Environmental Engineering Reference
In-Depth Information
Table 4.11 Calibration Curves for Whatman No. 42 and Schleicher and Shuell No. 589 Filter Papers
(a) Whatman No. 42 Filter Paper
References
Calibration curves
Hamblin (1981)
Chandler and Gutierrez (1986)
Chandler et al. (1992)
Greacen et al. (1987)
ASTM (1997b)
log
ψ
=
8
.
022
−
3
.
683 log
w
f
log
ψ
=
4
.
84
−
0
.
0622 log
w
f
w
f
<
47
w
f
log
ψ
=
6
.
05
−
2
.
48 log
w
f
47
w
f
<
45
.
3
w
f
≥
log
ψ
=
5
.
327
−
0
.
0779 log
w
f
log
ψ
=
2
.
413
−
0
.
0135 log
w
f
≥
45
.
3
(b) Schleicher and Schuell No. 589 Filter Paper
log
ψ
=
5
.
238
−
0
.
0723 log
w
f
w
f
<
54
w
f
McQueen and Miller (1968a)
log
ψ
=
1
.
8966
−
0
.
01025 log
w
f
54
w
f
<
85
w
f
≥
log
ψ
=
4
.
136
−
0
.
0337 log
w
f
log
ψ
=
2
.
0021
−
0
.
009 log
w
f
≥
Al-Khafaf and Hanks (1974)
85
w
f
<
66
w
f
log
ψ
=
4
.
9
−
0
.
0624 log
w
f
McKeen (1980)
Greacen et al. (1987)
ASTM (1997b)
log
ψ
=
1
.
25
−
0
.
0069 log
w
f
66
w
f
<
54
w
f
≥
log
ψ
=
5
.
056
−
0
.
0688 log
w
f
log
ψ
=
1
.
882
−
0
.
0102 log
w
f
≥
54
Note
:
ψ
=
suction (kPa),
w
f
=
filter paper water content (%).
another equation can be written for the low-water-content
range (i.e., high suction range). Various proposed calibration
equations by different researchers yield quite similar values
for soil suction provided consideration is given to the break-
ing point on the calibration curve. Table 4.11 shows a sum-
mary of calibration equations that have been proposed for
Whatman No. 42 and Schleicher and Shuell No. 589 fil-
ter papers. In each case [except for the Hamblin (1981)
case], two equations are written for the filter paper cali-
bration curves.
There are several factors that appear to have an influence
on the calibration curves for filter paper: the quality of filter
paper, the method used to control the applied soil suction,
hysteresis, and equilibration time.
Quality of the Filter Paper.
Differences in filter paper
quality between batches of filter paper could possibly affect
the measured calibration curves. Hamblin (1981) found that
the calibration curves for batches of Whatman No. 42 filter
paper produced two years apart were almost identical. Simi-
lar results were found by Fawcett and Collis-George (1967),
Chandler and Gutierrez (1986), and Swarbrick (1995). The
quality of the filter paper was studied by comparing the cal-
ibration data using pressure plate and pressure membrane
apparatuses for soil suctions less than 1000 kPa. The vapor
pressure generated by salt solutions at high suctions was
used to establish suctions greater than 1000 kPa. Whatman
No. 42 and Schleicher and Schuell No. 589 filter papers are
shown in Figs. 4.77a and 4.77b, respectively. Measured data
are confined to a relatively narrow band even though the
results were obtained by different researchers, at different
times, with different batches of filter paper. The Whatman
No. 42 data appear to be slightly more consistent than the
Schleicher and Schuell No. 589 data. It can be concluded
that variations in the source of the filter paper do not appear
to be a serious problem.
Bacterial Growth on the Filter Paper.
Gardner (1937)
pretreated Schleicher and Schuell No. 589 filter papers with
0.2% HgCl
2
to prevent contamination by soil organisms prior
to installation. McQueen and Miller (1968a) and Al-Khafaf
and Hanks (1974) used a 3% solution of pentachlorophenol
in ethanol to prevent the growth of bacteria and other
organisms. Fawcett and Collis-George (1967) pretreated
Whatman No. 42 filter papers with 0.005% HgCl
2
and
oven dried the filter paper at 105
◦
C, while Hamblin (1981)
did it with 0.005% HgCl
2
. Hamblin (1981) and Chandler
and Gutierrez (1986) reported that there was no need to
pretreat the filter paper before usage. There does not appear
to be any reports of serious concern over bacteria or algae
growth on filter papers when using filter paper to measure
soil suction.
Control of Applied Suction during Calibration.
Several
methods have been used by various researchers to create a
controlled suction environment for filter paper calibration
with suggested methods listed in Table 4.12. Ridley (1995)
suggested that the calibration of filter paper be performed
under conditions close to expected experimental conditions.
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