Campbell Scientific 229 Water Matric Potential Sensor and CE4/CE8 - Manual (Page 28)

229 Heat Dissipation Matric Water Potential Sensor
2. With the sensor in place, use the T Δ
from the in situ measurement
along with the
dry
T Δ
,
wet
T Δ
values for the particular sensor to
calculate
norm
T Δ
.
3. Implement the iterative temperature correction as presented in
datalogger example program #4 to obtain a corrected
norm
T Δ
.
4. Use the corrected
norm
T Δ
in the calibration equation, e.g. equation
[4].
6.3 Using Pressurized Extraction Methods
Pressurized extraction methods use a porous material (typically ceramic or
stainless steel) to separate the pressurized soil sample from atmospheric
pressure. One side of the porous material is in contact with the soil sample and
the other side is at atmospheric pressure. A simple configuration is a cylinder
which has the porous material as a bottom and a solid cap at the top which
provides a pressure-tight seal after the soil sample has been placed on the
porous bottom. This allows pressurization of the soil sample which will force
water from the soil. The air entry pressure of the porous material is dependent
on the effective pore diameter and must be greater than the maximum pressure
applied during calibration. When a specified pressure is applied, all soil water
at water potentials greater than -1*(applied pressure) will move through the
soil sample and through the porous bottom. The time required for the soil
water to leave the sample system depends on the pressure gradient and the
hydraulic conductivity of the soil and the porous bottom plate.
Equilibration of water potential throughout the system must be attained or the
accuracy of the water potential measurement using the derived calibration will
be reduced. The simplest way to confirm equilibration is to repeat the
measurements at a given applied pressure until readings do not change.
6.4 General Description of Calibration/Measurement Process
using Pressure Plate Extractor
A pressure plate extractor consists of a high-pressure vessel, a porous plate and
tubing to remove soil water from the soil sample. A ring with diameter slightly
less than the diameter of the ceramic plate can be used to hold the soil in which
the 229 sensors are buried. Smaller rings can also be used. The porous plate,
soil and 229 sensors must be thoroughly saturated at the beginning of the
calibration routine. Complete saturation of the ceramic plate and 229 sensors
is better achieved by applying vacuum. The number of sensors that can be
calibrated in a single pressure vessel will depend on the 229 cable lengths
because of the limited space in the vessel.
The general calibration sequence is:
1. The extractor plate, soil and sensors are saturated and positioned in the
pressure vessel.
2. The lowest calibration pressure chosen by the user (>10 kPa) is applied
and the soil solution, which is held by the soil at an energy level less than
that applied, is allowed to leave the pressurized system.
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