right amount of water. btw i am talking about the deep tank variation where you have the water tank 1m< above the crops.

trying to resume what others suggested. I deduced that the gold coated stainless steel is better choice.
Found those gold plated chain tabs, why are they not good, you think? I assume that behind the plate is stainless steel, or, and they have already the hole for a screw.

They are made from brass not stainless steel. If that info on that site you postet is correct. Its a copper zinc alloy.

[SeanB]

The chain ends are thin brass sheet with a very thin gold coat. It may be gold coloured but is likely to be very porous as it will be very thin. It will corrode green in a matter of weeks in salt water.

[Whales]

ok, ok,... today I came with another idea. Was just canibalizing an old computer for it's power supply and by curiosity unmounted the processor, and bingo! The pins are supposed to be gold plated or? It is an intel pentiom III. Now a fine wire could be soldere to the end that attaches to the processor. The terminals of the two pins (the soldering) should be covered with epoxy. The full thing into gipsum so that something provides the correct humidity grade to the small probe.

what is your opinion?

Probably too small compared to the size of dirt granules.  Contact will be intermittent.

[Witention]

It's not every metal that's a good conductor! There are metalloids and also non metals that are bad conductors.

[mahjongg]

want two carbon rods with metal caps? simple, just take two old batteries (non rechargeable) and remove the outer zinc shell and the lignite electrolyte, and whats left over is a carbon rod with a metal top!

[IanB]

indeed, that is correct, that is almost the same category as graphite. Just that it is too big, and I would need to mess up with several batteries. I also realizeed that I was not clear with the design forces. I am thinking here about small probes that are practical in very small gardening surfaces.

The probes will need to be inserted into the soil to a depth of 5-10 cm. The carbon rods from an AA cell are only about 4 cm long. They are barely large enough.