Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 151

and the pore-air in the soil specimen. Isothermal equilibrium
must be achieved between the soil and the air before the
psychrometer reading is taken. A temperature environment
controlled to within

The water vapor pressure or the relative humidity in the
calibration chamber under isothermal equilibrium conditions
is related to the osmotic suction of the salt solution. The psy-
chrometer can be calibrated at various suction values by using
different molalities (or osmotic suctions) for the salt solution.
Isothermal conditions are maintained by placing the chamber
in a constant-temperature bath, as illustrated in Fig. 4.68.

0.001 ◦ C is required in order ± to

measure total suctions to an accuracy of 10 kPa (Krahn

and Fredlund, 1972). Thermal equilibrium within the
psychrometer is assured by obtaining a zero reading on the
microvoltmeter.

±

4.3.1.3 Psychrometer Performance
In situ measurements of total suction using psychrometers
are generally not recommended because the thermal envi-
ronment is not sufficiently constant in the field. However,
laboratory measurements can be conducted under controlled
temperature environments using undisturbed soil specimens
from the field.
A soil specimen is placed into a stainless steel or Lucite
chamber together with the thermocouple psychrometer, as
illustrated in Fig. 4.69. The entire assembly is then placed

4.3.1.2 Calibration of Peltier-Type Psychrometers
The calibration of a psychrometer consists of determining
the relationship between microvolt outputs from the thermo-
couple for a salt solution with a known total suction value.
The psychrometer can be mounted within a sealed chamber.
Filter papers can be saturated with a solution of NaCl or
KCl and placed at the base of the chamber. The osmotic
suctions for NaCl and KCl solutions at different molalities
and temperatures are summarized in Tables 4.9 and 4.10.

Table 4.9 Osmotic Suctions for NaCl Solutions

0 ◦ C 7.5 ◦ C 15 ◦ C 25 ◦ C 35 ◦ C Molality

0 0.0 0.0 0.0 0.0 0.0

0.20 836 860 884 915 946

0.50 2070 2136 2200 2281 2362

0.70 2901 2998 3091 3210 3328

1.00 4169 4318 4459 4640 4815

1.50 6359 6606 6837 7134 7411

1.70 7260 7550 7820 8170 8490

1.80 7730 8035 8330 8700 9040

1.90 8190 8530 8840 9240 9600

2.00 8670 9025 9360 9780 10,160

Source : From Lange, 1967.

Table 4.10 Osmotic Suctions for KCl Solutions

10 ◦ C 15 ◦ C 20 ◦ C 25 ◦ C 30 ◦ C Molality

0 0.0 0.0 0.0 0.0 0.0

0.10 436 444 452 459 467

0.20 859 874 890 905 920

0.30 1277 1300 1324 1347 1370

0.40 1693 1724 1757 1788 1819

0.50 2108 2148 2190 2230 2268

0.60 2523 2572 2623 2672 2719

0.70 2938 2996 3057 3116 3171

0.80 3353 3421 3492 3561 3625

0.90 3769 3846 3928 4007 4080

1.00 4185 4272 4366 4455 4538

Source : From Campbell and Gardner, 1971.