Delta-T Devices Ltd SM300 User Manual

User Manual for the

SM300

Soil Moisture Sensor

Delta-T Devices Ltd

SM300-UM-1.2

Notices

Copyright

All parts of the SM300 design and documentation are the exclusive
right of Delta-T Devices and covered under copyright law.

Copyright © 2014 Delta-T Devices Ltd.

Patents

The SM300 is protected under international law by the following
patents:-

USA: Patent US7944220

Europe: Patent EP1836483

Australia: Patent AU2005315407

China: Patent CN101080631

EMC Compliance

See page 33.

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: 1.2

Delta-T Devices Ltd

130 Low Road, Burwell

Cambridge CB25 0EJ
UK

Tel: [phone]

Fax: [phone]
email: [e-mail]
web: www.delta-t.co.uk

SM300 User Manual 1.2  3

Contents

Introduction 5

Description 5

Features 5
Dimensions 6

Parts list 7
Care and Safety 8
How the SM300 works 9

Operation 10

Cable Connections 10
Installation 11
Logger connections and configuration 12

GP1 Logger 12
GP2 Logger Controller 13
DL6 Logger 14
DL2e Logger 15
Other data loggers 16
Logging Advice 16
Logger Grounding 17

HH2 Meter 20
Calibration 21

Soil calibration 21
Sensor calibration 24
Soil moisture reading 25

Troubleshooting 27

Technical Reference 29

Specifications 29

Volumetric water content 29
Temperature 29

Definitions 34

References 36

SM300 User Manual 1.2  4

Technical Support 37

Appendix 1 39

Soil-specific Calibration 39
Laboratory calibration for non-clay soils 40
Laboratory calibration for clay soils 43

Appendix 2: 46

The SM300 Temperature Sensor 46

SM300 Temperature Measurement 47
Effect of Temperature on Water Permittivity 49

Resistance to Temperature Lookup Table 50

Index 51

SM300 User Manual 1.2 Introduction  5

Introduction

Description

The SM300 measures soil moisture content and temperature.
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 ± 2.5%

 Soil temperature to ± 0.5°C over 0-40°C

 Low salinity sensitivity

 Excellent stability

 Minimal soil disturbance

 Easy installation at depth in augured holes

 Waterproof connector to IP68

 Rugged, weatherproof and can be buried.

 Good electrical immunity

 Choice of cabling system options

 Cable connector, cylindrical profile and extension tube

design simplifies removal for servicing.

See also Specifications on page 29

SM300 User Manual 1.2 Introduction  6

Dimensions

67 mm

92 mm

51mm

22mm

12

40 mm

Cable connector

sealed to IP68
M12, 5 pin, male

Thread ¾inch BSP

for connecting to
Extension Tube(s)

SM300 User Manual 1.2 Introduction  7

Parts list

Your shipment may include the following:

Part Sales Code

Description

SM300

SM300 sensor
with Quick Start Guide

SMSC/d-HH2

0.9m cable connects
to HH2 meter
via 25-way D­connector

SMSC/sw-05

5m cable with
100 mm flying leads
for GP1 or DL6 logger

SMSC/lw-05

5m cable with
200mm flying leads
for DL2e logger

EXT/5W-05
EXT/5W-10
EXT/5W-25

5, 10 and 25m
extension cables.
IP68 M12 connectors

ML/EX50
ML/EX100

50 and 100cm
Extension Tube

SM-AUG-100

45mm spiral auger

1.2m long

SM300 User Manual 1.2 Introduction  8

To prevent personal injury and damage to

the probe always store and transport the

SM300 in this protective tube

CAUTION

SHARP PINS

Care and Safety

 The rods of the SM300 are sharp in

order to ease insertion. Care must
be taken and handling precautions
followed.

 Avoid touching the rods or exposing

them to other sources of static
damage, particularly when powered up.
Keep the SM300 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 SM300 out of the soil by its cable.

 If you feel strong resistance when inserting the SM300 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.

SM300 User Manual 1.2 Introduction  9

How the SM300 works

When power is applied to the SM300...

...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...

ε

...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…

Vout

…which is detected by the SM300, resulting in a stable
voltage output that…

Soil Moisture

22 %

...acts as a simple, sensitive measure of soil moisture

content.

SM300 User Manual 1.2 Operation  10

Operation

Cable Connections

 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 – see Specifications on page 29

0.9m

5m
10m
25m

5m with

100 or 200
mm bare

leads

HH2

cable

Extension
cables

Logger
cables

brown
white
blue
black

grey
green

Power 0V
Power V+
Signal HI
Signal LO
Temperature +
Cable shield

SM300 User Manual 1.2 Operation  11

Installation

Surface installation and spot measurements

 Clear away any stones. Pre-form holes in

very hard soils before insertion.

 Push the SM300 into the soil until the rods

are fully inserted. Ensure good soil contact.

 If you feel strong resistance when inserting

the SM300, you have probably hit a stone.
Stop, and re-insert at a new location.

Note: The SM300 is not suitable for soil surface temperature
measurements. For soil temperature near the surface dig a trench and
install horizontally as shown below. Cover both SM300 and the first
30cm of cable with at least 5cm of soil.

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 SM300 into the soil until rods are

fully inserted. Ensure good soil contact.

Alternatively

 Dig a trench, and install horizontally.

SM300 User Manual 1.2 Operation  12

Logger connections and configuration

GP1 Logger

Two SM300s can connect to each GP1.
Each soil moisture sensor is wired as a
differential, powered sensor.

These details illustrate connection to
Channels 1 and 3:

SM300 wire Colour GP1 terminal

Power 0V

brown

CH1 (GND)
or Temp (GND)

Power V+

white

CH1 (PWR)

Signal HI

blue

CH1 (+)

Signal LO

black

CH1 (-)

Temperature +

grey

Temp3 (IN)

Cable shield

green

CH1 (GND)

Using the DeltaLINK

1

logger software, configure channel 1 or 2 as
sensor type SM300 and channel 3 or 4 as an SM300 Temperature
sensor.

See also GP1 Quick Start Guide and the DeltaLINK on-line Help.

1

You need the PC logger software DeltaLINK version 2.4 or later. A free upgrade can

be obtained from www.delta-t.co.uk

or from the Software and Manuals DVD.

SM300 User Manual 1.2 Operation  13

GP2 Logger Controller

Up to 6 SM300s can connect to a GP2.
Up to 12 can be connected if not using the temperature
sensor.
If using more than 9 you need expansion lid GP2-G5-LID.

These details illustrate connection to Channels 1 and 2:

SM300 wire Colour GP2 terminal

Power 0V

brown

CH1 (PGND)

Power V+

white

CH1 (PWR)

Soil Moisture Signal HI

blue

CH1 (+)

Soil Moisture Signal LO

black

CH1 (-)

Cable shield

green

CH1 (PGND)

Thermopile HI

grey

CH2(+) and CH2(-)
Fit wire link

For configuration details see the DeltaLINK2 software sensor Info Panel and
Help or the GP2 User Manual.

2

You need the PC logger software DeltaLINK version 3.0 or later. A free upgrade can

be obtained from www.delta-t.co.uk

or from the Software and Manuals DVD.

SM300 User Manual 1.2 Operation  14

DL6 Logger

6 SM300s can be connected to a DL6.
Each soil moisture sensor is wired as a differential,
powered sensor.
A DL6 logger can only read one SM300
temperature sensor.*

These details illustrate connection to channels 6
& 7:

SM300 wiring Colour DL6

terminal

Power 0V brown

0V

Power V+ white

V+

Signal HI blue

IN+

Signal LO black

IN-

Temperature + grey

RES IN+

Cable shield green

In DeltaLINK

3

configure channel 6 as type SM300 and channel 7 as a

type SM300 Temperature sensor.

See also the DL6 Quick Start Guide and the DeltaLINK online Help.

3

You need the PC logger software DeltaLINK version 2.4 or later. A free upgrade can

be obtained from www.delta-t.co.uk

or from the Software and manuals DVD.

SM300 User Manual 1.2 Operation  15

DL2e Logger

Up to 60 SM300s can be connected to a DL2e
logger (if not using the temperature sensor
channel).
Up to 30 SM300s can be connected if also
reading the temperature sensor.

Each moisture sensor is connected as a differential, powered sensor.

These details illustrate connection to Channels 57 and 58 using a
LAC1 input card configured in 15-channel mode, and warm-up channel
63:

SM300 wiring

Colour DL2e terminal

Power 0V brown

CH62- or 61-

Power V+ white

CH63 NO

Signal HI blue

CH58+

Signal LO black

CH58-

Temperature + grey

CH57+ and CH57-

Cable shield green

CH61- or 62-

Configure the chosen DL2e logger channels by selecting the
appropriate S3M and S3O sensor types for mineral and organic soils
and S3T for the temperature sensor type listed in the Ls2Win

4

sensor

library.

See the DL2e User Manual and the Ls2Win online help

4

You need a PC running Ls2Win version 1.0 SR6 or later. A free upgrade can be

obtained from www.delta-t.co.uk

or from the Software and manuals DVD.

SM300 User Manual 1.2 Operation  16

Other data loggers

 The SM300 soil moisture output should be connected as a differential,

powered sensor.

 Configure the logger to convert the SM300 readings from milliVolts into

soil moisture units by using either :-

Polynomial conversion on page 25 or
Linearisation table

conversion on page 26

Note: Output signals in the range 0 to 1.0 volts from the SM300,
corresponding to ~0 to 60% water content in mineral soils – see
Linearisation table conversion on page 26.

Note: The SM300 has been optimised for warm-up of 0.5 to 1 second
duration.
It is recommended that the sensor is not powered continuously.

 The temperature sensor output should be read as a resistance and the

logger configured with a look-up table to covert the measured resistance
to temperature.

See SM300 Temperature

Measurement on page 47

and Resistance to Temperature Lookup Table on page 50.

Logging Advice

Allow 20mins for the temperature readings to stabilise after installation

Do not log faster than 1 minute to avoid SM300 self-heating, which
could affect the accuracy of temperature readings.

SM300 User Manual 1.2 Operation  17

Logger Grounding

Sub-optimal grounding of a DC power supply and its cabling
to a logger can cause currents in the screen of the cable
connected to a soil moisture sensor, and so cause corrosion
at the M12 connector of an SM300 soil moisture sensor. In
addition, if used in an electrically noisy environment - such as
a farm or greenhouse - then the presence of a ground loop
can permit AC signals to inject noise and so disrupt readings.

To minimise sensor cable-screen currents that can result
from the DC power supply unit and its cabling to the logger,
earth the logger using a 1m long copper-clad earth stake

.

Figure

1 Showing the use of a 1m long copper-clad earthing rod connected to

logger earth

SM300 User Manual 1.2 Operation  18

Ungrounded system

In the example shown in Figure 2 both the logger and the power
supply are not grounded, so a current loop can exist between the
sensor and the mains power source, allowing noise to be injected
into readings, and also galvanic corrosion to the M12 connector on
the SM300.

Figure 2 This diagrams shows that if the logger is not well grounded then a
potential ground loop can exist between the sensor and the power supply and/or
mains power, and also the risk of galvanic corrosion to buried M12 connectors on
the SM300 and cable.

SM300 User Manual 1.2 Operation  19

Fully grounded system

Figure 3 This diagram shows the logger, power supply and sensor all grounded.
This minimises the possibility of galvanic corrosion of the SM300 connector. It also
minimises the possibility of nearby AC-powered machinery injecting noise into the
readings. The potential ground loop between logger and sensor is mitigated by the
differential wiring of the SM300

Logger grounded

Figure 4 This diagram shows a battery or high quality power supply with no
leakage current to ground. The logger is grounded. The potential ground loop
between logger and sensor is mitigated by the differential wiring of the SM300.
This is the best arrangement.

SM300 User Manual 1.2 Operation  20

Device:  SM300

Soil Type:
 Mineral

HH2 Meter

This assumes you have version 2.5 or later for both the PC
software HH2Read and the HH2 firmware (see foot of page).

 Connect the SM300 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 SM300:

► Press Set and scroll down to the Device option.
► Press Set again and scroll down to select

SM300.

► 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)

► 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.

 Remove the SM300 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.

See also: Support for the SM300 Soil Moisture Sensor with an HH2
and HH2 User Manual and HH2 User Manual Addendum to V4 - SM300.

Note: the HH2 does not display or store SM300 temperature readings.

Note: For an upgrade, contact Delta-T.

SM300 Store?

SM300 User Manual 1.2 Operation  21

Calibration

The SM300 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 the HH2 moisture meter. This section explains how these
calibrations work, how to adapt them for other soils and how to
provide calibrations for other data loggers.

The SM300 measures volumetric soil moisture

θ

, by detecting the

dielectric properties of the damp soil – the permittivity, ε, or more
conveniently the refractive index, which is closely equivalent to
√ε. The SM300 response is best understood in these stages:

SM300 User Manual 1.2 Operation  22

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 √ε. 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

SM300 is only affected by changes to the water content, θ. This
relationship simplifies to:

θε

⋅+=

1

0

aa

where the coefficients a0 and a1 conveniently parameterise the
dielectric properties of soils.

Note that:

soildry

a

_0

ε

=

is usually between 1.3 to 2.3

a

1

corresponds approximately to

1

−

water

ε

and usually takes a

value about 8.0. Real soil values for

a

0

and a1 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

.

0.0

2.0

4.0

6.0

8.0

0.0 0.2 0.4 0.6 0.8

Soil refractive index (√ε)

Soil moisture (m3.m-3)

Soil calibrations

Offset (a0)

Slope (a

1

)

SM300 User Manual 1.2 Operation  23

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).

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 a1 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 38.

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.

a a

Mineral soils 1.6 8.4

Organic soils 1.3 7.7

General soil calibrations

0.0

2.0

4.0

6.0

8.0

0.0 0.2 0.4 0.6 0.8

Soil Moisture

(m3.m-3)

Soil refractive index

(√ε)

Mineral

Organic

SM300 User Manual 1.2 Operation  24

Sensor calibration

Each SM300 is individually adjusted to provide consistent dielectric
performance:

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)

√

𝜖 = 1.0 + 14.868𝑉 − 33.56𝑉2+ 51.223𝑉3− 36.283𝑉4+ 9.715𝑉

5

where V is the SM300 output in Volts

0.0

2.0

4.0

6.0

8.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Soil refractive index (√ε)

SM300 output (Volts)

SM300 dielectric performance

SM300 User Manual 1.2 Operation  25

Linearisation table

(for use over the full range of SM300 readings)

V

√ε

V

√ε

V

√ε

V

√ε

V

√ε

0.000 1.000 0.300 3.556 0.600 4.956 0.900 6.471 1.200 7.961

0.075 1.963 0.375 3.913 0.675 5.327 0.975 6.841 1.275 8.411

0.150 2.641 0.450 4.254 0.750 5.708 1.050 7.205 1.350 8.971

0.225 3.149 0.525 4.598 0.825 6.092 1.125 7.571 1.425 9.724

Soil moisture reading

Polynomial conversion

Combining the Soil calibrations and Sensor calibration

steps, the conversion equation becomes:

𝜃 =

[

1.0 + 14.868𝑉 − 33.56𝑉

2

+ 51.223𝑉3− 36.283𝑉4+ 9.715𝑉

5

]

− 𝑎

0

𝑎

1

where a0 and a1 are the calibration coefficients.

For a generalised mineral soil this becomes:

𝜽

𝒎𝒊𝒏𝒆𝒓𝒂𝒍

= −0.071 + 1.77𝑉 − 3.995𝑉2+ 6.098𝑉3− 4.319𝑉4+ 1.157𝑉5

And for a generalised organic soil:

𝜽

𝒐𝒓𝒈𝒂𝒏𝒊𝒄

= −0.039 + 1.931𝑉 − 4.358𝑉2+ 6.652𝑉3− 4.712𝑉4+ 1.262𝑉5

SM300 User Manual 1.2 Operation  26

Linearisation table conversion

The conversion from SM300 reading (Volts) to soil moisture θ
(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

Mineral

soil

Organic

soil

Soil

moisture

Mineral

soil

Organic

soil

%vol

Volts Volts

%vol

Volts Volts

-4

-2.090

-2.090

52

0.801

0.670

0

0.044

0.021

56

0.867

0.731

4

0.074

0.045

60

0.934

0.791

8

0.108

0.072

64

1.003

0.852

12

0.147

0.102

68

1.072

0.913

16

0.193

0.137

72

1.140

0.976

20

0.248

0.178

76

1.204

1.039

24

0.313

0.226

80

1.260

1.102

28

0.384

0.281

84

1.309

1.163

32

0.457

0.343

88

1.351

1.220

36

0.530

0.410

92

1.388

1.270

40

0.600

0.477

96

1.419

1.314

44

0.669

0.544

100

1.447

1.352

48

0.735

0.608

104

2.090

2.090

SM300 User Manual 1.2 Troubleshooting  27

Troubleshooting

Always try to identify which part of the measurement system is the
source of the difficulty. For the SM300 this may fall into one of the
following areas:

The measurement device

What equipment is being used to read the probe output?

 A Delta-T HH2 Moisture Meter.

Note: the HH2 does not measure SM300 temperature.

 A Delta-T data logger such as the GP1, DL6 or DL2e

Check Versions

Check you have the correct versions:

HH2 Meter: Firmware version 2.5 and PC software HH2read
version 5 or later are recommended.

GP1 & DL6 Loggers: DeltaLINK version 2.4 or later is required.
GP2 Logger: DeltaLink 3 or later is required
DL2e Logger: Ls2Win 1.0 SR6 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 SM300 itself

Try to isolate the problem into one of the following areas

 The SM300 or the connecting cable

Then try to narrow down the area further

 Mechanical problems faults, or damage

 Electrical or electronic problems or faults

SM300 User Manual 1.2 Troubleshooting  28

Functional check

The following two simple checks can be used to establish whether
your SM300 is functioning within expected bounds:

Air reading

Hold the SM300 away from other objects and take a reading using
an HH2 meter, or voltmeter or logger.
The reading should be 0 ±4mV when used with a 5m cable.

Warning : Do not touch the pins

Do not touch the pins when the sensor is
attached to a cable. A typical electrostatic
discharge from your body can create a
temporary -10mV offset in sensor readings
lasting for up to one hour.

Mid range reading – dip rod tips in water

If you wish to take a quick reading to check the
sensor is working you can dip the sensor into
water.

With the pins half-immersed in tap water an
HH2 set to read an SM300 with soil type set to
Organic should give a reading in the range 80
to 100%vol.

SM300 User Manual 1.2 Technical Reference  29

Technical Reference

Specifications

Volumetric water content

Accuracy

±2.5% vol over 0 to 50% vol and 0-60°C

Measurement range 0 to 100% vol with reduced accuracy5

Salinity error (see p.30) ≤3.5%vol over 50 to 1000 mS.m-1 and 0-40% vol

Output Signal 0-1V differential ≈ 0 to 60% vol nominal

Output compatible with GP1, GP2, DL6, DL2e, HH2

Temperature

SM300 must be fully buried to
accurately measure soil temperature

Sensor accuracy

±0.5°C over 0-40°C

not including logger or cabling error

Output

Resistance6: 5.8kΩ to 28kΩ

Output compatible with

GP1, GP2, DL67, DL2e

Cabling error contribution
(to temperature readings)

Negligible for GP1,GP2, DL6 (any cable length)
Negligible for DL2e (with 5m cable)8

Maximum cable length

100m (GP1,GP2 & DL6 data loggers)
100m (DL2e: water content measurement)
25m (DL2e: temperature measurement)

Power requirement 5-14VDC, 18mA for 0.5 to 1s

Operating range -20 to +60°C

Environment IP68

Sample volume 55 x 70mm diameter

Dimensions 143 x 40 mm diameter

Weight 77 gm (without cable)

5 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 page 24.

6 See Appendix 2 on page 42.

7 Note: The DL6 has only one temperature channel. The DL6 error contribution to SM300 temperature
measurement is negligible compared to the accuracy of the SM300 temperature sensor itself. The two
only become comparable below -15C.

8 DL2e logger users can apply a correction in the Ls2Win logging software (for cable lengths >5m)

SM300 User Manual 1.2 Technical Reference  30

Conductivity response

This chart shows how salinity affects the output of the soil moisture sensor
at various soil moisture levels.

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

SM300 output (V)

Conductivity ECp(mS.m-1)

SM300 conductivity response at different water contents

100% (water)

~60%

~45%

~38%

~30%

~20%

0 200 400 800 1600600

1000800 1200 1400

non-

saline

slightly

saline

moderately

saline

strongly

saline

extremely

saline

mS.m

-1

0 2

4 6 8 10 12 14 16

dS.m

-1

Classification of salinity

SM300 User Manual 1.2 Technical Reference  31

Temperature response of soil moisture readings

The effect of temperature on the SM300 soil moisture readings in any
particular soil will depend on a combination of effects:

 The SM300 soil moisture electronics has very low temperature

sensitivity, and makes a negligible contribution to the overall
sensitivity.

 The refractive index of water (√ε, 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 SM300 will appear to have a
significant temperature response. This illustration is based on the model in
reference 7, see page 36.

Note: ice has a quite different refractive index from water, so SM300 soil
moisture readings cannot be interpreted reliably when inserted into soil
below 0°C.

0

0.1

0.2

0.3

0.4

0.5

0 10 20 30 40 50

Apparent soil moisture content (m

3

.m

-3

)

Soil temperature (°C)

Illustrating

temperature dependence

of SM300 readings in a clay soil

θ ~ 0 . 4

θ ~ 0 . 3

θ ~ 0 . 2

θ ~ 0 . 1

bound wa te r

SM300 User Manual 1.2 Technical Reference  32

Sampling Volume

The SM300 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.
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 negligeable

SM300s may interact if they are placed too close together – they
should be separated by at least 100mm.

If the SM300 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.

-6%

-5%

-4%

-3%

-2%

-1%

0%

1%

0 5 10 15 20 25 30 35 40

Error (% of reading)

Gap between SM300 body and wall of pot (mm)

SM300 Error close to wall of plant pot.

live pin nearest to pot wall

SM300 User Manual 1.2 Technical Reference  33

Electromagnetic Compatibility (EMC)

General information

SM300 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 SM300 EMC Guidance on the Software and Manuals CD
Issue 3.

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.

SM300 User Manual 1.2 Technical Reference  34

Definitions

Volumetric Soil Moisture Content is defined as

where Vw is the volume of water contained in the
sample

and Vs is the total volume of the soil sample.

The preferred units for this ratio are m3.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

where is the mass of water in the sample,

and is the total mass of the dry sample.

To convert from volumetric to gravimetric water content, use the
equation

where is the density of water (= 1g.cm

-3

),

and is the bulk density of the sample (

S

S

V

M

).

M

W

M

S

ρ

W

ρ

S

S

W

G

M

M

=

θ

g.g

-

S

W

V

V

V

=

θ

S

W

VG

ρ

ρ

θθ

×=

SM300 User Manual 1.2 Technical Reference  35

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

around
organic
content:

use for

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 = 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:

See also http://www.land.vic.gov.au/DPI/Vro/vrosite.nsf/pages/water_spotting_soil_salting_class_ranges#s1

0 200 400 800 1600600 1000800 1200 1400

non-

saline

slightly

saline

moderately

saline

strongly

saline

extremely

saline

mS.m

-1

0 2 4 6 8 10 12 14 16

dS.m

-1

Classification of salinity

SM300 User Manual 1.2 References  36

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
(TDR) for measuring soil moisture content.

Journal of Soil Sci. 44, 1-9

4.

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

5.

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

6.

Knight, J.H. 1992

Sensitivity of Time Domain Reflectometry measurements to
lateral variations in soil water content.

Water Resour. Res., 28, 2345-2352

7.

Or, D. and J.M. Wraith 1999

Temperature effects on soil bulk dielectric permittivity
measured by time domain reflectrometry: A physical model.

Water Resour Res., 35, 371-383

SM300 User Manual 1.2 Technical Support  37

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.

SM300 User Manual 1.2 Technical Support  38

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:

Technical Support
Delta-T Devices Ltd
130 Low Road
Burwell
Cambridge CB25 0EJ
England (UK)

Tel: [phone]
Fax: [phone]
E-mail: [e-mail]

[e-mail]

Web: www.delta-t.co.uk

SM300 User Manual 1.2 Appendix 1  39

Appendix 1

Soil-specific Calibration

This note provides details of 2 techniques for generating soil-specific
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 (θ) is proportional to the refractive index of the soil
(√ε) as measured by the SM300 (see Calibration section).

The goal of calibration is to generate two coefficients (a

0

, a1) which can be

used in a linear equation to convert probe readings into soil moisture:

θε

×+=

10

aa

SM300 User Manual 1.2 Appendix 1  40

Laboratory calibration for non-clay soils

This is the easiest technique, but it’s not suitable for soils that shrink or
become very hard when dry.

Equipment you will need:



SM300 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)

Process

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.

Ls = 463.5ml

Weigh the sample, including the beaker.

Ww = 743.3g

SM300 User Manual 1.2 Appendix 1  41

Insert SM300 into the sample and record its output in Volts.

Vw = 0.350V

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 and composts it’s usual to air-dry the
sample to avoid burning off any volatile factions.

Weigh the dry sample in the beaker.

W0 = 627.2g

Re-insert the SM300 into the dry sample and record this
reading.

V0 = 0.051V

Calculate a0

For the SM300,

In the dry soil V = V0 = 0.051 Volts
Substitute this into the equation

√

𝜖

= 1.0 + 14.868𝑉 − 33.56𝑉2+ 51.223𝑉3− 36.283𝑉4+ 9.715𝑉5

gives

68.10=

ε

Since

θ

0

= 0, this is the value needed for a0

a

0

= 1.68

SM300 User Manual 1.2 Appendix 1  42

Calculate

θ

w

The water content of the wet soil,

θ

w

, can be calculated from

the weight of water lost during drying, (W

w

– W0) and its

volume, L

s

:

( )

( )

25.05.4632.6273.743

0

=−=−=

sww

LWW

θ

θ

w

= 0.25

Calculate a1

In the wet soil V = Vw = 0.350 Volts and substituting gives

79.3=

w

ε

Finally

( )

( )

( ) ( )

44.8025.068.179.3

001

=−−=−−=

θθεε

ww

a

a

1

= 8.44

Result

a

0

= 1.68

a

1

= 8.44

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 SM300 readings
into volumetric water content θ using:

θε

×+=

10

aa

See also page Underlying principle on page 39

SM300 User Manual 1.2 Appendix 1  43

Laboratory calibration for clay soils

This technique is adapted to avoid the near-impossibility of inserting
the SM300 into completely dry clay soil. It requires taking
measurements at 2 significantly different, but still damp, moisture
levels.

Equipment you will need:

 SM300 and meter
 Soil corer
 Heat-resistant beaker (≥ 500ml)
 Weighing balance (accurate to < 1g)
 Temperature controlled oven

Process

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.

Ls = 463.5ml

Weigh the wet sample, including the beaker.

Ww = 743.3g

SM300 User Manual 1.2 Appendix 1  44

Insert SM300 into the wet sample and record its output in
Volts.

Vw = 0.349V

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.

Wm = 693.2g

Re-measure with the SM300.

Vm = 0.180V

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.

W0 = 627.2g

SM300 User Manual 1.2  45

Calculations Substituting in the SM300 equation

√

𝜖 = 1.0 + 14.868𝑉 − 33.56𝑉2+ 51.223𝑉3− 36.283𝑉

4

+ 9.715𝑉5

provides two dielectric values, √εw and √εm,
at two known water contents,

θ

w

and

θ

m

For the wet
soil

Substituting Vw = 0.349 gives

ww

aa

θε

⋅+==

10

79.3

for

( )

25.05.4632.6273.743 =−=

w

θ

For the moist
soil

Substituting Vm = 0.180 gives

m

m

aa

θε

⋅

+==

1

0

85.

2

For

( )

14.0

5

.4632

.6272.

693 =

−=

m

θ

Calculate a1

Then

( )

( )

69.81=−−=

mwmw

a

θθεε

a

1

= 8.69

Calculate a0

and

( )

61.110=⋅−=

ww

aa

θε

a

0

= 1.61

Result

a

1

= 8.69

a

0

= 1.61

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 SM300 readings
into volumetric water content θ using:

θε

×+=

10

aa

See also page Underlying principle on page 39

SM300 User Manual 1.2 Appendix 2:  46

Appendix 2:

The SM300 Temperature Sensor

Soil moisture content is used with the measurement of soil
temperature in several major application areas including the
following:

Global warming and climate studies

Soils contain more than four times as much carbon as the CO2 in
the atmosphere, and each year they release about ten times as
much carbon through soil respiration as the combined release
through burning fossil fuels. Soil respiration rates are particularly
sensitive to changes in both temperature and the moisture content
of the soil.

Soils also have a significant interaction with climate as they store
and release heat – soil temperature provides a measure of the
energy partitioning, which in turn is strongly influenced by the effect
of soil moisture on thermal conductivity.

Civil engineering

Most civil engineering projects depend critically on the mechanical
properties of soils. Those properties are effected by many different
parameters, but moisture content and temperature are the two
variables that are most likely to change over time, so may be
measured together in order to assess their impact.

Soil contamination and hydrogeology

Soil moisture is the main determinant for the movement of
contaminants and solutes through soils, but temperature also has a
significant influence so they are often measured together.

Agriculture

Temperature may be measured alongside soil water content for
studies of evapotranspiration, soil water balance and irrigation. Soil
strength and seedling emergence depend on soil moisture and
temperature, and both need to be taken into account when deciding
when to sow.

SM300 User Manual 1.2 Appendix 2:  47

SM300 Temperature Measurement

The SM300 Temperature sensor uses a thermistor with a 10K
resistance at 25 ˚C. However:

A. This sensor has a different response curve from the more

widely used 10K3A1B type. The response curve is given in the
Resistance to Temperature Lookup Table on page 50.

B. The Thermistor circuit shares the Power 0V wire. If the

thermistor is measured when the SM300 is powered, the
measured resistance measurement may need to be corrected
for 18 mA SM300 supply current.

Allow 20 minutes for the temperature reading to stabilise after
installation.

Do not log at freqencies under 1 minute - in order to prevent
thermal self-heating errors.

SM300 User Manual 1.2 Appendix 2:  48

GP1, GP2 and DL6 loggers

The ‘SM300 Temperature’ sensor type in DeltaLINK performs the
supply current correction.

DL2e Logger

The linearization table for the ‘S3T’ sensor code (‘SM300
Temperature’) provides supply current correction for the

SMSC/lw-05 5m logger cable ONLY.

Extension cables and other cable lengths

Create your own custom sensor type(s) and linearization tables as
described in Ls2Win Help topic, How to… ‘Add or modify a sensor
type in the sensor library’.

Enter corrected resistance values (R) for each linearization table
point:

R = R5 + (0.059 x Lex) kΩ (See footnote

9

)

or R = R5 + (0.9 x Rc – 0.297) kΩ

where

R5 = value supplied in the table for the ‘SM300 Temp, 5m’ sensor type.

Lex = length of extension cable, excluding the 5m of SMSC/lw-05 cable.
Rc = total cable resistance, including resistance of SMSC/lw-05 cable, if

fitted.

Other loggers

If your logger can be programmed so that the soil moisture and
temperature readings can be taken sequentially (i.e. the sensor is
not powered during the temperature reading), then the temperature
can be obtained directly from the response curve on page 50.

Otherwise, correct the resistance reading before applying the
response curve.

You need to know the resistance of the Power 0V wire in the
SM300 cable (Rc) and establish whether your logger uses voltage
or current excitation for resistance measurement.

9

Note: This equation only applies to Delta-T SM300 cables

SM300 User Manual 1.2 Appendix 2:  49

Voltage Excited

You need to know the excitation voltage (Vref), reference resistance (Rref).

The correct resistance is given by the equation:

R = a0 + a1 * Rmeas

Where:

a0 = – Ic.Rc.Rref / Vref

a1 = 1 – Ic.Rc / Vref

Ic = 18 mA (SM300 sensor supply current)

For Delta-T EXT/5W-xx series cables:

Rc = 0.066 Ω.m

-1

For the SMSC/lw-05 5m logger cable

Rc = 0.33 Ω

Current Excited

You need to know the excitation current (Iex).

The corrected resistance is given by the equation (using terms defined
above):

R = Rmeas – Ic.Rc/Iex

Effect of Temperature on Water Permittivity

See Temperature response of soil moisture readings on page 31

SM300 User Manual 1.2 Appendix 2:  50

Resistance to Temperature Lookup Table

Note: This table has been optimised for use as a look-up table.
To minimise linear interpolation errors the data points fall either side of the
manufacturers’ specified sensor response curve. This helps optimise the
overall accuracy of readings.

Temperature Resistance

degrees C Kohms

-25 90.538

-22 77.683

-19 66.854

-16 57.713

-13 49.968

-10 43.379

-7 37.759

-4 32.957

-1 28.844

2 25.299

5 22.244

8 19.608

11 17.321

14 15.334

17 13.606

20 12.098

23 10.780

26 9.623

29 8.611

32 7.720

35 6.935

38 6.241

41 5.627

44 5.080

47 4.595

50 4.162

53 3.775

56 3.430

59 3.121

62 2.843

65 2.593

0

10

20

30

40

50

60

70

80

90

100

-40 -20 0 20 40 60 80

Resistance

(kOhms)

Temperature (degrees C)

SM300 Resistance to Temperature

Conversion Chart

SM300 User Manual 1.2 Index  51

Index

A

Agriculture, 46

Air reading, 28
Auger, 7

C

Cable Connections, 10
Cable length correction, 47
Calibration

check, 28
generalised, 25
sensor, 24, 25
soil, 22, 25
Soil, 39

soil-specific, 39
Care and safety, 8
Care and Safety, 8
Certification

emc, 33

Civil engineering, 46
climate, 46

Conductivity response, 30
Connections, 12
contamination, 46
Conversions

linearisation table, 23, 24

polynomial, 23, 24

D

Data logger, 16, 26, 27, 28

DL2e, 26, 27

DL6, 27

GP1, 14, 27

other, 16

Definitions, 34
Description, 5
Dielectric

performance, 24

refractive index, 21, 22, 39
Dimensions, 6
DL6, 14

E

EMC, 33
Extension cables, 10

F

FCC compliance, 33
Features, 5

G

Global warming, 46

GP1, 12, 14, 27
GP2 Logger Controller, 13

H

HH2, 7, 20, 21, 27, 28

hydrogeology, 46

I

Installation

buried, 11

insertion rod, 11

surface, 11

SM300 User Manual 1.2 Index  52

L

Linearisation table, 25

volts to %vol, 16, 26

Logger Grounding, 17

M

Meter, 27
Moisture content, 5, 9, 36, 39

O

Organic and Mineral, 35

P

Parts, 7
Permittivity, 9, 21, 36
Polynomial conversion, 25

R

References, 36
Regulatory information, 33
Rods, 8, 9, 11

S

Sales code, 7

Salinity, 35

Sampling Volume, 32
Servicing, 38
SMSC/lw-05, 48
Soil

clay, 22, 23, 34, 39, 40, 43

mineral, 21, 23, 25, 40, 41, 44

organic, 21, 23, 25, 35, 41

stony, 11
soil calibrations, 23
Specifications, 2, 29

T

Technical support, 37, 38
Temperature

cable length correction, 16, 47
Temperature response of soil moisture

readings, 31
Temperature sensor cable length

correction, 48
Temperature Sensor Lookup Table, 50
Troubleshooting, 27

V

Volumetric Soil Moisture, 34

W

Warning : Do not touch the pins, 28