3
On these operating instructions
If the text follows a mark (as shown on the left), this means that an important instruction follows.
If the text follows a mark (as shown on the left), this means that an important warning follows
relating to danger to the user or damage to the apparatus.The user is always responsible for its
own personal protection.
Italic indicated text indicates that the text concerned appears in writing on the display (or must be
typed).
1. Introduction
This Sandbox (acc. to ISO 11274) (art. no.: 0801) can be used to apply a range of pressures from pF 0 (saturation)
to pF 2.0 (-100 hPa). Sand is used to convey the suction from the drainage system to the soil samples. The
surface of the sand is flexible, which makes it easier to restore the contact between it, and the samples, after
they have been removed for weighing. This quality makes sand a better suction material than a stiff porous
plate, for this instrument.
If the influence of higher pF-values needs to be measured, then additional equipment is required. The Sand/
Kaolin box (art. no: 0802SA) can be used to determine moisture percentages at pF-values from 2.0 (-100 hPa)
to 2.7 (-500 hPa), while the pressure membrane apparatus (art. no.: 0803) can create pressures from pF 3.0
to pF 4.2. A pF-value of 4.2 is equal to -15,000 hPa of pressure, which is often taken as the lower limit of soil
moisture availability to plants.
Results of measurements taken with this sandbox correspond with points on the drying curves of the relevant
samples; associated with decreasing pressure. These pressure values are usually standard water potential
increments. The wetting curve, on the other hand, is determined by graphing the water content against
increasing pressure values. This curve is not identical to the drying curve, because the water content does
not respond instantaneously to changes in pressure (Hysteresis).
2.
Description of the sandbox
The assembled sandbox can be seen in Figure 1. In the bottom of the box (1) is a PVC-pipe drainage system
(4). This box is filled with fine synthetic sand, which is covered with a nylon filter cloth. The assemblage stage
(Chapter 4) is only necessary prior to the initial use of the sandbox: After which, with proper maintenance
(Chapter 9), the sandbox can be used for several years. Soil sample core rings are placed on top of the filter
cloth to take measurements.
If the sandbox has already been assembled then begin at Chapter 5.
The ‘Hanging Water Principle’ is used to apply suction to the soil samples. The difference in height between the
suction regulator (11) and the middle of the soil samples determines the amount of pressure. Pressure heads
(h) between 0 and -100 cm can be applied. The suction regulator (11) is adjusted to apply specific pressures to
the soil samples. The samples are weighed after they have reached equilibrium at a specific pressure. Finally,
the samples are dried and weighed to deduce the water content at each specific pressure.
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