AVR-Chat

Some Geophisics Theory:
 
Some time ago one scientist called Archie found:
 
F = Ro/Rw 

F= Rock formation factor
Ro=Rock Resistivity
Rw= Resistivity of fluid in the rock
 
also F =        a   
           -----------
                  b
                P
 
where: a = 0.65 for sandstone or 0.81 for limestone
           b = 2 for sandstone  or 2.15 for limestone
           P = Rock Porosity
 
in summary: the rock resistivity depend on the fluid in the rock 
porosous space .
 
Then if you want, you can "Calibrate" your meter with a water 
saturated rock and the same rock dried, and measured the water 
resistivity.
 
this procedure is used in Oil Logging Services for more the 50 
years...
 
 
sorry for my bad english...
 
 








--- In AVR-Chat@yahoogroups.com, Sandu Teo <teodorstv@...> wrote:
>
> Hello again,
>    
>    
>   For my project,  I need low accuracy. 
>   I just want to know when my little apple tree or plants that I 
have  in the house need water.
>   Just one type of soil.
>    
>    FTDI USB to serial converter  sounds very cool.
>   
> 
> 
> Zack Widup <w9sz@...> wrote:
>           
> I haven't spoken out about this yet but I thought I'd throw in a 
few 
> words. I work for a geotechnical engineering firm that has a soils 
> testing lab. I spend quite a bit of time doing all sorts of soil 
and rock 
> tests, including moisture contents.
> 
> I never saw it mentioned exactly why the electronic means of 
measuring 
> soil moisture content was needed or what kind of accuracy was 
desired. 
> But I can tell you this - there are so many different kinds of 
soils which 
> all have different characteristics and read differently that you 
just 
> can't do it with much accuracy.
> 
> I've worked with silts, clays, silty clays, clayey silts, clays 
with 
> little to some sand, etc. - I've seen everything you will find on 
the 
> Burmister and Unified Soil Classification charts. I once had the 
idea 
> that we might be able to measure moisture content electronically. 
After 
> spending some time at it, I concluded it wouldn't work. If it did, 
ASTM 
> would have a techniquue outlined for it. They don't.
> 
> You might be able to calibrate ONE sample of ONE type of soil for 
> electronic moisture content measurement, but take some soils from a 
few 
> feet away and it will be dfferent. The only way to accurately get a 
> measurement of moistuire content is to apply the ASTM standards, 
which 
> involve weighing the soil with moisture, baking the water out of it 
and 
> reweighing.
> 
> I hate to be the bearer of bad news but I do this profesionally and 
I've 
> been there, done that.
> 
> Zack
> 
> On Thu, 18 Oct 2007, Thomas Keller wrote:
> 
> > Show me soil with pure, distilled water in it.
> >
> > Look, I am just telling you what will work. There is no way for a 
simple
> > test unit with a probe to tell the difference between water or 
high metallic
> > content in the soil. I can tell you that dry soil, regardless of
> > mineral makeup
> > will have a MUCH higher resistance than wet soil.
> >
> > Ralph Hilton wrote:
> >>
> >> On Thu, 18 Oct 2007 16:16:18 -0500 you (Thomas Keller
> >> <tjkeller1@... <mailto:tjkeller1%40alltel.net>>)
> >> wrote:
> >>
> >>> Basicly, the resistance of soil is based on moisture content, 
because it
> >>> is the moisture
> >>> that provides the conductivity from particle to particle, not 
the
> >>> mineral or other
> >>> dry content of the soil.
> >>>
> >>
> >> I'd suggest testing it in practice. It doesn't work that way. 
Pure
> >> distilled
> >> water has a very low conductivity. The dissolved salts in the 
water
> >> bring about
> >> the conductivity. That's the principle on which a TDS meter is 
based.
> >> (total
> >> dissolved solids).
> >>
> >>> avrFreak
> >>>
> >>> Ralph Hilton wrote:
> >>>>
> >>>> On Thu, 18 Oct 2007 12:27:59 -0500 you (Thomas Keller
> >>>> <tjkeller1@... <mailto:tjkeller1%40alltel.net>
> >> <mailto:tjkeller1%40alltel.net>>)
> >>>> wrote:
> >>>>
> >>>> I was wondering how a simple resistance measurement could
> >>>> differentiate between
> >>>> a moist soil with low nutrient content and a drier soil 
containing a
> >>>> lot of
> >>>> soluble nutrients.
> >>>>
> >>
> >> --
> >> Ralph Hilton
> >> http://www.ralphhilton.org <http://www.ralphhilton.org>
> >> C-Meter: http://www.cmeter.org <http://www.cmeter.org>
> >> FZAOINT http://www.fzaoint.net <http://www.fzaoint.net>
> >>
> >>
> >
> >
> 
> 
>                          
> 
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