EIJKELKAMP SM150-UM-1 User Manual
EIJKELKAMP SM150-UM-1 is a soil moisture sensor used to measure soil moisture content and can be inserted directly into the soil for taking readings. It has a differential analogue DC voltage output which is converted to soil moisture by a data logger or meter. Here are some of its features:
- Accurate soil moisture reading to ±3%
- Low salinity sensitivity
- Excellent stability
- Minimal soil disturbance
- Easy installation at depth in augured holes
- Waterproof connector for IP68
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User Manual for the
SM150
Soil Moisture Sensor
P.O. Box 4, 6987 ZG Giesbeek
Nijverheidsstraat 30,
6987 EM Giesbeek,
The Netherlands
T +31 313 880200
F +31 313 880299
I http://www.eijkelkamp.com
SM150-UM-1
Notices
Copyright
All parts of the SM150 design and documentation are the exclusive right of Delta-T Devices and covered under copyright law.
Copyright © 2011 Delta-T Devices Ltd.
Patents
The SM150 is protected under international law by the following patents:-
USA: Patent US7944220
Europe: Patent EP1836483
Australia: Patent AU2005315407
China: Patent Application CN101080631
EMC Compliance
See page 31.
Design changes
Delta-T Devices Ltd reserves the right to change the designs and specifications of its products at any time without prior notice.
User Manual Version: SM150-UM-1
Nov 2011
P.O. Box 4, 6987 ZG Giesbeek
Nijverheidsstraat 30,
6987 EM Giesbeek,
The Netherlands
T +31 313 880200
F +31 313 880299
I http://www.eijkelkamp.com
Contents
Introduction
Description
Features
Dimensions
Parts list
Care and Safety
How the SM150 works
Operation
Cable Connections
Installation
HH150 Meter
HH2 Meter
Logger connections and configuration
GP1 Logger
DL6 Logger
DL2e Logger
Other data loggers
Calibration
Soil calibration
Sensor calibration
Soil moisture reading
Troubleshooting
Technical Reference
Specifications
Definitions
SM150 User Manual 1.0
24
26
26
32
Introduction 3
6
7
8
9
5
5
5
10
10
11
12
13
14
14
15
16
17
18
19
21
22
References
Appendix 1
Soil-specific Calibration
Laboratory calibration for non-clay soils
Laboratory calibration for clay soils
Technical Support
Index
34
35
35
36
39
42
44
SM150 User Manual 1 Introduction 4
Introduction
Description
The SM150 measures soil moisture content.
Its sealed plastic body is attached to two sensing rods which insert directly into the soil for taking readings.
A waterproof plug connects to a choice of signal cables.
Both extension cables and extension tubes can be used.
The soil moisture output signal is a differential analogue DC voltage. This is converted to soil moisture by a data logger or meter using the supplied general soil calibrations.
It can also be calibrated for specific soils.
Features
Soil moisture accurate to ± 3%
Low salinity sensitivity
Excellent stability
Minimal soil disturbance
Easy installation at depth in augured holes
Waterproof connector to IP68
Rugged, weather-proof and can be buried.
Good electrical immunity
Choice of cabling system options
Cable connector, cylindrical profile and extension tube design simplifies removal for servicing
Dedicated HH150 meter kit for simple readings
HH2 meter, GP1, DL6 or DL2e loggers compatible
See also Specifications on page 26
SM150 User Manual 1.0 Introduction 5
Dimensions
Cable connector
sealed to IP68
M12, 5 pin, male
Thread ¾
inch BSP
for connecting to
Extension Tube(s)
SM150 User Manual 1
22mm
40 mm
Introduction 6
Parts list
Your shipment may include the following:
HH150 soil moisture sensor
HH150 + SM150 Kit
HH150 includes 1m cable,
Connects to SM150
SMCS/d-HH2 1.5m cable
Connects SM150 to HH2
SMSC/sw-05 5m cable
100mm flying leads
Connect to GP1 & DL6 loggers
SMSC/lw-05 5m cable
200mm flying leads
Connect to DL2 data logger
L ogger extension cables
EXT/5W-05 5m
EXT/5W-10 10m
EXT/5W-25 25m
Extension tubes
ML/EX50 50cm
ML/EX100 100cm
SM-AUG-100 Spiral Auger 1.2m
SM150 User Manual 1.0 Introduction 7
Care and Safety
The rods of the SM150 are sharp in order to ease insertion. Care must be taken and handling precautions followed.
To prevent personal injury and damage to the probe always store and transport the
SM150 in this protective tube
CAUTION
Avoid touching the rods or exposing them to other sources of static charge, particularly when powered up.
SHARP PINS
Keep the SM150 in its protective tube when not in use.
Take care when attaching cables to ensure that the connectors are clean, undamaged and properly aligned before pushing the parts together.
Do not pull the SM150 out of the soil by its cable.
If you feel strong resistance when inserting the SM150 into soil, it is likely you have encountered a stone. Stop pushing and re-insert at a new location.
Do not touch the pins, particularly when the sensor is attached to a cable. An electrostatic discharge from your body can typically cause a temporary -10mV offset in sensor readings for up to one hour. At worse it may permanently damage the sensor.
SM150 User Manual 1 Introduction 8
How the SM150 works
When power is applied to the SM150 ...
...it creates a 100MHz waveform (similar to FM radio).
The waveform is applied to a pair of stainless steel rods which transmit an electromagnetic field into the soil.
The water content of the soil surrounding the rods...
V
out
...dominates its permittivity .
(A measure of a material‟s response to polarisation in an electromagnetic field. Water has a permittivity
81, compared to soil
4 and air
1)
The permittivity of the soil has a strong influence on the applied field…
…which is detected by the SM150, resulting in a stable voltage output that…
Soil Moisture
22 %
...acts as a simple, sensitive measure of soil moisture content .
SM150 User Manual 1.0 Introduction 9
Operation
Cable Connections
HH150 includes 1m cable,
Connects to SM150
SMCS/d-HH2 1.5m cable
Connects SM150 to HH2
L ogger extension cables
EXT/5W-05 5m
EXT/5W-10 10m
EXT/5W-25 25m
For wiring colour codes see Logger connections and configuration on page 14
Take care when attaching cables to ensure that the connectors are clean, undamaged and properly aligned before pushing the parts together.
Screw together firmly to ensure the connection is water-tight.
Extension cables* can be joined up to a recommended maximum of 100m (for
GP1, DL6 or DL2e data loggers) – see Specifications on page 26.
*Note: for full accuracy, do not use extension cables with the HH150
SM150 User Manual 1 Operation 10
Installation
Surface installation and spot measurements
Clear away any stones. Pre-form holes in very hard soils before insertion.
Push the SM150 into the soil until the rods are fully inserted. Ensure good soil contact.
If you feel strong resistance when inserting the
SM150, you have probably hit a stone. Stop, and re-insert at a new location.
Installing at depth
Make a 45mm diameter hole, preferably at about
10° to the vertical using the SM-AUG-100 auger.
Connect an extension tube e.g. ML/EX50
Push the SM150 into the soil until rods are fully inserted. Ensure good soil contact
.
Alternatively
Dig a trench, and install horizontally.
SM150 User Manual 1 Operation 11
HH150 Meter
Connect the SM150 to the HH150 meter.
With the meter OFF, press the right off – menu button. This wakes and allows you to set the meter to display readings - either as % volumetric water content of Mineral or Organic soils, or to show the sensor output in Volts.
Press off to save the current Setting and turn the meter off.
With the meter off, press the left on – read button to take a reading.
Repeat as required. You may wish to write down the readings.
Turn on, take and display reading
Organic
32.2%vol
Read
Organic
34.3%vol
Read
Organic
▼
OFF
Read
Mineral
▼
Read
OFF
Volts
▼
Read
Turn on, show current setting
SM150 User Manual 1 Operation 12
HH2 Meter
This assumes you have version 2.6 or later for both the PC software HH2Read and the HH2 firmware (see foot of page).
Connect the SM150 to the HH2 meter.
Press Esc to turn the meter on, and if necessary press again until the HH2 displays the start-up screen.
Set the meter to read from an SM150:
►
Press Set and scroll down to the Device option.
►
Press Set again and scroll down to select
SM150.
Device: SM150
►
Press Set to confirm this choice.
Make sure the HH2 is correctly configured for your soil type:
►
At the start-up screen, press Set and scroll down to the
Soil Type
option.
►
Press Set again and scroll down to the appropriate soil type (use
Mineral
for sand, silt or clay soils or
Organic
for peaty soils)
Soil Type:
Mineral
►
Press Set to confirm this choice.
Choose the units you want for displaying readings.
►
At the start-up screen, press Set and scroll down to the
Display
option.
►
Press Set again and scroll down to select units.
►
Press Set to confirm this choice.
Press Read to take a reading.
Press Store to save or Esc to discard the reading.
SM150 Store?
20.3 %vol
Remove the SM150 from the soil and move to a new location...
If you have saved data, connect your HH2 to a PC and run
HH2Read to retrieve the readings.
For an upgrade contact Delta-T.
See also: HH2 User Manual and
HH2 User Manual Addendum to V4 – SM150 Support.
:
SM150 User Manual 1 Operation 13
Logger connections and configuration
GP1 Logger
4 SM150s can connect to a GP1, but results recorded via channels 3&4 are less accurate.
Requirements
GP1 logger (with v1.47 firmware or later)
PC running DeltaLINK (version 2.6 or later)
SM150 with SMSC/sw-05 cable
Channel 1 and 2 wiring
SM150 wire Colour GP1 terminal
Power 0V
Power V+
Signal HI
Signal LO brown CH1 /2 (GND) white CH1 /2 (PWR) blue black
CH1 /2 (+)
CH1 /2 (-)
Cable shield green
Not used grey
CH1 /2 (GND)
Not connected
Channel 3 and 4 wiring
SM150 wire Colour GP1 terminal
Power 0V
Power V+
Signal HI
Signal LO
Cable shield green
Not used brown CH1 /2 (GND) or WET GND white CH1 /2 (PWR) or WET PWR blue black grey
Temp3
Temp3 /4 (GND)
Temp3
/4
/4
(IN)
(GND)
Not connected
or WET GND
In the DeltaLINK sensor menu configure each sensor type SM150 .
Note: channels 1&2 are wired differently to 3&4. Note also that the relative positions of the terminals change on the left and right sides of the GP1
See also GP1 Quick Start Guide and the DeltaLINK on-line Help.
SM150 User Manual 1 Operation 14
DL6 Logger
6 SM150s can be connected to a DL6.
Each is wired as a differential, powered sensor.
These details illustrate connection to channel 6
SM300 wiring Colour DL6 terminal
Power 0V
Power V+
Signal HI
Signal LO
Cable shield
Not used brown white blue black green grey
0V
V+
IN+
IN-
Not connected
In DeltaLINK
1
configure channel 6 as type SM150 .
See also the DL6 Quick Start Guide and the DeltaLINK online Help.
1
You need the PC logger software DeltaLINK version 2.6 or later obtainable online at www.delta-t.co.uk
or from the Software and manuals CD Issue 5
SM150 User Manual 1 Operation 15
DL2e Logger
Up to 60 SM150s can be connected to a DL2e.
Each moisture sensor is connected as a differential, powered sensor.
These details illustrate connection to channel 58 using a LAC1 input card configured in 15-channel mode, and warm-up channel 63:
SM150 wiring Colour DL2e terminal
Power 0V
Power V+
Signal HI
Signal LO
Cable shield
Not used brown CH62- or 61- white blue black
CH63 NO
CH58+
CH58- green grey
CH61- or 62-
Not connected
Note: If using channel 58 ensure the LAC1 card ribbon is attached to the connector block opposite terminal groups 46-60.
See page 3 of DL2e Quick Start Guide .
Configure the chosen DL2e logger channels by selecting the appropriate S1M and S1O sensor types for mineral and organic soils listed in the Ls2Win
2 sensor library.
See also the DL2e User Manual and the Ls2Win online help
2
You need a PC running Ls2Win version 1.0 SR8 or later. A free upgrade can be obtained from www.delta-t.co.uk
or from the Software and manuals CD Issue 5.
SM150 User Manual 1 Operation 16
Other data loggers
The SM150 soil moisture output should be connected as a differential, powered sensor.
Configure the logger to convert the SM150 readings from milliVolts into soil moisture units by using either :-
Polynomial conversion on page 22 or
Linearisation table conversion on page 23
.
Note: Output signals in the range 0 to 1.0 volts from the SM150, corresponding to ~0 to 60% water content in mineral soils – see
Linearisation table conversion on page 23.
Note: The SM150 has been optimised for warm-up of 0.5 to 1 second duration. It is recommended that the sensor is not powered continuously.
SM150 User Manual 1 Operation 17
Calibration
The SM150 is provided with general calibrations for mineral
and organic
soils which can be used to convert the output from the sensor directly into soil moisture when used with Delta-T loggers and moisture meters. This section explains how these calibrations work, how to adapt them for other soils and how to provide calibrations for other data loggers.
The SM150 measures volumetric soil moisture
, by detecting the dielectric properties of the damp soil – the permittivity,
, or more conveniently the refractive index
, which is closely equivalent to
.
The SM150 response is best understood in these stages:
SM150 User Manual 1 Operation 18
Soil calibration
Damp soil is essentially a mixture of soil particles, air and water, and together these components determine its dielectric properties, including the refractive index
. The refractive index of the mixture is approximated simply by adding the contributions from the individual components [ref 4.].
For a particular soil, the contribution from the soil particles can be assumed to be constant, so the refractive index measured by the
SM150 is only affected by changes to the water content,
. This relationship simplifies to:
a
0
a
1
where the coefficients a
0
and a
1
conveniently parameterise the dielectric properties of soils.
Soil calibrations
8.0
6.0
4.0
Slope (a
1
)
2.0
Offset (a
0
)
0.0
0.0
0.2
0.4
Soil moisture (m
3
.m-
3
)
0.6
0.8
Note that: a
0
dry _ soil
is usually between 1.3 to 2.3 a
1 corresponds approximately to
water
1 and usually takes a value about 8.0. Real soil values for a
0
and a
1
can vary significantly from these guidelines when they are affected by other factors – in particular, bound water in clay may result in higher values of a
1
.
SM150 User Manual 1 Operation 19
General soil calibrations
Most soils can be characterised simply by choosing one of the two general calibrations we supply, one for mineral soils (predominantly sand, silt and clay) and one for organic soils (with a high organic matter content).
Mineral soils
Organic soils a
0
1.6
1.3 a
1
8.4
7.7
General soil calibrations
8.0
6.0
Mineral
Organic
4.0
2.0
0.0
0.0
0.2
0.4
Soil Moisture (m
3
.m
-3
)
0.6
0.8
These values have been used to generate the polynomial conversions and linearisation tables in the Soil moisture reading section.
Soil-specific calibration
Instead of adopting these general calibrations, you may wish to determine specific calibration values of a
0
and a
1
for your soil. This procedure is fairly straightforward if you can get access to standard laboratory equipment and is described in detail in Appendix 1 on page 35.
Soil specific calibration can significantly improve the accuracy of individual readings - but make less of an improvement to readings where installation and sampling errors are high.
SM150 User Manual 1 Operation 20
Sensor calibration
Each SM150 is individually adjusted to provide consistent dielectric performance:
SM150 dielectric performance
8.0
6.0
√ε
4.0
2.0
0.0
0.00
0.20
0.40
0.60
0.80
SM150
output (V)
1.00
1.20
This response can be approximated either by a polynomial (below) or by a linearisation table (see next page):
Polynomial (for use over the full range of SM300 readings)
√ where V is the SM150 output in Volts
SM150 User Manual 1 Operation 21
Linearisation table
(for use over the full range of SM150 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
Soil moisture reading
Polynomial conversion
Combining the
Soil calibrations
and
Sensor calibration steps, the conversion equation becomes:
[ ] where a
0
and a
1 are the calibration coefficients
For a generalised mineral
soil this becomes:
And for a generalised organic
soil:
SM150 User Manual 1 Operation 22
Linearisation table conversion
The conversion from SM150 reading (Volts) to soil moisture
(m
3
.m
-3
or %vol) can be accomplished by a look-up table.
The following table lists the values used for the DL2e data logger:
Soil moisture
%vol
12
16
20
24
28
-4
0
4
8
32
36
40
44
48
Mineral soil
Volts
Organic soil
Volts
-2.090 -2.090
0.046 0.022
0.076 0.046
0.110 0.074
0.149 0.105
0.195 0.140
0.248 0.180
0.308 0.226
0.373 0.279
0.440 0.336
0.507 0.397
0.573 0.458
0.636 0.520
0.699 0.580
Soil moisture
%vol
68
72
76
80
84
52
56
60
64
88
92
96
100
104
Mineral soil
Volts
Organic soil
Volts
0.758 0.638
0.818 0.695
0.876 0.750
0.933 0.805
0.987 0.859
1.039 0.910
1.087 0.962
1.130 1.010
1.170 1.056
1.206 1.099
1.238 1.138
1.267 1.174
1.294 1.207
2.090 2.090
SM150 User Manual 1 Operation 23
Troubleshooting
Always try to identify which part of the measurement system is the source of the difficulty. For the SM150 this may fall into one of the following areas:
The measurement device
What equipment is being used to read the probe output?
An HH150 or HH2 Moisture Meter.
A data logger such as the GP1, DL6 or DL2e
Check Versions
Check you have the correct versions:
HH2 Meter: Firmware version 2.6 or later and PC software
HH2Read version 2.6 or later are recommended.
GP1 & DL6 Loggers: DeltaLINK version 2.6 or later is required.
DL2e Logger: Ls2Win 1.0 SR8 or later is required
Consult the user manuals or the on-line help for these devices and their related software.
Try alternative types of equipment if you have them available.
Check that you are using an appropriate soil calibration and the correct conversion method – see Calibration section.
The SM150 itself
Try to isolate the problem into one of the following areas
The SM150 or the connecting cable
Then try to narrow down the area further
Mechanical problems faults, or damage
Electrical or electronic problems or faults
SM150 User Manual 1 Troubleshooting 24
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.
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
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.
SM150 fully immersed in water - as read by HH150 and HH2 meters
"TOO WET"
Volts
% vol (mineral)
%vol (organic)
100 200 300 400
Conductivity EC p
(mS.m
-1 )
500
100.00%
90.00%
80.00%
70.00%
60.00%
50.00%
40.00%
30.00%
20.00%
10.00%
600
0.00%
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
Graph: showing the effect of salinity on SM150 sensor output when fully immersed in water with no soil present.
SM150 User Manual 1 Troubleshooting 25
Technical Reference
Specifications
Accuracy
Measurement range
Salinity error
Conductivity response
Temperature sensitivity
Sampling volume
Output signal
Output compatible with
Maximum cable length
Power requirement
Operating range
Environment
Sample volume
Dimensions/Weight
3.0% vol over 0 to 70 % vol and 0-60°C
0 to 100% vol but less accurate above 70%vol
3
5% vol over 100 to 1000 mS.m
-1
and 0-60% vol
See page 27
See page 28
See page 29
0-1 V differential
0 to 60% nominal
HH150, HH2, GP1, DL6, DL2e
1m (HH150 meter)
100m (GP1, DL6 and DL2e data loggers)
5-14VDC, 18mA for 1s
-20 to +60°C
IP68
4
55 x 70mm diameter
143 x 40 mm diameter/ 0.1 kg
3
In water (no soil present) the reading may not be 100% vol. It depends on a0 and a1 but can still be used as a quick check that the unit is working. See also page 25
4
With Delta-T supplied cables
SM150 User Manual 1 Technical Reference 26
Conductivity response
This chart shows how salinity affects the output of the soil moisture sensor at various soil moisture levels.
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
SM150 conductivity response at different water contents
100% (water)
~60%
~45%
~38%
~30%
~20%
100
Typical artificial substrate conditions
Typical soil conditions
200 300 400
Conductivity EC p
(mS.m
-1 )
500 600
0 nonsaline
200 slightly saline
400 moderately saline
600
0 2 4
1000 strongly saline
1200 1400 1600
16 extremely saline mS.m
-1 dS.m
-1
6 8 10 12
Classification of salinity
14
SM150 User Manual 1 Technical Reference 27
Temperature response of soil moisture readings
The effect of temperature on the SM150 soil moisture readings in any particular soil will depend on a combination of effects:
The SM150 soil moisture electronics has very low temperature sensitivity, and makes a negligible contribution to the overall sensitivity.
The refractive index of water (
, see Calibration section) reduces as the temperature increases. This produces a negative temperature response particularly in soils or substrates with high water content.
Water that is bound to the surface of soil particles has a much lower refractive index than free water. The percentage of bound water decreases with temperature and this produces a positive temperature response particularly in clay soils at lower water contents.
The last two effects partially offset each other, but in soil conditions where one or the other effect dominates, the SM150 will appear to have a significant temperature response. This illustration is based on the model in reference 7, see page 34.
0.5
Illustrating temperature dependence of SM150 readings in a clay soil
0.4
0.3
θ ~ 0.4
θ ~ 0.3
θ ~ 0.2
θ ~ 0.1
bound water
0.2
0.1
0
0 10 20 30
Soil temperature ( ° C)
40 50
Note: ice has a quite different refractive index from water, so SM150 soil moisture readings cannot be interpreted reliably when inserted into soil below 0°C.
SM150 User Manual 1 Technical Reference 28
Sampling Volume
The SM150 is most sensitive to signals very close to the two rods, but a small proportion of the signal reaches up to 50mm from the rods.
SM150 field of sensitivity surrounding the rods
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
-25 -20
-15 -10
Distance parallel to plane of
-5
0
5
10 rods from a centre line between the rods (mm)
15
20
25
Minimum soil sample size: Full accuracy requires a soil volume of one litre but the additional error from taking a reading in a 0.5 litre sample is negligible
SM150s may interact if they are placed too close together – they should be separated by at least 100mm.
SM150 User Manual 1 Technical Reference 29
If the SM150 is inserted too close to the wall of a plant pot the sensing field can “see” outside the pot. This behaviour is shown in the graph below.
For best results keep a gap of at least 25mm (1 inch) between the body of the sensor and the wall of the plant pot.
SM150 User Manual 1 Technical Reference 30
Electromagnetic Compatibility (EMC)
General information
SM150 is a Class A product, intended for operation in nonresidential environments.
Only use cables and accessories authorised by Delta-T (sensor cables from other sources for example may adversely affect product performance and affect quality of results).
If possible route cables along the soil surface or bury them – this also reduces possible trip hazard and animal damage.
Do not modify the product or its supplied accessories.
See also SM150 EMC Guidance on the Software and Manuals CD
Issue 5
Regulatory information
Europe
This device conforms to the essential requirements of the EMC directive 2004/108/EC, based on the following test standards:
EN61326-1:2006 Electrical requirement for measurement, control and laboratory use. EMC requirements: Group 1, Class A equipment – (emissions section only).
EN61326-1:2006 Electrical requirement for measurement, control and laboratory use. EMC requirements: Basic Immunity (immunity section only).
FCC compliance (USA)
This device conforms to Part 18 of FCC rules – Industrial, Scientific
& Medical Equipment.
Note: with reference to FCC Part 18.115 Elimination and investigation of harmful interference.
(a) The operator of the ISM equipment that causes harmful interference to radio services shall promptly take appropriate measures to correct the problem.
SM150 User Manual 1 Technical Reference 31
Definitions
Volumetric Soil Moisture Content is defined as
V
V
W
V
S where V w
is the volume of water contained in the sample and V s
is the total volume of the soil sample.
The preferred units for this ratio are m
3
.m
-3
, though %vol is frequently used.
Soil Moisture Content varies from approx. 0.02 m
3
.m
-3
for sandy soils at the permanent wilting point, through approx. 0.4 m
3
.m
-3
for clay soils at their field capacity, up to values as high as 0.85 m
3
.m
-3 in saturated peat soils.
Gravimetric Soil Moisture Content is defined as g.g
where M
W
is the mass of water in the sample, and M
S
is the total mass of the dry sample.
To convert from volumetric to gravimetric water content, use the equation
G
V
W
S where
W
is the density of water (= 1g.cm
-3
), and
S
is the bulk density of the sample (
M
S
V
S
).
SM150 User Manual 1 Technical Reference 32
Organic and Mineral soil definitions:
The general calibrations have been optimised to cover a wide range of soil types, based on the following definitions:
Soil type optimised use for around organic content: organic contents: bulk density range:
(g.cm
-3
) use for bulk densities:
(g.cm
-3
)
Mineral ~ 1 %C* < 7 %C 1.25 - 1.5 > 1.0
Organic ~ 40 %C > 7 %C 0.2 - 0.7 < 1.0
* Note: %C denotes “percentage Carbon” and is a measure of organic content
Salinity
The preferred SI units for ionic conductivity are mS.m
-1
(where S is
Siemens, the unit of electric conductance. Dimensionally it is equivalent to the inverse of resistance i.e. Ohm
-1
).
The following conversions apply:
1 mS.m
-1
= 0.01 dS.m
-1
= 0.01 mS.cm
-1
= 10 µS.cm
-1
Soil salinity can be classified using the following descriptive categories:
0 nonsaline
200 slightly saline
400 moderately saline
600 1000 strongly saline
1200 1400 1600 extremely saline mS.m
-1
0 2 4 6 8 10 12 14 16 dS.m
-1
Classification of salinity
See also http://www.land.vic.gov.au/DPI/Vro/vrosite.nsf/pages/water_spotting_soil_salting_class_ranges#s1
SM150 User Manual 1 Technical Reference 33
References
1.
Gaskin, G.J. and J.D. Miller, 1996
Measurement of soil water content using a simplified impedance measuring technique.
J. Agr. Engng Res 63 , 153-160
2.
Topp, G.C., J. L. Davis and A. P Annan 1980
Electromagnetic determination of soil water content .
Water Resour. Res 16 (3) 574-582
3.
Whalley, W.R. 1993
Considerations on the use of time-domain reflectometry
4.
5.
6.
7.
(TDR) for measuring soil moisture content.
Journal of Soil Sci. 44 , 1-9
White, I., J.H. Knight, S.J. Zegelin, and Topp, G.C. 1994
Comments on „Considerations on the use of time-domain reflectometry (TDR) for measuring soil water content‟ by W R
Whalley
Journal of Soil Sci. 45 , 503-508
Roth, C.H., M.A. Malicki, and R. Plagge, 1992
Empirical evaluation of the relationship between soil dielectric constant and volumetric water content as the basis for calibrating soil moisture measurements.
Journal of Soil Sci. 43 , 1-13
Knight, J.H. 1992
Sensitivity of Time Domain Reflectometry measurements to lateral variations in soil water content.
Water Resour. Res., 28 , 2345-2352
Or, D. and J.M. Wraith 1999
Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: A physical model.
Water Resour Res., 35 , 371-383
SM150 User Manual 1 References 34
Appendix 1
Soil-specific Calibration
This note provides details of 2 techniques for generating soilspecific calibrations:
Laboratory calibration for substrates * and non-clay soils
Laboratory calibration for clay soils
* We use the term substrate to refer to any artificial growing medium.
Underlying principle
Soil moisture content (
) is proportional to the refractive index of the soil (
) as measured by the SM150 (see Calibration section).
The goal of calibration is to generate two coefficients ( a
0
, a
1
) which can be used in a linear equation to convert probe readings into soil moisture:
a
0
a
1
SM150 User Manual 1 Appendix 1 35
Laboratory calibration for non-clay soils
Process
This is the easiest technique, but it‟s not suitable for soils that shrink or become very hard when dry.
Equipment you will need:
SM150 and meter
Soil corer (if doing a calibration for a cohesive soil rather than sand or a substrate)
Heat-resistant beaker (
0.5 litre)
Weighing balance (accurate to < 1g)
Temperature controlled oven (for mineral soils or substrates)
Notes and example
Collect a damp sample of the soil or substrate.
This sample needs to be unchanged from its in-situ density, to be
0.5 litre, to have the correct dimensions to fit the beaker, and to be generally uniform in water content.
For cohesive soils this is most easily done with a soil-corer.
Sandy soils can be poured into the beaker, but you should take the subsequent measurements immediately, as the water will quickly begin to drain to the bottom of the beaker.
Compressible soils and composts often require measurement of the in-situ density and then need to be carefully reconstituted at that density within the beaker.
Measure the volume occupied by the sample.
L s
= 463.5
ml
Weigh the sample, including the beaker .
W w
= 743.3
g
SM150 User Manual 1 Appendix 1 36
Insert SM150 into the sample and record its output in Volts.
V w
= 0.350
V
Dry the sample thoroughly.
With mineral soils this is usually achieved by keeping it in the oven at 105°C for several hours or days (the time required depends on the sample size and porosity).
For organic soils a nd composts it‟s usual to air-dry the sample to avoid burning off any volatile factions.
Weigh the dry sample in the beaker.
W
0
= 627.2
g
Re-insert the SM150 into the dry sample and record this reading.
V
0
= 0.051
V
Calculate
a
0
For the SM150,
In the dry soil V = V
0
= 0.051 Volts
Substitute this into the equation
√ gives √
Since
0
= 0, this is the value needed for a
0 a
0
= 1.66
Calculate
w
The water content of the wet soil,
w
, can be calculated from the weight of water lost during drying, ( W w
–
W
0
) and its
SM150 User Manual 1 Appendix 1 37
Calculate
a
1
Result volume, L s
:
w
W w
W
0
L s
743 .
3
627 .
2
463 .
5
0 .
25
w
= 0.25
In the wet soil V = V w
= 0.350 Volts and substituting gives
w
3 .
79
Finally
a
1
w
0
w
0
3 .
79
1 .
66
0 .
25
0
8 .
51 a
1
= 8.51
a
0
= 1.66
a
1
= 8.51
I n this example this soil is now calibrated.
You can now use these two numbers in place of the standard mineral or organic calibration factors to convert SM150 readings into volumetric water content
θ using:
a
0
a
1
See also page Underlying principle on page 35
SM150 User Manual 1 Appendix 1 38
Laboratory calibration for clay soils
Process
This technique is adapted to avoid the near-impossibility of inserting the SM150 into completely dry clay soil. It requires taking measurements at 2 significantly different, but still damp, moisture levels.
Equipment you will need:
SM150 and meter
Soil corer
Heat-resistant beaker (
500ml)
Weighing balance (accurate to < 1g)
Temperature controlled oven
Notes and example
Collect a wet sample of the clay soil: 25 to 30% water content would be ideal.
This sample needs to be unchanged from its in-situ density, to be
500ml, to have the correct dimensions to fit the beaker, and to be generally uniform in water content.
This is most easily done with soil-corer.
Measure the volume occupied by the sample
.
L s
= 463.5
ml
Weigh the wet sample, including the beaker
.
W w
= 743.3
g
SM150 User Manual 1 Appendix 1 39
Insert SM150 into the wet sample and record its output in
Volts.
V w
= 0.349
V
Dry the sample until still moist, ~15% water content. Gentle warming can be used to accelerate the process, but take care not to over-dry in places, and allow time for the water content to equilibrate throughout the sample before taking a reading.
Reweigh.
W m
= 693.2
g
Re-measure with the SM150.
V m
= 0.180
V
Dry the sample thoroughly.
With clay soils this is usually achieved by keeping it in the oven at 105°C for several hours or days (the time required depends on the sample size and porosity).
Weigh the dry sample in the beaker
.
W
0
= 627.2
g
SM150 User Manual 1 Appendix 1 40
Calculations Substituting in the SM150 equation
√ provides two dielectric values,
w
and
m
, at two known water contents,
w
and
m
For the wet soil
For the moist soil
Substituting Vw = 0.349 gives
√ for
( )
Substituting Vm = 0.180 gives
√
f
or
( )
Calculate
a
1
Then
√ √ a
1
= 9.00
Calculate
a
0
and
√ ( ) a
0
= 1.58
Result a
1
= 9.00 a
0
= 1.58
In this example this soil is now calibrated.
You can now use these two numbers in place of the standard mineral or organic calibration factors to convert SM150 readings into volumetric water content θ using:
a
0
a
1
See also page Underlying principle on page 35
SM150 User Manual 1 41
Technical Support
Terms and Conditions of Sale
Our Conditions of Sale (ref: COND: 1/07) set out Delta-T's legal obligations on these matters. The following paragraphs summarise Delta T's position but reference should always be made to the exact terms of our Conditions of Sale, which will prevail over the following explanation.
Delta-T warrants that the goods will be free from defects arising out of the materials used or poor workmanship for a period of twelve months from the date of delivery.
Delta-T shall be under no liability in respect of any defect arising from fair wear and tear, and the warranty does not cover damage through misuse or inexpert servicing, or other circumstances beyond their control.
If the buyer experiences problems with the goods they shall notify Delta-T
(or Delta-
T‟s local distributor) as soon as they become aware of such problem.
Delta-T may rectify the problem by replacing faulty parts free of charge, or by repairing the goods free of charge at Delta-T's premises in the UK during the warranty period.
If Delta-T requires that goods under warranty be returned to them from overseas for repair, Delta-T shall not be liable for the cost of carriage or for customs clearance in respect of such goods. However, Delta-T requires that such returns are discussed with them in advance and may at their discretion waive these charges.
Delta-T shall not be liable to supply products free of charge or repair any goods where the products or goods in question have been discontinued or have become obsolete, although Delta-T will endeavour to remedy the buyer‟s problem.
Delta-T shall not be liable to the buyer for any consequential loss, damage or compensation whatsoever (whether caused by the negligence of the
Delta-T, their employees or distributors or otherwise) which arise from the supply of the goods and/or services, or their use or resale by the buyer.
Delta-T shall not be liable to the buyer by reason of any delay or failure to perform their obligations in relation to the goods and/or services if the delay or failure was due to any cause beyond the Delta-
T‟s reasonable control.
Service, Repairs and Spares
Users in countries that have a Delta-T distributor or technical representative should contact them in the first instance.
Spare parts for our own instruments can be supplied and can normally be despatched within a few working days of receiving an order.
Spare parts and accessories for products not manufactured by Delta-T may have to be obtained from our supplier, and a certain amount of additional delay is inevitable.
No goods or equipment should be returned to Delta-T without first obtaining the return authorisation from Delta-T or our distributor.
On receipt of the goods at Delta-T you will be given a reference number.
Always refer to this reference number in any subsequent correspondence.
The goods will be inspected and you will be informed of the likely cost and delay.
We normally expect to complete repairs within one or two weeks of receiving the equipment. However, if the equipment has to be forwarded to our original supplier for specialist repairs or recalibration, additional delays of a few weeks may be expected. For contact details see below.
Technical Support
Users in countries that have a Delta-T distributor or technical representative should contact them in the first instance.
Technical Support is available on Delta-T products and systems. Your initial enquiry will be acknowledged immediately with a reference number.
Make sure to quote the reference number subsequently so that we can easily trace any earlier correspondence.
In your enquiry, always quote instrument serial numbers, software version numbers, and the approximate date and source of purchase where these are relevant.
Contact details:
P.O. Box 4, 6987 ZG Giesbeek
Nijverheidsstraat 30,
6987 EM Giesbeek,
The Netherlands
T +31 313 880200
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I http://www.eijkelkamp.com
Index
A
Air reading, 25
C
Cable Connections, 10
Calibration, 18 check, 25 generalised, 22 sensor, 21, 22 soil, 19, 22
Soil, 35 soil-specific, 35
Care and safety, 8
Care and Safety, 8
Certification emc, 31
Conductivity response, 27
Connections , 10, 14
Conversions linearisation table, 20, 21 polynomial, 20, 21
Copyright, 2
D
Data logger, 17, 23, 24, 25
DL2e, 16, 23, 24
DL6, 15, 24
GP1, 14, 15, 24 other, 17
Definitions, 32
Description, 5
Dielectric performance, 21 refractive index, 18, 19, 35
Dimensions, 6
DL6, 15
EMC, 31
Extension cables, 10
E
FCC compliance, 31
Features, 5
F
G
GP1, 14, 15, 24
H
HH150 meter, 12
HH2, 25
HH2 Meter, 13
Installation buried, 11 insertion rod, 11 surface, 11
I
L
Linearisation table, 22 volts to %vol, 17, 23
M
Moisture content, 5, 9, 34, 35
Pair of sensor rings
O
Organic and Mineral , 33
P
Parts, 7
Patent, 2
Permittivity, 9, 18, 34
Polynomial conversion, 22
R
References, 34
Regulatory information, 31
Rods, 8, 9, 11
S
Salinity, 33
Sampling Volume, 29
Servicing, 43
Soil clay, 19, 20, 32, 35, 36, 39 mineral, 18, 20, 22, 36, 37, 40 organic, 18, 20, 22, 33, 37 stony, 11 soil calibrations, 20
Specifications, 2, 26
T
Technical support, 42, 43
Temperature response of soil moisture readings, 28
Troubleshooting, 24
V
Volumetric Soil Moisture , 32
W
Warning : Do not touch the pins , 25
Water reading, 25

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