Go Back   Chemistry Help Forum > Lobby > New Users

New Users New to CHF? Post up here and introduce yourself!

Reply
 
Thread Tools Display Modes
Old Mar 29th 2013, 08:21 AM   #1
chemHelpTim
Junior Member
 
Join Date: Mar 2013
Posts: 3
chemHelpTim is on a distinguished road
anode erosion of a moisture probe

I'm sorry, but I'm not a chemistry student so I'm not sure where to post this question but hope that someone here will be able to help me out.

I'm working on a project in which I'm using a basic moisture sensor that I've made by sliding an insulated wire inside of a copper tube in such a way that the insulated wire with it's insulation extends past the tube about 1/8". That is then poked into the ground. On the other end of the copper tube I connect a wire with a 2k resistor to ground. I send 5 volts through the wire and take a voltage reading at a point before the 2k resistor - a basic voltage divider. All that's fine and is not a problem. When the ground is moist I get a reading and when it's not I get zero volts because what voltage does make it through goes to ground.

The problem I'm having is the decay of the anode (the copper wire tip). In this case, the copper wire is acting as an anode, with the soil moisture (because of the salt content) acting as an electrolyte. I tried various metals to replace the copper wire and they all eventually erode. So then I replaced it with piece of .5mm mechanical pencil lead (graphite) which I crimped into a tiny copper tube and sealed with petroleum jelly and shrink wrap to protect the copper tube from moisture so that all anode activity is upon the graphite that extends past the shrink wrap and wa-la, no corrosion and all works great. The only problem is the graphite tip is too fragile so it can be easily broken off.

My question for you all, since I know little about the ion exchange or whatever is going on to deteriorate all but the graphite tip, is what else can I use that would conduct electricity in the same way to form my voltage divider circuit without being fragile like the graphite yet also not corrode.

You can kinda see how I'm using this in one of my YouTube videos at http://www.youtube.com/watch?v=QembR...Lm87uj&index=2 If you go to 2:14, you'll see a typical moisture meter probe which I started with before using the copper wire inside the tube, both of which preceded the use of the graphite. If you go to the picture at 2:34 and look to the lower right hand side of the controller box, you'll see the probe inserted through the wood frame into the soil.

Please help, or direct me to a place in this forum where I might receive a solution.

Thanks,
Tim

Last edited by Unknown008; Mar 29th 2013 at 10:38 AM. Reason: Youtube link not working
chemHelpTim is offline   Reply With Quote
Old Mar 29th 2013, 10:41 AM   #2
Unknown008
Senior Member
 
Unknown008's Avatar
 
Join Date: May 2010
Location: Mauritius
Posts: 1,685
Unknown008 will become famous soon enough
What other metals did you try? I would suggest stainless steel, but that's only from theory and I never really tested it in-life as a probe.
__________________
Jerry (Working, studying, writing and drawing :3)
It is easier to protect your feet with slippers than to cover the earth with carpet.
No one can go back and change a bad beginning; but anyone can start now and create a successful ending.

If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
Thank you for leaving a reputation to my post

Unknown008 is offline   Reply With Quote
Old Mar 29th 2013, 12:37 PM   #3
chemHelpTim
Junior Member
 
Join Date: Mar 2013
Posts: 3
chemHelpTim is on a distinguished road
Thanks. As a non-chemist but having worked with metal, stainless steel was my first thought too but it still eroded but not nearly as fast as the copper. The graphite doesn't seem to erode even the slightest.
chemHelpTim is offline   Reply With Quote
Old Mar 30th 2013, 06:52 AM   #4
Unknown008
Senior Member
 
Unknown008's Avatar
 
Join Date: May 2010
Location: Mauritius
Posts: 1,685
Unknown008 will become famous soon enough
Yes, graphite is very inert for its purpose here. The thing is that metals react with electrolysis, or other elements do. With a usually inert metal, the metal will instead dissolve. With a too reactive metal (on the complete opposite of the scale I guess you can say), it will dissolve fast enough without needing the electricity.

Have you tried aluminium?

Aluminium, while there's a possibility it doesn't dissolve/corrode into the soil might instead get an insulating layer after some time, so you will end up cleaning it now and then. But I would assume that's better than having to replace it completely.

I am not much familiar with alloys, but from this wikipedia list, it seems you could try Kennethinium.

Good luck!
__________________
Jerry (Working, studying, writing and drawing :3)
It is easier to protect your feet with slippers than to cover the earth with carpet.
No one can go back and change a bad beginning; but anyone can start now and create a successful ending.

If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
Thank you for leaving a reputation to my post

Unknown008 is offline   Reply With Quote
Old Apr 2nd 2013, 06:24 AM   #5
chemHelpTim
Junior Member
 
Join Date: Mar 2013
Posts: 3
chemHelpTim is on a distinguished road
Thanks for the additional info. No, I didn't even think to try aluminum. That could present some other problems though since I need to connect a copper wire to it at some point and that point could create a thermo-couple effect more so than with perhaps with stainless steel? Something that someone elsewhere suggested, which was pretty basic yet had eluded me for some reason, was to just increase the values of the resistors forming my voltage divider. Increasing by a factor of just 100 would also decrease the erosion rate of the anode by the same. So, I could probably go back to the stainless steel with that in place.
chemHelpTim is offline   Reply With Quote
Old Apr 2nd 2013, 08:49 AM   #6
Unknown008
Senior Member
 
Unknown008's Avatar
 
Join Date: May 2010
Location: Mauritius
Posts: 1,685
Unknown008 will become famous soon enough
Ah, yes, that does make sense, and I haven't thought about it either. I'd be glad to hear about how it goes about
__________________
Jerry (Working, studying, writing and drawing :3)
It is easier to protect your feet with slippers than to cover the earth with carpet.
No one can go back and change a bad beginning; but anyone can start now and create a successful ending.

If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
Thank you for leaving a reputation to my post

Unknown008 is offline   Reply With Quote
Reply

  Chemistry Help Forum > Lobby > New Users



Thread Tools
Display Modes



Facebook Twitter Google+ RSS Feed