Subject: homebrew moisture meter plans ---------- Forwarded message ---------- Date: Fri, 20 May 1994 17:26:29 +0200 From: Willy A Verheulpen Hello this is an attempt to give you some info on how to build a MOISTURE METER! BASIC PRINCIPLE When a metal electrode is put into an electrolyte, then the electrode builds up an electric potential. This is positive or negative depending on the proper physico-chemical property of the metal. Any book on physico-chemistry will let you find the classification of metals according to their "potential". You need two electrodes to make the meter so you have to choose two metals for which the potentials are far enough away from each other because you will connect the top of the two electrodes which will make a tiny current flow between the two. This current is what we are going to measure. Choice of the metal for the electrodes is mostly restricted to what is currently available in your "junk box" or the shop around the corner! PRACTICALS Do not worry to much about the choice of the metals. For one electrode Aluminum or Iron(Steel) should be OK. The other one can be Copper or Brass. Preferences are for Al & Cu. Copper will be the positive pole. You will also need a (small) micro ampere meter. The ones that are used in small audio equipment as a signal strength meter are OK & probably rather cheap. If you want a better reading scale you will have to buy a larger one. price will be accordingly I guess. Range 50 100 or 250 micro amp Full Scale Deflection (FSD) will do. A trimming potentiometer could be usefull for calibration of the FSD. The Cu electrode is connected to the + of the meter (soldered or bolted) the Al to the -. Eventually connect the resistor in between. The general layout of the instrument is the tricky part. Remember however you will have to push it fairly deep into some soil thus using some muscle strength so some mechanical rigidity is needed. The commercially available meters have a central Al electrode with a somewhat sharpened tip. The tubular brass electrode & the insulation slide over this central shaft to make an electrode set of equal diameter over it's whole length. For calibrating just put the set in wet (soaked) soil & trim the resistor for a full scale reading. You should never leave the equipment into the soil as the electrodes will soon "dissolve" due to electric corrosion. The value of the resistor is difficult to give here as it depends on the type/quality of the electrodes as well as the meter. You should have a friend with an electronics hobby :-) I do not know if this is enough info. I never had expected to receive any querries for the making of the instrument as the commercially available ones are rather cheap. BTW electrodes on my commercial one are Al and Brass. The Brass was apparently electro-plated with a "shiny" metal to give it an attractive look I guess HTH, ----------------------------------------------------------- Willy Verheulpen Internet = waverheu@vub.ac.be Brussels Free University Institute for Molecular Biology Phone : +32 2 3590259 Fax : +32 2 3590399 90 Paardenstr 65 B-1640 Sint Genesius Rode Belgium