6
MAINTENANCE AND TROUBLE
MPS-2 & MPS-6
6
Maintenance and Troubleshooting
The MPS-2 and MPS-6 sensors measure the water potential of two
engineered ceramic discs sandwiched between stainless steel screens
and the circuit board. The ceramic discs are somewhat brittle and
can chip or crack if abused. The metal screens afford the discs some
amount of protection, but sharp trauma on the disc edges or massive
impact (such as dropping the sensor onto a hard surface) can cause
the ceramic to break. One or two small chips on the edge of the disc
does not affect the sensor accuracy significantly. However, a cracked
ceramic disc creates a loss of accuracy.
For the MPS-2 and MPS-6 to accurately measure water potential,
the ceramic discs must readily take up water. If you expose the ce-
ramic discs to oils or other hydrophobic substances, it compromises
the ability of the discs to take up water from the soil. This inability
to take up water leads to slow equilibration times and loss of accu-
racy. We recommend you minimize exposure of the ceramic material
to skin oils and that you do not handle the discs with greasy hands,
or expose the discs to synthetic oils or other hydrophobic compounds.
6.1
MPS-2 & MPS-6 in Frozen Soils
The MPS-2 and MPS-6 measure the dielectric permittivity of two
ceramic discs to measure their water content and then derive their
water potential. The dielectric permittivity of water in the ceramic
discs is 80 compared to a dielectric permittivity of
∼
5 for the ceramic
material, and 1 for air. When water freezes to ice, the dielectric
permittivity drops to 5 at the frequency of the sensor measurement
meaning that the MPS-2 or MPS-6 can no longer accurately measure
the water in the ceramic. The MPS-2 and MPS-6 do not accurately
measure the water potential of soil in frozen soil conditions. How-
ever, you can measure the water potential of the soil under frozen soil
conditions by measuring the soil temperature accurately (Koopmans
and Miller, 1966). For each 1
◦
C decrease in temperature below 0
◦
C, the water potential in the soil decreases by
∼
1200 kPa. Spaans
and Baker (1996) showed that this relationship is valid in field soils
for water potentials below about
−
50 kPa.
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