MPS-2 & MPS-6
4
RANGE AND ACCURACY
4
Range and Accuracy
4.1
Measurement Range
The MPS-2 and MPS-6 measure the water content of porous ceramic
discs and convert the measured water content to water potential us-
ing the moisture characteristic curve of the ceramic.(See Section 3) It
is important for sensor function that the ceramic drains over a wide
water potential range.(See Figure 2) The size of the pore determines
the water potential at which a pore drains (the air entry potential or
bubble pressure), so the ideal ceramic would have pores that range
from very small to relatively large. Decagon specially designed the
ceramic discs to approach this ideal, but they have a total pore vol-
ume that is weighted toward the larger pores, which drain at water
potentials within the plant-available range. However, the MPS-2 and
MPS-6 measurement range extends all the way to air-dry (
−
100,000
kPa).
As the sensor dries past the plant-available range, the total pore vol-
ume that drains at a given water potential decreases. At these low
water potentials, the measured water potential can become somewhat
noisy because small changes in measured water content of the ceramic
translate into large changes in water potential. This phenomenon is
most pronounced when the sensor is air-dry. It is expected that the
measured water potential of an uninstalled, air-dry sensor can jump
around throughout the range of
−
50,000 kPa to
−
100,000 kPa. The
noise level is much lower when the sensor is installed in the soil, even
at air-dry water potential.
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