Australian Water Management Review Vol. 1 2014 | Page 78

Soil Water Probe Installation –
Who Has The Time?

By Michael Dalton, Research Scientist, Sentek Pty. Ltd.

If you want to measure anything well, then
it is important to install your measuring
equipment with the minimum of disruption
to the medium you are testing. Sounds
simple, but as an agricultural field
operative I am constantly surprised by the
mistakes that I see made.
At the outset we are disadvantaged by field and soil
variability. It is not always practical to install multiple
probes due to cost and time constraints. To get ahead
of this we need to think hard about the best possible
installation methods and site locations for the
available time and budget.
Since 1991 Sentek has espoused a standard Direct
Installation method for insertion of its soil moisture
monitoring equipment. This technique involves augering
through the centre of an access tube fixed in the centre
of a tripod which is pinned to the ground. The installer
alternately augers inside and sledge hammers the access
tube until full depth is attained. The access tube must
then be cleaned internally and sealed at the bottom
before the electronics are installed. This has remained
the gold standard for undisturbed installation of access
tubes into the ground for 22 years. However, this
technique requires a skilled operator with sufficient time
and specialised tools. In the commercial setting, this
technique is often modified to suit available resources
and time constraints.

Figure 1

72 | Australian water man age m e nt re v ie w

An alternative method often employed in the
commercial arena is the slurry technique. This is
where a larger hole is augered and liquid slurry of
either clay/cement or native soil is used to set the
access tube into place. The water in the slurry may
take a period of weeks to dissipate before relevant
trending data can be interpreted. Often the prepared
slurry forms a conduit for water or plant roots that
would not normally travel to these depths at such
high rates. Trending of the water movement may be
clearly visible, but this may not truly reflect the plant
root zone environment. There are some situations
where this is the only practical method available,
such as installations in stony soils or for probes
longer than two metres. Sometimes this is done for
convenience and speed, but it is not the preferred
option due to the increased potential for interference
in the normal flow of water within the soil.
Both of the above methods require a skilled operator
to perform them. Common errors that must be
avoided with the Direct Installation method include
the introduction of excessive gaps from misdirected
hammer blows or inadequately loose tripod support,
dirty internal access tubing from inadequate cleaning
and leakage from an inadequately tightened bottom
stopper. In the case of a slurry installation, the
thickness of the slurry must be minimised for the best

Figure 2

trending data to be achieved from the sensors. This
is due to the close sphere of influence of the sensor,
which extends outward to 14cm from the access
tube and covers a soil depth of 10cm. This sphere of
influence means that narrow bands of moisture may
be differentiated, giving the sensor high resolving
power to see detailed soil interactions.
With these potential installation difficulties in mind
the company has recently developed a faster Direct
Installation method for its standard sized (56mm)
access tubes down to a depth of 1.5m. This method
uses a new rigid tripod as a guide for the auger
(Figure 1). An undersized hole is drilled by hand to
a depth of 16cm deeper than the length of the access
tube (Figure 2). The water-lubricated end-sealed
access tube is then inserted into the tripod and forced
into the hole using a steel tube and drop hammer.
Excess soil shaved from the sides of the hole by the
metal cutting ring on the glued PVC bottom stopper
falls to the base of the hole as the access tube forms
a tight fit with the soil (Figures 3 & 4). Once finished,
the access tube requires no internal cleaning and is
ready to accept the electronic components.
This new procedure reduces the installation time to 1/3
(10-15 minutes) of the standard technique. The time
savings are primarily in augering, which can be done
freely, rather than the restrictive augering ahead of the

Figure 3