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John Johnston wrote:
>>I don't think we covered whether it would be better to insulate
>>them with heat-shrink yet - any offers on this?
>They are contact sensitive, as is any TDR based sensor, and any
>material placed between the rods and the soil will effect
>contact. As the field effect decay is exponential it is the
>worst case situation to place a foreign material on the rods. It
>is probable that a calibration, or post process of the data can
>correct for this effect, but heat shrink tubing is not well
>known to have a constant thickness, so "can a constant for
>correction be established"?
>Additionally, with the error introduced by breaking contact, the
>device is measuring a dielectric effect. The plastic of the
>tubing will distort the corrections established for soil in some
>unknown way. I don't know what the overall effect is, or if it
>may be within the error budget of the "sensor system" as a
>whole. Just thoughts, not answers.
Yes, agreed, but perhaps, with the required calibration
adjustment, surely this is better than having the
salinity-related problems (of 190% soil water content)?
I have heard that some are loking at placing the TDR wave guides
into a ceramic tube - thereby reducing the soil's influence on
changes in the dielectric - and restricting the sensor to
measuring the water content of the ceramic in hydrologic
equilibrium with the soil. The pore size distribution of the soil
has then to be matched to the soil, but progress is being made on
this front too, I believe. But soil sensor contact problems are
alleviated. Perhaps this system will be more suitable to
insulating the waveguides - prior to casting them into ceramic
(stone :> ?)
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
brooz@pobox.com
Bruce.Metelerkamp@pobox.com
URL http://www.icfrnet.unp.ac.za/~metele
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