© 2005 Delta-T Devices Ltd
Page
7
a
0
a
1
Mineral 1.6 8.4
Organic 1.3 7.7
Example
Take a reading with the SM200
V
= 0.294 volts
either
Convert the reading to
√ε
using the polynomial or
linearisation table below.
√ε
= 3.62
Then convert
√ε
to soil moisture,
θ
, using the soil
calibration values (
a
0
,
a
1
).
θ
=
24.0%
or
(for mineral soil)
Convert directly for mineral or organic soils
θ
= 24.0%
5
Convert SM200 voltages to soil moisture readings
Note
: The
SM200 output is automatically converted to soil moisture when using an
HH2 meter or Delta-T data loggers.
Conversion to
√ε
Polynomial
(use for SM200 readings from 0 to 1.1 Volts)
5
4
3
2
536
.
13
032
.
46
881
.
60
725
.
38
103
.
16
0
.
1
V
V
V
V
V
+
−
+
−
+
=
ε
where
V
is the SM200 output in volts
Linearisation table
(use
for SM200 readings from 0 to 1.2 Volts)
V
√ε
V
√ε
V
√ε
V
√ε
V
√ε
0.000 1.011 0.120 2.461 0.260 3.442 0.820 5.936 1.150 7.661
0.030 1.447 0.150 2.718 0.310 3.700 0.960 6.586 1.200 8.039
0.060 1.832 0.180 2.946 0.400 4.105 1.040 6.991 1.220 8.264
0.090 2.168 0.220 3.211 0.600 4.953 1.100 7.333 1.800 9.000
Conversion from
√ε
to Soil Moisture
Soil moisture
(
)
1
0
a
a
−
=
ε
θ
Use these generalised soil calibration values for
mineral and organic soil types, or carry out a soil-
specific calibration to derive your own values – see
SM200 User Manual
.
Multiply x100 to convert soil moisture from m
3
.m
-3
to % volumetric.
Direct conversion for Mineral and Organic soils
5
4
3
2
mineral
611
.
1
48
.
5
248
.
7
61
.
4
917
.
1
071
.
0
V
V
V
V
+
−
+
−
+
−
=
θ
5
4
3
2
organic
758
.
1
978
.
5
907
.
7
029
.
5
091
.
2
039
.
0
V
V
V
V
V
+
−
+
−
+
−
=
θ
