Article 18764 of rec.gardens:
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Newsgroups: rec.gardens
Subject: Re: soil Electrical Conductivity -- measure of salinity?
Message-ID: <1993Jun18.144738.1@rhea.arc.ab.ca>
From: thacker@rhea.arc.ab.ca
Date: 18 Jun 93 14:47:38 MDT
Followup-To: rec.gardens
References: <1v5kc1INN5c4@network.ucsd.edu> <1993Jun11.091419.1@rhea.arc.ab.ca> <1993Jun16.001530.18343@miavx1.acs.muohio.edu>
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In article <1993Jun16.001530.18343@miavx1.acs.muohio.edu>, vgblackwell@miavx1.acs.muohio.edu (VIC BLACKWELL) writes:
> 
> Hmmm!  Lets see now, conductivity is the recipical of resistance.
> Can I use an ohmmeter, take an ohm reading and then   G=1/r ?
> What about the cross-sectional area of the sample.  That would have affect
> my readings.
> 
> Could you provide a little more detail of the actual test?

I've always just used a conductivity meter and read the value directly.
(And I must confess that it's been a few years since I've made the 
actual measurements myself.)  If you want to play with the procedure 
and substitute an ohmmeter for a conductivity meter, I'll give you 
this procedure from the research branch of Agriculture Canada 
(Analytical Methods Manual 1984, B.H. Sheldrick, Ed.).  
I think you can modify this to use your ohmmeter:

"Most conductivity meters have a logarithmic scale marked in micromhos
and also a range switch giving a number of multiplication factors which 
are powers of 10 so that the meter can measure specific conductivities
from 1 to 100,000 micromhos.

"Check the cell constant and meter performance by obtaining scale readings
for each of the standard KCl solutions (0.002 N, 0.005 N, 0.01 N, 0.05 N).

"Calculation of cell constant:
Let B be the range or multiplication factor.
Let E be the dial reading.
Let R be the electrical resistance in ohms, obtained with a standard
KCl solution be B x E.
Let S the electical conductance in mhos be 1/R
Let M be the published conductivity value in mhos (see table of specific
conductivity values for KCl solutions) of the standard KCl solution at the 
temperature of the reading.
Let C be the cell constant

Example: (For conductivity bridge, model RC)
B=100
E=65
R=100x65 = 6500
Temp = 28C
KCl = 0.002 N
S = 1/6500 = 153.8 E-6
M (from table) = 310 E-6

C = 310 E-6/153.8 E-6 = 2.02 mmhos/cm (or dS/m)

Calculations of unknown solutions:
Let B, E, S, and C have the same meaning as above
Let R the electrical resistance in ohms, obtained from the unknown solution
be B x E
Let T be the temperature of the unknown solution.
Let F be the temperature factor (this is obtained from a table, in 
this example it is 1.112 at 20C, but it is designed only to convert all values
to a standard reference temp of 25 C - much easier to just make sure
your solution is near 25C, and then the conversion will be close to 1.0.)

Conductivity at standard 25C = SxFxC = FxC/(BxE) mhos/cm
values are usually reported in mmhos/cm (or dS/m) so add a conversion
factor of 1000, then Conductivity = FxCx1000/(BxE)

Example:
E=245
B=10
Temp. = 20C
F (from table = 1.112)
C=2.02
   dS/m = 1.112 x 2.20 x 1000/(245x10) = 0.92 dS/m (or millimhos/cm)

Have fun!

Don


