Re: tension measurements in sandy soils

[Paul Hutchinson]

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With respect to Bob's difficulty in getting moisture to flow from a fine
sand into a granular matrix sensor, my guess is that over a range of
tensions there is a capillary barrier between the fine sand and the porous
material of the sensor.  The moisture won't enter because the hydraulic
conductivity of the coarse sensor material, over a range of tensions, is
much less than that of the fine sand.  To test this hypothesis, one could
pack the granular matrix sensor in a container of dry fine sand and put it
on a tension plate.  If the tension is slowly raised (becoming wetter),
then the granular matrix sensor will start to respond at the tension where
the conductivities are similar.    

I don't know of a cure, other than to change the sensor material to
something finer.  Other packing materials, such as a coarse material, won't
help because the capillary barrier will then form between the fine sand and
the coarse packing material. My guess is that a finer packing material will
make the problem worse.  

Paul Hutchinson.

At 21:45 30/07/99 GMT, you wrote:
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>There has been some recent discussion on materials used to backfill
>ceramic porous cup devices that are in contact with the surrounding
>soil/water environment - bentonite being mentioned as one.
>
>I am investigating the use of a connecting media between granular matrix
>sensors (for measuring soil-water tension) and sandy soils.  Sandy soils
>with a small percentage of fines are often not suitable for granular
>matrix sensors due to, I believe, some difficulty in achieving
>soil-water continuity between the porous media of the sensor and the
>surrounding soil.
>
>Is there a fine material that could be used as a slurry, having good
>hydraulic conductivity, so that the sensor could communicate with the
>surrounding soil without compromising the response time?
>
>I would sure appreciate some suggestions.
>
>Thank you,
>
>Bob Mittelstadt
>Othello, WA
>
>
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