Re: sensor to measure the conductivity of mud

[Martin Parkes]

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[Thank you, Martin, for your constructive and worthwhile post - (you might
find it making up the text of a sowacs page real soon, if that is alright
with you?!?)
Bruce]

Hello Wendy

Standard soil conductivity measurements are made on a saturated paste.
Typically 300 g of air dry soil is placed in a beaker with distilled water
added until the soil is near saturated. The mixture is allowed to stand for
a few hours and more water is added to form a saturated paste. This
saturation point is a bit subjective but described by:
* paste glistens as it reflects light
* it flows slightly when beaker is tipped
* it slides cleanly and freely off a spatula
* it is consolidated easily by tapping if a groove is formed in the surface.

A solution is then normally extracted from the saturation paste, which is
called a saturation extract.

Obtaining the saturation extract

Place a Whatman No 50 filter paper in a Buchner funnel. Place the paste in
the funnel and apply a vacuum. The filtration is finished when air starts
to pass through the paper. Then add 0.1% sodium hexametaphosphate to the
filtrate ( one drop per 25 ml filtrate). This prevents preciptation of CaCO3.

Measuring electrical conductivity

This is measured using a conductivity meter that determines the current
passing through a solution between two electrode in a 
conductivity cell.
Equipment: Electrical conductivity meter, conductivity cell and
conductivity standards

Making standards. Dry NaCl at 105oC for 1 hour then cool in desiccator.
Dissolve 2.922 g in water and make up to 1 litre. This is 50 mM NaCl.
Dilute by 10 and by 100 to give 5 and 0.5 mM solutions. The resulting EC
values are 5.550, 0.604 and 0.0625 dSm-1 at 25oC or 4.995, 0,543 and 0.0562
dSm-1 at 20oC.

Use the 5 mM solution to calibrate the meter and check it works at the
other two standards. Then the conductivity of the sample can be measured.
Best to work at 25oC, otherwise convert to ambient temperature.

Ref Rowell D L 1994 Soil Science, Methods and Applications, Longman

Meters
(i) Camlab www.camlab or info@camlab.co.uk
  portable meter on its own £520,
  or with case and cell £700
reads conductivity, salinity and TDS/temperature - stores 50 values
(ii) ELE International www.eleint.co.uk or ele@eleint.co.uk
portable conductivity meter code EA430-270

Hope that helps

Martin Parkes

WasteTrim at Technology Transfer Centre, Alrick Building,
Mayfield Road, Edinburgh EH9 3JL, SCOTLAND 
tel (44) 131 472 4708
fax (44) 131 662 4678