Soil water measurement instruments can be divided as follows.
Soil water
measurement
Soil water content Soil water potential
-Tensiometer
-Electric resistance
Volumetric Gravimetric -Thermal dissipation
-Neutron scatting -Psychomotor
These instruments have the ability to
measure matric water potential with using of sufficiently less expensive and
reliable methods.
(1)Tensiometer
Tensiometer is composed of a porous
ceramic cup and a sealed plastic cylindrical tube. The pressure measuring device is
connected to the ceramic cup by the cylindrical tube at top of the cylindrical tube.
Working principle:
Semi-equilibrium is existed with soil water by tensiometer.
Saturation of ceramic cup is essential to get readings
accurately because it acts as cell membrane and water move through into the
system.
With beginning of process, the tensiometer is adjusted to get
negative pressure because water move from tensiomter to soil.
Difference of water potential is recorded as a difference of
pressure by using a U tube which is filled with water or mercury.
If water potential increases, water moves from soils to
tensiomter therefore it causes for less negative potential.
Negative potential will be caused for gasses which are
released from solution to form air bubbles. It affects to the reading of the
tensiomter.
Tensiomter has a practical operating range of 0-0.75Bar.If
the pressure is above 1bar at ambient temperature, water start to form water
vapor it also affects to alter pressure readings.
The cylinders of tensiomter must be existed under a vacuum,
so cylinders must be frequently needed to de-air.
Large numbers of roots are often accumulated in ceramic cup.
Before and after installation of tensiomter, ceramic cup is
existed under wet condition. Even though during the transportation of
tensiomter must be covered with water.
The hole which is prepared for installation of tensiomter
must have suitable size and depth. Suitable depth is important for extending
tensiomter to refill and de-air. Suitable size is important for tensiomter to
fir enough with every side.
Thermal expansion of the upper tensiometer cylinder may be
induced by interception of solar radiation. The pressure gradient from soil to
tensiomter causes for thermal expansion.
The cylinder must be shaded or the cylinder must be made by
non-conducting materials such as plastics to avoid thermal expansion.
Advantages:
(1)Tensiometer is inexpensive equipment for soil water
potential measurement.
(2)It is reliable to use in field-scale monitoring programs.
Disadvantages:
(1)Tensiomter can be only used under wet conditions.
(2)The pressure range is limited.
(3)The tensiomter occur slow process even rapid change of
soil matric potential.
(2)Resistance blocks
Electrodes are encased within a
porous material such as gypsum, plastics and fabrics.
Working principle:
Semi- equilibrium is occurred soil after two days.
Water potential is measured as a function of electrical
resistance.
It tends to process at lower water contents which have
relatively higher tension.
Resistance is decreased according to the increasing of
saline. But gypsum blocks are less sensitive to saline.
Gypsum blocks are needed to correct for temperature.
Before make for hole to install, a drain must be prepared for
lead wires to minimize to flow water along wires to the blocks.
For calibration, the blocks must be saturated with distilled
water and adjusted the predetermined pressure in pressure-plate equipment.
The resistance block must be frequently calibrated before and
after installation because calibration curves change with time.
Disadvantages:
(1)It usually takes two days to semi-equilibrium with soil.
(2)It can be used only under dry conditions.
(3)Some types of resistance blocks are sensitive to saline.
It causes for decreasing resistance.
(4)Resistance blocks are needed to calibrate individually.
Advantages:
(1)Resistance blocks are less expansive
(2)Relatively minimum maintenance is needed for resistance
blocks.
(3)Psychrometer
A Psychrometer is composed of mini
thermocouple within a small chamber which has a porous wall.
Working principle:
The thermocouple is cooled by the evaporation of water from the junction. It causes the production of current.
Measurements are responded according to the changes in soil
water potential quickly.
A psychrometer is most suitable for lower matric potentials
which are less than -300kPa.
Readings must be obtained at the same time each day to minimize
to thermal expansion and extractions according to changes in temperature.
Disadvantages:
(1)A psychrometer is sensitive to sensitivity.
(2)It can be measured by only pressures of less than -300kPa.
(3) Readings can be changed according to the temperature difference
of the surrounding environment.
Advantages:
(1)It’s very sensitive to temperature differences.
(2)Phsychroomter is a less expensive device.
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