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 Welcome to the second message of the Soil
  Water Content Sensor discussion group!

**********     (SoWaCS group )    ************

     V O L T A M M E T R I C S E N S O R S
           (NB: not volumetric!)
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                      23 April 1996

Over and above the TDR discussion, this file details a new type of
sensor that seems to have not yet been "discovered".

I first read of it in the TWRI newsletters, and my inquisitiveness
(and net surfing) finally paid off when the author sent me a copy of
his paper:

>Huiliang Huang and Purnendu K Dasgupta.  1995.  >Fast voltametric
[NOT volumetric} sensors for the measurement of soil water
>activity.  Electroanalysis 7:7 pp 626-632

I quote from the abstract:" The sensor consists of a blunt ended
hypodermic needle cathode containing concentric PTFE insulating tube
and therein, a Pt-wire anode.  The sensing film is formed by
dip-coating a 1:7 blend of a perflourosulfonate and a
polyestersulfonate ploymer at the tip of the needle.  Atop this
sensing film is a composite protective film of cellulose acetate and
nylon that inhibits the passage of ionic or low MW constituents and
allows the transport of water....  The sensor is capable of making
reliable moisture measurements between the wilting points and the
field capacity of the soil sample: the response behaviour consists
of the combination of two nearly linear segments, the sensor output
increasing steeply between wilting point and ca.  60% of the FC and
then much more gradually to the FC.  The sensors have been field
tested.  No effect on sensor performance is observed even after
burial under soil for several months."

The paper itself gives some indication of the excellent prospects of
the sensor: there is no hysteresis, and the repeatability appears
excellent.

According to Prof Dasgupta - there has been no attempt at
commercialisation as yet, but only due to his time constraints.  He
urged me to publicise this point through this forum.  (Companies
take note!)


Apparently the technology has not yet been well accepted by
Agronomists, but there are numerous other applications where the
technology could have good use.  High humidity measurements (in the
95 to 100% area) are difficult any other way, yet this method
appears to be able to be tweaked to cover this range well.


A further use of the sensor has been reported but has more to do
with the petrochemical and related industry:

Anal.  Chim.  Acta, Amy E. Clough: Measuring water content of
Synthetic Lubricants with polymer coated sensors, 315:15-26 (1995)


Prof Dasgupta can be contacted at:

Purnendu K. Dasgupta
Department of Chemistry and Biochemistry
Texas Tech University
Lubbock, TX 79409-1061
Ph (806) 742-3064 Fax (806) 742-1289

END
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
Bruce Metelerkamp              SOIL WATER RESEARCH OFFICER
Institute for Commercial Forestry Research,
University of Natal, PO Box 100281
Scottsville, ZA3209 Rep.  of South Africa
Voice: 27 331 62314            E-mail:bruce@icfr.unp.ac.za
FAX:   27 331 68905
URL http://www.icfrnet.unp.ac.za/~metele
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