�From:�	Bruce Metelerkamp (BRUCE)
�To:�		SOWACS@aqua.ccwr.ac.za
�Date:�	Monday, July 1, 1996  4:52 pm
�Subject:�	Cracking clays and radio sensors! sowacs

Sowacs :  Two points for discussion today:

1) Can a pocket of sand be used in the immediate vicinity of
TDR/FDR probes to prevent swell/shrink sensor-contact problems in
clayey soils?

2) How can we avoid having miles of cable in the field
when using electronic sensors?
(Does this sound a bit like the much talked about promise
of the "paperless office" that never happened?)

1)
Johan Smit and John Johnston have both mentioned the
dangers of using soil water sensors in cracking soils.
Without intimate contact, various sensors are liable to
different sources of error. Those that rely on being in
hydraulic continuity with the soil solution obviously no
longer are when the soil moves so that it is no longer in
contact with the porous parts.
TDR and FDR type instruments have very small spheres of
influence and their measurement is affected most very close
to the waveguides. Thus even minor shrinking away from the
sensor will introduce [an unknown amount] of error.

Johan suggested using a volume of sand into which the
sensor is placed rather than risk the above problems. Sand
is about the furtherest opposite extreme of the soil water
retention characterisitic (WRC).

The sand would tend towards the same matric potential as
the clay, but definitely not the same volumetric water
content  - which is what sensors based on the TDR and FDR
principles measure.

Two alternatives come to mind:

a) could one correct for the change in volumetric water content
(with knowledge of the two soils' WRC).                or

b) assuming it therefore advisable not to use sand, but rather a
clay material with as similar hydraulic properties as possible,
just one chosen for lacking swell and shrink tendencies?  Such as
diatomatious earth or kaolinite?

=====
 2) HELP!
My field sensor installation recently got
vandalised when thieves attempted to steal the cable. (They
broke the sensors in their efforts, but fortunately didn't
get away with my logger box!)

Cables are also problematical when it comes to lightning;
ploughs; rats; water; etc etc...

I am looking at two possible solutions: both hi-tech and
expensive (but cheaper than loosing an entire set of custom
made imported sensors and weeks of data!?!)

a) dedicated mini-dataloggers that can be buried (or placed
in vegetation) where they are unlikely to get
found/damaged. These will need their own stable power
source and memory. These will require dedicated programmed
loggers and as far as I know are currently only available
for common measurements such as temperature and humidity
(and so come with the sensor as part of the unit). For
example Hobo and TinyTalk dataloggers are of this form. I
know of none that are a datalogger as such without sensor
attached.

b) low power mini radio transmitters on each sensor and a
receiver at the datalogger. This would allow the datalogger
to be placed in a secure environment and run off mains
power. This still means that the minidatalogger as
discussed in a) is required, in addition to the transmitter.

Option a) would need to be visited fairly often to collect
the data, which is probably just as well - as most of us
don't visit the field enough?!? The radio option would
allow real-time data - as needed for scheduling for example
to be collected by modem, particularly if the logger is
housed in a building serviced by phone-lines.

DOES ANYONE KNOW OF EXISTING OR SYSTEMS UNDER DEVELOPMENT?

Thanks Regards

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-. 
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                       
                                    /SoWaCS.html
                                    /RR.html
Host of SoWaCS (Soil Water Content Sensor) Discussion List. SEND 
info sowacs  OR  subscribe sowacs  TO majordomo@aqua.ccwr.ac.za