LaMotte CONDUCTIVITY/TDS METER DA-DS 1939 - Manual (Page 3)

4. Set the “TEMP” knob at 25
o
C, and switch the control knob to the
“CAL” position. The meter should indicate exactly 5,000
micromhos/cm. If not, carefully adjust by using the “STD” knob.
5. Measure the temperature of the sample solution, set the “TEMP” knob
at the mark indicating the temperature. (Note the markings are in
centigrade.) Temperature has a pronounced effect upon conductivity,
and must be monitored accurately.
6. Immerse the probe into sufficient solution to at least cover the bottom
groove of the probe. Make sure all bubbles are removed from the probe.
7. Switch the control knob to the “ON” position and read either
Conductivity in micromhos/cm (top scale) or Total Dissolved Solids in
ppm (bottom scale).
8. After completing the conductivity measurement, switch control knob
to the “OFF” position and rinse probe thoroughly with water, preferably
distilled water and drain.
DETERMINATION OF CONDUCTIVITY &
TOTAL DISSOLVED SALTS FOR SOIL
An approximation of the Total Dissolved Salts (TDS) in soil is obtained by
making a soil extraction with distilled water and measuring the TDS of the
soil extract. For purposes of comparison, all TDS values are recorded at
25
o
C.
PROCEDURE
1.
Fill a 50 mL beaker full with the soil to be tested. Tap it lightly to
eliminate any air trapped. Strike the excess soil off the surface.
2. Empty the contents of the beaker into a 250 mL wide mouth flask and
add 100 mL of distilled water.
3. Stopper and shake vigorously. Allow to stand for thirty minutes. During
the thirty minute waiting period the sample should be shaken
vigorously three or four times.
4. Filter the contents of the flask and collect filtrate in suitable
conductivity chamber.
5. Take TDS reading according to method given for General Operating
Procedure.
If the Soluble Salts as determined above are greater than 1,000 ppm, the
chlorides and sulfates should be determined in order to learn whether the
Soluble Salts are chlorides or sulfates. Usually in calcareous soils, the
sulfates represent gypsum and cause little injury to plants, while high
concentrations of chlorides might result in damage to the plants.
Greenhouse and sensitive plants may be damaged if the Soluble Salts are
over 500ppm of chlorides. Most other plants will grow well in soils
containing less than 1,000 ppm chlorides.