© 2017 Delta-T Devices Ltd
Page
9
6
How to calculate soil moisture
Conversion to
Polynomial (for use over the full range of SM150T readings)
√𝜖 = 1.0 + 14.4396𝑉 − 31.2587𝑉
2
+ 49.0575𝑉
3
− 36.5575𝑉
4
+ 10.7117𝑉
5
………
.(1)
where V is the SM150T output in volts
Linearisation table
(for use over the full range of SM150T readings)
V
V
V
V
V
0.000
1.000
0.300
3.576
0.600
5.101
0.900
6.778
1.200
8.924
0.075
1.942
0.375
3.964
0.675
5.503
0.975
7.232
1.275
9.743
0.150
2.620
0.450
4.337
0.750
5.917
1.050
7.720
1.350 10.808
0.225
3.144
0.525
4.713
0.825
6.342
1.125
8.270
1.425 12.242
Conversion from
to Soil Moisture
◼
Soil moisture
𝜃 = (√𝜀 − 𝑎
0
)/𝑎
1
◼
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
SM150T User Manual
.
◼
Multiply x100 to convert soil moisture from
m
3
.m
-3
to % volumetric.
Direct conversion for Mineral and Organic soils
𝜽
𝒎𝒊𝒏𝒆𝒓𝒂𝒍
= −0.0714 + 1.7190𝑉 − 3.7213𝑉
2
+ 5.8402𝑉
3
− 4.3521𝑉
4
+ 1.2752𝑉
5
… (2)
𝜽
𝒐𝒓𝒈𝒂𝒏𝒊𝒄
= −0.0390 + 1.8753𝑉 − 4.0596𝑉
2
+ 6.3711𝑉
3
− 4.7477𝑉
4
+ 1.3911𝑉
5
… (3)
Example
◼
Take a reading with the SM150T
V = 0.294 volts
either
◼
Convert the reading to
using equation 1 or the
linearisation table below.
= 3.52
◼
Then convert
to soil moisture,
, using the soil
calibration values (
a
0
,
a
1
).
= 22.9% for mineral soil
or
◼
Convert directly for mineral or organic soils using
equations 2 or 3
= 22.9%
a
0
a
1
Mineral
1.6
8.4
Organic
1.3
7.7
Peat mix
1.16
7.09
Coir
1.16
7.41
Min. wool
1.04
7.58
Perlite
1.06
6.53
