Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 150

the air phase of the soil pores or the environment immedi-
ately around the soil surface. The relative humidity can be
expressed in terms of relative vapor pressure (

Gauge (AWG) copper lead wires to form a reference junction.
The highly conductive copper wires have a large diameter
(i.e., large thermal mass), in order to serve as heat sinks that
can maintain a constant temperature at the reference junction.
The heat sink absorbs the heat generated near the reference
junction when the measuring junction is being cooled.
The maximum degree of cooling generated by the chromel-
constantan thermocouple is about 0.6 ◦ C below the ambient
temperature (Brown and Bartos, 1982). This maximum cool-
ing represents the lowest relative humidity (i.e., 94%) or the
upper limit of the total suction (i.e., 8000 kPa), which can be
measured using the thermocouple psychrometer. The low-
est suction which can be measured using a thermocouple
psychrometer is somewhat less than 100 kPa provided the
temperature environment is controlled to within

u v / u v 0 ), which

is related to total suction.
There are two basic types of thermocouple psychrometers:
(i) the wet loop type (Richards and Ogata, 1958) and (ii) the
Peltier type (Spanner, 1951). Both types of psychrometers
operate on the basis of temperature difference measurements
between a nonevaporating surface (i.e., dry bulb) and an
evaporating surface (i.e., wet bulb). The difference in the
temperatures between these surfaces is related to relative
humidity. The wet-loop and the Peltier-type psychrometers
differ in the manner by which the evaporating junction is
wetted in order to induce evaporation.

0.001 ◦ C
(Krahn and Fredlund, 1972). The lower limit corresponds to
a relative humidity approaching 100%. A slight lowering of
the temperature as the 100% relative humidity is approached
will produce condensation on the thermocouple.
A protective housing is usually provided around the ther-
mocouple wires. The protective cover may take the form
of a ceramic cup, a stainless steel screen, or a solid tube
(stainless steel or teflon), with a screen end window. The
selection of the type of the protective cover depends on the
application of the psychrometer. The time required for water
vapor equilibration is significantly affected by the type of
the protective cover. The ceramic cup appears to have the
longest equilibration time and may not be practical in some
situations.
Measurements of total suction are conducted by sus-
pending a psychrometer near a soil specimen in a closed
environment. The relative humidity is measured after equi-
librium is attained between the air near the thermocouple

±

4.3.1.1 Peltier-Type Psychrometers
A Peltier current causes a thermocouple junction to cool
below the dew point with the result that a minute quantity
of water is condensed onto the thermocouple junction. The
Peltier-type psychrometer has been quite commonly used
in geotechnical engineering. The Seebeck and the Peltier
effects constitute the main principles behind the operation
of the Peltier psychrometer.
Seebeck (1821) discovered that an electromotive force
(emf) was generated in a closed circuit of two dissimilar
metals when the two junctions of the circuit are at different
temperatures. This phenomenon is referred to as the Seebeck
effect. The Seebeck effect allows the use of two dissimilar
wires (i.e., a thermocouple) to measure temperature.
Peltier (1834) discovered that when a current is passed
through a circuit of two dissimilar metals, one of the junc-
tions becomes warmer, while the other junction becomes
cooler. This phenomenon allows the use of thermocouples
for the measurement of relative humidity.
The Peltier effect can be used to cool a thermocouple
junction to reach the dew-point temperature corresponding
to the surrounding atmosphere. As a result, water vapor con-
denses on the junction. After terminating the passage of the
applied current, the condensed water tends to evaporate to
the surrounding atmosphere, causing a further reduction in
the temperature at the junction. The temperature reduction is
a function of the evaporation rate, which is in turn affected
by the water vapor pressure in the atmosphere. If the ambient
temperature and the temperature reduction due to evapora-
tion are measured using the Seebeck effects, the relative
humidity of the atmosphere can be computed. There is a
maximum degree of junction cooling that can be achieved
using the Peltier current (Spanner, 1951).
The Peltier psychrometer is often called a Spanner
psychrometer and details of the design of the sensor are
shown in Fig. 4.67. The thermocouple consists of 0.025-mm-
diameter wires of constantan (i.e., copper-nickel) and
chromel (i.e., chromium-nickel). The wires are welded
together to form an evaporating or a measuring junction. The
other ends of the wires are connected to 26 American Wire

Figure 4.67 Screen-caged single-junction Peltier thermocouple
psychrometer (after Brown and Collins, 1980).