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[Jason Smesrud]

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The air gap is not a huge problem as it is with electrical devices (i.e.
TDR, FDR). Neutrons will pass through the air gap, get thermalized by
hydrogen in the adjacent soil and return to the neutron counter through the
air gap with only a slight affect on sensitivity as the soil volume measured
is moved further away from the source and counter. The hydrogen content in
the air will be negligible in comparison to that of water bound by soil
particles. The biggest problem with the air gap is the potential for
preferential flow down the annular space between the soil and the tube. If
any part of the profile is expected to reach saturation, this will be a
problem and the hole should probably be bored again at the correct diameter.
If unsaturated conditions are expected, the native soil could be pressed
into the annular space from the surface to seal near the top of the tube to
prevent surface run-off, precipitation, or irrigation from flowing down the
annular space. You will have to check that plug periodically to make sure
the soil does not settle further into the annular space opening up the
potential flow channel to the surface.

Filling the hole with bentonite is not a good solution as the bentonite will
retain a lot of water. This will bias your measurements and would be
difficult to impossible to separate out the water retained by the bentonite
from the water retained in the soil you are concerned with.

Good Luck.
Jason Smesrud
CH2M HILL
825 N.E. Multnomah, Suite 1300
Portland, OR 97232-2146
(503-235-5022, ext.4372)


-----Original Message-----
From: owner-sowacs@aqua.ccwr.ac.za [mailto:owner-sowacs@aqua.ccwr.ac.za]
Sent: Thursday, August 26, 1999 12:06 PM
Subject: neutron probe aluminium tube

Dear subscribers,

I have a slight dilemma. I have recently installed a neutron probe aluminium
access tube and the hole drilled for installation is slightly larger than
the tube (only about < 5mm diameter). Should I leave the small air gap as it
is or fill it with bentonite? Which will affect my readings more?

Thanks


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Magdeline Pokar,		Tel: 0113-2335231 (O)
School of Earth Sciences,	     0113-2343795 (H)	
University of Leeds,		Fax: 0113-2335259
Woodhouse Lane,			email: M.Pokar@earth.leeds.ac.uk
Leeds, West Yorkshire,
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