MPS-2 & MPS-6
4
RANGE AND ACCURACY
Dry End
As described in the section above, both the MPS-2 and MPS-6 rely
the linear relationship between water content and the logarithm of
water potential for calibration in the dry end. Lab testing and field
evaluations have shown that additional calibrations in this region are
not necessary, but could improve accuracy some.
Decagon strongly discourages dry end calibrations in the pressure
plate apparatus. Our early attempts to improve MPS-1 and MPS-2
sensor dry-end performance in the pressure plate apparatus actually
decreased accuracy, likely because of pressure plate dry end equilib-
rium issues pointed out in the literature.
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4.4
Soil Type Dependence
The MPS-2 and MPS-6 calibration is not affected by soil type because
the sensors only measure the water potential of the ceramic discs in
equilibrium with the soil. The MPS-2 and MPS-6 work in any soil
type or other porous media as long as you install them correctly
with adequate hydraulic contact (to ensure timely water potential
equilibrium between the sensor and the medium of interest).
4.5
Hysteresis
The amount of water that a soil holds at a given water potential is
greater if the material is dried to that water potential than if the ma-
terial is wet up to that water potential; a phenomenon known as hys-
teresis. Because the MPS-2 and MPS-6 essentially make a dielectric
measurement of water content and convert that to water potential,
sensor measurements have some hysteresis. In most situations, soil
undergoes brief periods of wet up (precipitation or irrigation events)
followed by longer dry down periods where water potential measure-
ments are most useful. We perform MPS-2 and MPS-6 calibration
on the drying leg of the hysteresis loop, so the measurements are
4
e.g.
Campbell (1998), Gee et al.
(2002), Bittelli and Flury (2009), and
Frydman and Baker (2009)
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