Campbell Scientific 253-L and 257-L (Watermark 200) Soil Matric Potential Sensors - Manual (Page 23)

253-L and 257-L Soil Matric Potential Sensors
( ) ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
=
X
X
R R
s 1 1
Where X = V
s
/V
x
(output from Instruction 5).
A multiplier of 1, which represents the value of the reference resistor in k Ω,
should be used to output sensor resistance (R
s
) in terms of k Ω.
7.2.3 Calculate Soil Water Potential
The datalogger can calculate soil water potential (kPa) from the sensor
resistance (R
s
) and soil temperature (T
s
). See TABLE 7-3.
The need for a precise soil temperature measurement should not be ignored.
Soil temperatures vary widely where placement is shallow and solar radiation
impinges on the soil surface. A soil temperature measurement may be needed
in such situations, particularly in research applications. Many applications,
however, require deep placement (12 to 25 cm) in soils shaded by a crop
canopy. A common practice for deep or shaded sensors is to assume the air
temperature at sunrise will be close to what the soil temperature will be for the
day.
7.2.3.1 Linear Relationship
For applications where soil water potential is in the range of 0 to –200 kPa,
water potential and temperature responses of the 257 can be assumed to be
linear (measurements beyond –125 kPa have not been verified, but work in
practice).
The following equation normalizes the resistance measurement to 21°C.
( )
dT * 018 . 0 1 21 −
704 . 3 * 407 . 7
21
R
R s =
where
R
21
= resistance at 21°C
R
s
= the measured resistance
dT = T
s
– 21
T
s
= soil temperature
Water potential is then calculated from R
21
with the relationship,
= − R SWP
where SWP is soil water potential in kPa
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