PVC or Polyethylene access tubes for neutron probe measurements

[Berglind]

Can anyone tell me where I can get PVC or Polyethylene access tubes for
neutron probe measurements on soil water content. I have been
unsuccessful so far to find tubes with proper diameter (ca. 50 mm) and
wall thickness (ca. 1.6mm).

Thanks,
Berglind Orradottir.

[Jac le Roux]

Hi Berglind,
I use PVC all the time and prefer it over aluminum for cost and ease of use.
If you are in the USA, buy 1.5" 125psi irrigation pipe and cut into lenghts
of tubing. It is slightly smaller than 50mm but works well.

If you are metric, use 50mm class 4 (the lowest pressure produced).

All PVC absorbs some radiation and therefore the neutron probe must be
re-calibrated to use it.
It you are switching form aluminum, take a set of readings, remove the tube,
insert PVC and calibrate right there.

The get the same answer you will need to up the calibration by about 15%
from alu to 125psi or 22% to 160psi.

Go to the farmers co-op for PVC.
Good luck.
Jac

[Martin Hodnett]

Berglind,

PVC is not recommended for access tubes as chlorine is
a strong neutron absorber and will reduce count rates.
Polyethylene is ok - you could use tubing with a
slightly thicker wall, but the hydrogen in it will
also affect the count rates. Thin walled platic tubes
are difficult to install as they are not very strong.
You should make your calibrations using the same type
of tubing as you use for measurements (ie do not use
the manufacturers calibration).

Aluminium is the standard for access tubes though. Can
you use that instead? It is available in the right
diamters and wall thicnesses.

Martin Hodnett
VU Amsterdam








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[Richard G. Allen]

I concur with Jac's advise that PVC tubing can work well, provided the meter is
calibrated for the specific diameter and thickness of tubing to be used.


Jac's advice on switching tubes in the same hole is good, HOWEVER, it is important
that the outside diameters of the tubes are the same. Otherwise, there will be an air
gap outside the PVC tube that will affect the calibration. One can get around the tube
size problem by starting with which ever tube is smallest and then reauger the hole to
fit the larger tube.


If they are useful, there are PDF files for nine proceedings papers on calibration of
neutron meters and intercomparison of bulk density sampling methods that were
presented at a 1993 ASCE conference in Park City, Utah (seven of the papers) and at
a 1990 ASCE conference in Durango, Colorado. The url for these papers is:
http://www.kimberly.uidaho.edu/water/papers/index.html


One of the 1990 papers by Allen and Segura compares calibrations for aluminum vs.
PVC and shows the impact of air gap and tube diameter on the calibration. That
paper is found specifically at:
http://www.kimberly.uidaho.edu/water/papers/neutron_meters/Allen_Segura_1990%20
ASCE%20Access_Tube_Char.pdf


(all one line)


One of the 1993 papers by Dickey et al. also shows comparisons among PVC and
aluminum tubes as well as comparisons of tube type / meter type interactions for two
common meter types (CPN and Troxler). This paper's URL is:
http://www.nwisrl.ars.usda.gov/pubs/811.pdf


Also, remember that the shield count, if taken each day of operation, must be made
with the meter at least 1.2 m or more above the ground and away from moisture,
including that in the human body. The ASCE Task Committee that created the 1993
sessions on neutron meters strongly recommended positioning the meter on top of a
tall access tube that was at least 1.2 m high (above the ground) and made of the
same material as used for access tubes.


Regards


Rick Allen
Univ. Idaho

[metzorcon]

Hello Berglind,

I used stainless steel - yes it was expensive, but I needed 9m deep access tubes
and any other would not have been strong enough.. it didn't seem to
affect calibration ...

The fun was getting the tube in and auguring down the tube. At first I
tried a Venturi suction system to get the sandy soil out, but later
resorted to blowing the soil out as I pushed the pipe down.
Occasionally we would run into a heavier layer and would have to get
the auger out again! It was in 2m lengths that had to be connected
and disconnected each time we inserted it - without dropping it down
the hole.

(I am glad this is all ancient history now :)

Bruce

Thursday, May 12, 2005, 11:48:28 PM, you wrote:


B> Can anyone tell me where I can get PVC or Polyethylene access tubes for
B> neutron probe measurements on soil water content. I have been
B> unsuccessful so far to find tubes with proper diameter (ca. 50 mm) and
B> wall thickness (ca. 1.6mm).

B> Thanks,
B> Berglind Orradottir.





--
Best regards,
metzorcon mailto:br...@pobox.com

[Martin Hodnett]

Hi Richard

I agree that PVC tubes can be used, as long as a
calibration is carried out in that type of tube AND
that the standard counts are also carried out in the
same type of tube.

In the UK, and France, the standard adopted is a large
drum of water. This way, no need for tall tubes and
the risk of human body water content affecting count
rates - with associated radiological hazard. The water
drum isn't portable, but you can have them at the
various sites that are being worked at. The really big
advantage is that there is no need for specific
calibrations for each and every probe. All probes of
the same design (source and detector type and
geometry) have the same calibration (for a given soil)
- which can save a great deal of work! The drum is
also large enough that conditions outside the drum do
not affect the readings.

This is discussed in the following paper:

HODNETT, M.G., BELL, J.P. 1991. Neutron probe
standards: Transport shields or a large drum of
water?' Soil Science. 151, 113-120.

With best wishes

Martin Hodnett