On Sat, Mar 17, 2012 at 10:38:23AM -0700, Jerry Scharf wrote: > Mads, > > I prefer RF impedance measurements of total available water over the old > tension or gypsum systems. This is where the research community has > moved. My problem with the older technology is that it measures the > instantaneous water availability rather than the soil stored water. You > really want to know if you will have enough water for the next day if > you don't irrigate now, and water capacity is a better measure of that. > With the high frequency model, they are both volume and salinity > insensitive, what you need for your pot measurements. They also read in > 1-2 seconds, which is nice if you want to move them for a quick reading. > > The cheapest ones I have found are vegatronix > (http://vegetronix.com/Products/VH400/) at $37 each. They are a 3.3-20V > drive and 0-3v out with published curves, so it should take very little > to connect them up to a 1-wire voltage reader and apply the curves in > software. You can even read them off a multimeter. > > With any of these, you have to still learn a bit about your soil, > climate and plants to understand how to turn a reading into a meaningful > decision. 25% water volume could be 2 days worth or not make it a day > depending on soil, climate and plant demand.
I'm pretty sure these use the same technique as this: http://njhurst.com/electronics/watersensor/ with a little thought you can build one of those for about $1 + a microcontroller. At some point when I've got time (hehe) I'm going to reimplement that with an arduino or tinyAVR. If you don't care for high precision and low drift you can implement the whole thing in software. njh ------------------------------------------------------------------------------ This SF email is sponsosred by: Try Windows Azure free for 90 days Click Here http://p.sf.net/sfu/sfd2d-msazure _______________________________________________ Owfs-developers mailing list Owfs-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/owfs-developers
