4
RANGE AND ACCURACY
MPS-2 & MPS-6
Figure 2:
Moisture characteristic curve derived from mercury
porosimeter data for MPS-2 and MPS-6 ceramic
The air entry potential of the largest pores in the ceramic is about
−
9
kPa. However, the ceramic dis must have access to air for the large
pores to begin draining and the response of the sensor to change. If
the soil around the sensor has an air entry potential less (drier) than
−
9 kPa, air will not reach the sensor until reaching the air entry
potential of the soil. So, in this scenario, the air entry potential
of the soil limits the wet end range, rather than the ceramic discs
themselves. This is generally only an issue when using the sensor in
poorly structured soils with high clay content.
4.2
Measurement Accuracy
Wet end
The MPS-2 and MPS-6 sensors are identical in construction, with
the only difference between the two sensor types being the calibra-
tion method applied. We calibrate the MPS-2 and MPS-6 at a vac-
uum saturated state (0 kPa), and at an air-dry state (
−
100,000 kPa).
These calibration techniques result in accuracy of
±
(25% of reading
+ 2 kPa) over the range of
−
9 to
−
100 kPa. The MPS-6 has an
additional four calibration points between 0 and
−
100 kPa, resulting
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