1139
Technical Committee 106 /
Comité technique 106
tensiometer is used (Figure 3) and not the whole interpolation
function.
Figure 3. Principle of extending the soil suction range using the AEV of
the tensiometer’s ceramic porous cap (Schindler et al., 2010).
3.2
Dew point potentiometer WP4-T
Vapour pressure methods are ideal for measuring suctions at the
dry end of SWRC. The suction is measured when the
equilibration of vapour pressure above the soil specimen is
achieved. If vapour pressure is measured, the suction can be
calculated using the Kelvin equation:
0
ln /
w
w
RT p p
M
(1)
where ψ is total suction,
R
is gas constant (8.314 J/mol),
T
is
temperature (K),
M
w
is molar mass of water (18 g/mol)
, ρ
w
is
water density,
p
is vapour pressure and
p
0
is saturated vapour
pressure,
p/p
0
is relative humidity.
Potentiometer WP4-T measures vapour pressure through the
dew point temperature. This is done by cooling a mirror the
reflectance of which is changed when dew appears. From the
measured mirror temperature vapour pressure above the sample
can be calculated and from the measured air temperature above
the sample saturated vapour pressure can be calculated.
Detailed description of the dew point device can be found
elsewhere (Leong et al.,2003, Campbell et al., 2007).
Figure 4. Schematic cross section through dew-point potentiometer
WP4-T (Bulut et al., 2002).
3.2.1
Specimen preparation
When measuring suction with vapour pressure methods, dry
density and structure have little effect on the suction value
(Campbell and Gardner, 1971, Thakur et al., 2006, Birle et al.,
2008). Therefore, only disturbed specimens were used. All
samples were air dried in and then sieved through a 2 mm sieve.
A known amount of water was added to the 5-10 g of soil; soil-
water mixture was homogenously mixed and then it was
compacted in stainless steel cup forming a single specimen.
more to ensure that the water content and the suction
equilibrate.
To measure
Prepared specimen was sealed and then rested for 24 hour or
the whole SWRC, specimens with water contents
3.2.2
Measurement
urements started, potentiometer WP4-T
continuous mode for about
ha
ur equilibration is more important at low suctions
(<
from the air dried soil to the soil in soft consistency were
prepared. The procedure is similar to that described by
Campbell et al. (2007).
Before the regular meas
had been calibrated each day by using standard solution of 0.5
M KCl. In case a deviation from the suction of 2.22 MPa for
more than 100 kPa was recorded, the device was cleaned and
recalibrated. In case of smaller deviations, a deviation was used
to correct readings at low suctions.
All measurements were done in
lf an hour or longer and the measurements were recorded via
Hyperterminal program. When the vapour equilibration was
observed, an average of measurements after equilibration was
calculated.
The vapo
1 MPa), as it can be seen from the diagrams in Figures 5 and
6.
Figure 5. Suction measurements with potentiometer WP4-T in
continuous mode. Black horizontal lines are average suction of the last
measurements.
Figure 6. Comparison between SWRC obtained with potentiom r
n of the measurements is 100 kPa and this should
ete
WP4-T from the first measurements and the average value after
equilibrium.
The resolutio
be acknowledged, but with this measurement technique the
error due to vapour equilibration is eliminated. With this
technique good measurements can be performed at as low as
300 kPa, which is significantly lower than 1000 kPa as reported
