TM 5-818-7
Table B-1. Calibration Salt Solutions
Suction, tsf
Measured
for cited molality of sodium chloride solution
temperature
0.053
0.100
0.157
0.273
0.411
0.550
1.000
15
3.05
4.67
7.27
12.56
18.88
25.29
46,55
3.10
20
4.74
7.39
12.75
19.22
25.76
47.50
3.15
4.82
7.52
25
13.01
19.55
26.23
48.44
3.22
4.91
7.64
30
13.22
19.90
26.71
49.37
quired for sodium chloride salt solutions to provide the
measured by the psychrometer and the relative humid-
soil suctions given as a function of temperature.
ity in the soil specimen depends on the volume and ini-
tial relative humidity in the container. Equilibrium
b. Thermocouple psychrometer technique. The ther-
time may require up to 7 days, but may be reduced to 2
mocouple psychrometer measures relative humidity in
or 3 days by repeated testing of soils with similar suc-
soil by a technique called Peltier cooling. By causing a
tions.
current to flow through a single thermocouple junction
(c) After equilibrium is attained, the microvolt-
in the proper direction, that particular junction will
meter is set on the 10- or 30-microvolt range and
cool, then water will condense on it when the dew-
zeroed by using a zeroing suppression or offset control.
point temperature is reached. Condensation of this
The cooling current of approximately 8 millimeters is
water inhibits further cooling of the junction. Evapo-
applied for 15 seconds and then switched to the micro-
ration of condensed water from the junction after the
voltmeter circuit using the switch of item 6 in figure
cooling current is removed tends to maintain a differ-
B-2, The maximum reading on the microvoltmeter is
ence in temperature between the thermocouple and
recorded. The cooling currents and times should be
the reference junctions. The microvoltage developed
identical to those used to determine the calibration
between the thermocouple and the reference junctions
curves.
is measured by the proper readout equipment and re-
(d) The readings can be taken at room tempera-
lated to the soil suction by a calibration curve.
ture, preferably from 20 to 25 degrees Centigrade, and
(1) Apparatus. Laboratory measurements to eval-
corrected to a temperature of 25 degrees Centigrade
uate total soil suction may be made with the apparatus
by the equation
illustrated in figure B-1. The monitoring system in-
cludes a cooling circuit with the capability of immedi-
(B-4)
ate switching to the voltage readout circuit on termi-
nation of the current (fig. B-2). The microvoltmeter
where
(item 1, fig. B-2) should have a maximum range of at
microvolt at 25 degrees Centigrade
least 30 microvolt and allow readings to within 0.01
microvolt at t degrees Centigrade
microvolt. The 12-position rotary selector switch
Placement of the apparatus in a constant temperature
(item 2) allows up to 12 simultaneous psychrometer
room will increase the accuracy of the readings.
connections. The 0-25 millimeter (item 3), two
1.5-volt dry cell batteries (item 4), and the variable
by placing approximately 50 millilitres of the salt solu-
potentiometer (item 5) form the cooling circuit, Equip-
tions of known molality (table B-1) in the metal con-
ment is available commercially to perform these meas-
tainers and following the procedure in b(2) above to de-
urements of soil suction.
termine the microvolt output. Equilibration time may
(2) Procedure.
be reduced to 2 or 3 days by surrounding the psy-
chrometer with filter paper soaked with solution. The
(a) Thermocouple psychrometer are inserted
into 1-pint-capacity metal containers with the soil
suctions given for the known modalities are plotted
versus the microvolt output for a temperature of 25
specimens, and the assembly is sealed with No. 13-1/2
degrees Centigrade. The calibration curves of 12 com-
rubber stoppers. The assembly is inserted into a 1- by
mercial psychrometer using the equipment of figure
1- by 1.25-foot chest capable of holding six l-pint
B-1 were within 5 percent and could be expressed by
containers and insulated with 1.5 inches of foamed
the equation
polystyrene, Cables from the psychrometer are
(B-5)
To
passed through a 0.5-inch-diameter hole centered in
the chest cover, The insides of the metal containers are
foot. The calibration curves using other equipment
coated with melted wax to inhibit corrosion of the con-
may be somewhat different.
tainers.
(b) The apparatus is left alone until equilibrium
c. Filter paper technique. This method involves en- --
is attained. Temperature equilibrium is attained with-
closing filter paper with a soil specimen in an airtight
in a few hours after placing the chest cover. Time to
container until complete moisture equilibrium is
reach equilibrium of the relative humidity in the air
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