(sowacs) Re: neutron probe aluminium tube

[Rick Allen <RALLEN@Kimberly.UIdaho.edu>]

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Magdeline,

I agree with Jason Smesrud that a 5 mm gap is probably not a major problem.  
If you use bentonite, you may need to recalibrate your probe, as 
background elements in the bentonite may absorb neutrons.  It is 
possible that 5 mm of bentonite will not have enough mass to pose a 
problem to calibration.

In about 1989-1991, a M.S. student, Mr. Daniel Segura, at Utah 
State Univ. conducted a thesis study on the impact of air gap on 
calibration of neutron probes.   He also evaluated impact of tube type 
and thickness.  His thesis (quite thick) is on file in the Dept. of 
Biological and Irrigation Engineering at USU.  A summary paper on 
his results was published in the proceedings of the 1990 ASCE 
Specialty Conference on Irrigation and Drainage held in Durango, 
Colorado.  The paper is entitled "Access tube characteistics and 
neutron meter calibration" by Richard G. Allen and Daniel Segura., 
pages 21 - 31.  The air gaps in Segura's study were 10 and 20 mm 
(on each side of the tube).

What we found was that air gaps between tube walls and auger holes 
reduced the slopes of count ratio to moisture.  The slopes reduced 
by 5% (ie. counts were 5% less) for the 10 mm air gaps and 12% 
less for the 20 mm air gaps for PVC access tubes, as compared to 
snug fitting tubes.    The counts were only 1% lower for 10 mm air 
gaps and 9% lower for 20 mm air gaps when aluminum access tubes 
were used (relative to snug fitting aluminum tubes).  Therefore, the 
type of tube material appears to impact the effect of the air gap.  This 
is because tube type impacts the radius of the sphere of influence.  
PVC absorbs neutrons.

In another aspect of Segura's study, the impact of tube diameter was 
evaluted.  We found that neutron meter counts decreased by an 
average of 15% for each 20% increase in diameter of PVC tube, and 
counts were decreased by an average of 11% for a 33% increase in 
diameter of aluminum tube.  These numbers are similar to the effect 
of the presence of air gaps around tubes, as one might expect.

A similar analysis was incorporated into a study conducted in 1991 
(the Neutron "shootout" held in Logan, Utah among seven teams of 
experts) where we evaluated the impact on calibration of varying tube 
types, gaps, probes and bulk density sampling techniques.  The 
results were written up in seven papers published in the proceedings 
of the 1993 ASCE Irrigation and Drainage Specialty Conference held 
in Park City, Utah.  However, the impact of gap was not summarized. 
 However, Jack Stone does include in one of his papers at the 
conference, the influence of a "cavity" along the length of the access 
tube.

In summary, if you are calibrating the neutron gauge at the site, as 
one should, the gap does not present a problem in the statistical 
fitting between counts and water content.  If you are not calibrating at 
the site, then other problems besides the presence of air gap could 
very well plague you.

Best of luck.

Rick Allen
Professor, Univ. Idaho, Kimberly


> 
> 
> 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
> 
> 
> ****************************************************************
> 
> 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,
> LS2 9JT. UK	
> 
> 
> 
> 



=================================================
Dr. Richard G. Allen                     <IXOYE>< 
Professor of Water Resources Engineering
Dept. Biological and Agricultural Engineering
Dept. Civil Engineering
University of Idaho Research and Extension Center
3793 N. 3600 E.
Kimberly, Idaho 83341
Tel (208)423-6601
Fax (208) 423-6559
RALLEN@Kimberly.UIdaho.edu
=================================================