-----Original Message----- From: Paul B. Webster VK2BZC To: Lawrence Lile ; PICLIST@MITVMA.MIT.EDU Date: Monday, May 10, 1999 5:27 PM Subject: Re: "Plant out of water project" >Lawrence Lile wrote: > >> Problems I encountered when I built one of these: >> 1. Soil resistance changes over time. I would plug my two probes >> into the wet soil, then an hour later it would start beeping. I made >> a little circuit to plot this on my datalogger, and noticed a dramatic >> change in soil resistance. Of course I was measuring with an AC >> signal to prevent corrosion. > Paul; Replied > Do I take this to mean that the (continuous) measurement current (AC >notwithstanding) is causing "polarization" at the electrodes, drying the >soil locally? > I never did understand the phenomemon completely. I set up a 12C508 so it sleeps, takes a measurement, sleeps, etc. etc., and it would invariably alarm after an hour, even in obviously wet soil. A profile of the soil resistance over time looked suspiciously like an exponential RC charge curve (although many things in nature exhibit exponential behavious and this resemblence proves nothing) . It was wierd. > Surely the advantage of the 12C508 is the ability to sleep most of the >time (and even then, execute the test cycle only every so many >"sleeps",) and apply current and make a measurement only every five >minutes or so? >-- > Cheers, > Paul B. I wish this had worked.