SM150 User Manual 1
Troubleshooting
25
Functional check
Air reading
Hold the SM150 in air and away from other objects and take a
reading using an HH150 or HH2 meter or voltmeter or a logger with
no more than 5m of cable.
Warning: Do not touch the pins
A typical electrostatic discharge from your body can create a
temporary -10mV offset in sensor readings lasting an hour.
In air an SM150 gives an output of 0 ±4mV.
Note: the HH150 reports under-range if the reading is
less than zero.
Water reading
Immerse the pins in water and measure the output in volts.
In the UK the sensor will typically read about 1.5 volts in tap water
(because the salinity is typically 50mS.m-1).
The “water reading” you get will depend on the salinity of your local
water.
Note: HH150 meter indicates “TOO WET” above 1.5V or 85% vo
l.
Graph: showing the effect of salinity on SM150 sensor output when
fully immersed in water with no soil present.
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
100
200
300
400
500
600
O
u
tp
u
t
(%v
ol
)
O
u
tp
u
t
(V
ol
ts
)
Conductivity EC
p
(mS.m
-1
)
SM150 fully immersed in water - as read
by HH150 and HH2 meters
Volts
% vol (mineral)
%vol (organic)
"TOO WET"
Soil moisture
readings are not
correct when no
soil is present i.e. at
100% vol.
SM150 tables and
polynomial
constants are
optimised at 220
mS.m
-1
for soil
moisture values
below 70%vol
