Campbell HydroSense II User manual

HydroSense II
User Guide
10/11

HydroSense II
Soil Moisture Measurement System

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Warranty
The HydroSense II Soil Moisture Measurement System is warranted by
Campbell Scientific Australia to be free from defects in materials and
workmanship under normal use and service for twelve (12) months from date of
shipment unless specified otherwise. Batteries have no warranty. Campbell
Scientific Australia's obligation under this warranty is limited to repairing or
replacing (at Campbell Scientific Australia’s option) defective products. The
customer shall assume all costs of removing, reinstalling, and shipping defective
products to Campbell Scientific Australia. Campbell Scientific Australia will
return such products by surface carrier prepaid. This warranty shall not apply to
any Campbell Scientific Australia products which have been subjected to
modification, misuse, neglect, accidents of nature, or shipping damage. This
warranty is in lieu of all other warranties, expressed or implied, including
warranties of merchantability or fitness for a particular purpose. Campbell
Scientific Australia is not liable for special, indirect, incidental, or consequential
damages.
Products may not be returned without prior authorization. The following contact
information is for Australian and International customers residing in countries
served by Campbell Scientific Australia directly. Affiliate companies handle
repairs for customers within their territories. Please visit www.campbellsci.com
to determine which Campbell Scientific group company serves your country.
To obtain a Returned Materials Authorization (RMA), contact your local agent.
After an applications engineer determines the nature of the problem, an RMA
number will be issued. Please write this number clearly on the outside of the
shipping container.
For all returns, the customer must fill out a “Declaration of Hazardous Material
and Decontamination” form and comply with the requirements specified in it.
The form is available from Campbell Scientific Australia. A completed form
must be either emailed to [email protected]m or faxed to 07 47720555.
Campbell Scientific Australia will not process any returns until we receive this
form. If the form is not received within three days of product receipt or is
incomplete, the product will be returned to the customer at the customer’s
expense. Campbell Scientific Australia reserves the right to refuse service on
products that were exposed to contaminants that may cause health or safety
concerns for our employees.

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Table of Contents
1General Description ..........................................................................................................6
1.1 Introduction .............................................................................................................................. 6
2Specifications.......................................................................................................................7
2.1 HydroSense II Display specifications ............................................................................... 7
2.2 HydroSense II sensor specifications................................................................................. 7
3What’s included ..................................................................................................................8
4Setup and First Use............................................................................................................9
5Measurements.....................................................................................................................9
5.1 Measurement Display ............................................................................................................ 9
5.2 Water Content Display........................................................................................................... 9
5.3 Water Deficit Display ...........................................................................................................10
5.3.1 Soil types......................................................................................................................................... 11
5.3.2 Selecting a soil type and setting “wet” and “dry” values............................................. 11
5.3.3 Storage............................................................................................................................................. 13
6User Interface ................................................................................................................... 13
6.1 Buttons......................................................................................................................................13
6.2 Splash Screen ..........................................................................................................................14
6.3 Main Screen .............................................................................................................................14
6.3.1 GPS information........................................................................................................................... 14
6.3.2 Status information ...................................................................................................................... 15
6.3.3 Measurement display................................................................................................................. 15
6.3.4 Deficit Display............................................................................................................................... 16
7Data Storage and Retrieval ..........................................................................................17
7.1 Storing Data.............................................................................................................................17
7.2 What is stored?.......................................................................................................................17
7.3 Software....................................................................................................................................17
7.4 Connecting via Bluetooth....................................................................................................18
7.5 Collecting data ........................................................................................................................18
8GPS........................................................................................................................................ 18
8.1 GPS synchronisation.............................................................................................................18
8.2 Geo-tagging & Zones.............................................................................................................19
9Proper measurement Technique & Limitations ..................................................20
9.1 Measurement Principle.......................................................................................................20
9.2 Probe Insertion ......................................................................................................................21
9.3 Soil factors which can affect the Measurement ..........................................................21
9.4 Measurements in Special Materials................................................................................22
10 Maintenance..................................................................................................................22
10.1 Batteries................................................................................................................................22
10.1.1 Battery Types................................................................................................................................ 22
10.1.2 How to replace the battery pack........................................................................................... 23
10.2 Rod installation/replacement ......................................................................................23
11 Configuration Menus..................................................................................................25
11.1 Main Menu............................................................................................................................25
11.2 Deficit Mode.........................................................................................................................25

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11.3 Bluetooth Power................................................................................................................25
11.4 Time/Date Menu................................................................................................................26
11.5 Time Set ................................................................................................................................26
11.6 Date Format.........................................................................................................................26
11.7 Time Zone.............................................................................................................................27
11.8 Time Synchronisation......................................................................................................28
11.9 Display Settings..................................................................................................................28
11.10 Contrast.................................................................................................................................28
11.11 Brightness ............................................................................................................................29
11.12 Backlight On Time.............................................................................................................29
11.13 Light Sensing .......................................................................................................................29
11.14 GPS Settings .........................................................................................................................30
11.15 GPS Power ............................................................................................................................30
11.16 Coordinate format.............................................................................................................30
11.17 System Settings...................................................................................................................31
11.18 System On Time..................................................................................................................31
11.19 Restore Settings .................................................................................................................31
11.20 Serial Number.....................................................................................................................32
12 Soil Physics Primer .....................................................................................................33
12.1 Soil Bolus Testing ..............................................................................................................33
12.2 Typical Soil Water Levels................................................................................................34
13 Troubleshooting ..........................................................................................................35
14 Operating System Updates .......................................................................................36

HydroSense II Soil Moisture Measurement System
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1General Description
1.1 Introduction
The HydroSense II is an easy to use, portable device for measuring soil moisture
in terms of volumetric water content (in percent). The three major components of
the system are the display, the sensor and the software. The sensor is available in
two different versions –12cm (CS659) and 20cm (CS658).
As the name suggests the HydroSense II is an updated and improved successor
to the established HydroSense product. Below is a comparison of the features of
the different versions.
Feature
HydroSense (CD620)
HydroSense II
Portable
Yes
Yes
Battery life
2000+ readings
1000+ readings
Probe length
12cm, 20cm (interchangeable)
12cm, 20cm (not interchangeable)
Accuracy
±3% VWC (EC <2dS/m)
±3% (EC≤4dS/m) CS658
±3% (EC≤6.5dS/m) CS659
Water Content mode
Yes
Yes
Period Display
Yes in msec
Yes in μsec
Water Deficit mode
Yes 5 soil profiles
Yes 10 soil profiles
User configurable
Wet & Dry references
Yes
Yes
LCD
2 line x 16 char alphanumeric
128x64 pixel graphic
LCD Backlight
No
Yes
Data Storage
No
Yes 1000+ readings
GPS & Geotagging
No
Yes
GPS Zones
No
Yes
Date & Time
No
Yes GPS accuracy where possible
Bluetooth
No
Yes
Firmware updates
Factory only
User updateable
Field carry case
No
Yes

HydroSense II Soil Moisture Measurement System
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Please note that unlike the previous generation HydroSense, the 12cm and 20cm
sensor rods are not interchangeable.
Along with volumetric water content the HydroSense II can also display relative
water content based on “wet” and “dry” reference measurements previously set
by the user. Along with relative water content, the unit can also display water
deficit indicating how much water is required (in mm of rain) to bring the soil
back to the previously saved “wet” value.
Measurements are made by fully inserting the probe rods into the soil and
pressing the Read/OK button. The process takes between 1 and 2 seconds after
which the value can be stored and later downloaded to a computer for display
and analysis. Communication between the computer and HydroSense II are
carried out using Bluetooth. The HydroSense II also includes an GPS (Global
Positioning System) received which enables data to be stored with position
information. This technique is known as geo-tagging.
The HydroSense II has many power saving features –the device turns off when
it has been idle for a configurable period, the backlight brightness and on-time
are adjustable, the backlight is automatically disabled in bright conditions, as
well as the GPS and Bluetooth can each be forced off to extend battery life,
albeit with reduced functionality.
2Specifications
2.1 HydroSense II Display specifications
HydroSense II measurement and display unit
Display
128x64 pixel graphic LCD
Backlight
Blue/White LED brightness adjustable
GPS Accuracy
±5m position (typ), ±1ms time with GPS sync
Bluetooth range
Approx. 10m
Data storage
>1000 records (oldest data overwritten first)
Zone storage
>100 records
Batteries
4 x AA alkaline (included)
Power consumption (approx.)
20μA asleep
+2mA backlight off
+18mA backlight 60%
+30mA backlight 100%
+35mA GPS
+30mA Bluetooth
Expected Battery Life
6 –12mths (depends on usage)
Size (LxWxH)
200mm x 100mm x 58mm
Weight (g)
340g
2.2 HydroSense II sensor specifications
HydroSense II sensor –CS658
Measurement principle
Time Domain Reflectometry
Communication
SDI-12
Rod length
20cm
Rod diameter
4.7mm (nominal)
Measurement range
0% - 50%
Precision
<0.05%
Accuracy
±3% VWC in mineral soils with solution EC ≤4dS/m
Size (LxWxH)
100mm x 92mm x 40mm (sensor body)
Weight (g)
450g

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HydroSense II sensor –CS659
Measurement principle
Time Domain Reflectometry
Communication
SDI-12
Rod length
12cm
Rod diameter
4.7mm (nominal)
Measurement range
0% - 50%
Precision
<0.05%
Accuracy
±3% VWC in mineral soils with solution EC ≤6.5dS/m
Size (LxWxH)
100mm x 92mm x 40mm (sensor body)
Weight (g)
450g
3What’s included
The HydroSense II Soil Moisture Measurement System consists of many parts.
Standard parts are listed in the table below.
Qty
Description
1
HydroSense II measurement and display unit (pn: CD660)
1
CS658 (20cm) or CS659 (12cm) water content sensor
4
AA batteries (factory installed inside the display)
1
Spare battery holder (spare batteries not included)
1
Wrench for installing/replacing rods
1
Tube of thread locking compound for rod replacement
1
Philips screwdriver
1
Carry case
1
Hydrosoft PC software on CD
Some users may also wish to purchase optional extras such as spare rods for the
CS658 (pn: 26483) or CS659 (pn: 10184) in case of breakage or a USB
Bluetooth adapter for those who do not already have a Bluetooth adapter.

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4Setup and First Use
The steps to setup the HydroSense II are as follows:
Remove the HydroSense II display from the carry case.
Remove the blue protective strip from the display window and discard.
Remove the probe from the carry case.
Connect the probe to the display by mating the connector on the cable
to the connector at the bottom of the display.
Please note that the connector is a push-pull type with locators to align the
connectors. The connectors can be damaged if the user attempts to screw or
unscrew them. To mate the connectors, simply bring them together and turn until
you feel that they are aligned, then applying firm pressure, slide them together
until they click. To disconnect, pull back on the connector collar with the thumb
and forefinger and pull the two connectors apart.
The HydroSense II stores time, date and positional information with any stored
measurements. To take advantage of this, it is recommended to use the
HydroSense outdoors with a clear view of the sky. Measurements can be made
and data stored without a GPS signal, however it is important to note that the
software will be unable to group and display data geographically later.
The steps to taking a measurement are as follows:
Ensure that the HydroSense II is turned on. Start up the HydroSense II
by holding the Menu/PWR button for 3 seconds.
In order to store data with positional information, it may take up to 1
minute to establish GPS synchronisation. When the GPS icon is
displayed, synchronisation has been successful.
Insert the probe into the soil ensuring that the probes are fully inserted.
To take a measurement, press the Read/OK button. The measured VWC
and period are shown on the screen.
5Measurements
5.1 Measurement Display
The HydroSense II can display two different sets of data on the main screen. The
main data set contains volumetric water content (VWC) in percent and period (in
μsec). This is termed the Water Content Display and it is shown whenever a
successful measurement is made.
The second data set represents Water Deficit. This is helpful for irrigation
planning because it provides an estimate of the soil water content relative to the
pre-selected “wet” and “dry” reference levels and the amount of rain (in mm)
required to return the soil to the “wet” level.
By default, only Water Content data is shown on the main screen, however using
the configuration menus Water Deficit can be enabled if desired.
5.2 Water Content Display
The water content data set displays the volumetric water content (VWC) and
period (PER) retrieved from the CS658/9 sensor. The soil moisture sensor uses a
proprietary technique to determine water content over widely varying soils while
correcting for a range of bulk electrical conductivity.

HydroSense II Soil Moisture Measurement System
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In simple terms, soil consists of three main constituents –mineral particles
(sand, loam, clay), water and air. The air and water occupy the spaces or pores
formed between the mineral particles. Typically in agricultural soils these pore
spaces make up approximately 50% of the soil by volume, with water and air
together making up the remaining 50% in varying quantities. As a result, water
content normally ranges from 0% to 50%. In some conditions, the sensor is
unable to determine the soil water content and in these cases, an out of range
value is displayed (“---”).
N.B. even when the sensor cannot determine the VWC, the signal period
measurement will always be displayed. In some special media this value can be
used along with a soil specific calibration to estimate water content.
5.3 Water Deficit Display
The purpose of the HydroSense II Water Deficit display is to help with water
management decisions for irrigators.
In Water Deficit mode, the HydroSense II stores “wet” and “dry” values for up
to 10 different soil types and compares the current measurement to those
reference values. It then shows relative water content (RWC) on a scale where
0% represents the “dry” value stored for that soil and 100% represents the “wet”
value. While any two volumetric water content measurements can be stored as
“wet” and “dry” references, it is recommended to store the wilting point of your
crop as the “dry” value and field capacity as “wet”. The relative water content is
calculated as follows:
100x
VWCVWC
VWCVWC
RWC
drywet
dry
In this equation, VWC is the current measurement of volumetric water content,
VWCdry is the volumetric water content of the “dry” reference and VWCwet is the
volumetric water content of the wet reference point.
For example, a particular clay-loam soil may have an 18% wilting point and
35% field capacity. Using these values a VWC measurement of 24.5% would
display as a relative water content of 38.2%. Similarly a VWC measurement of
30% would show an RWC of 70.6%.
An RWC value greater than 100% indicates that the soil has a water content
value greater than the stored “wet” value and a negative value reflects that the
water content is below the “dry” point.
Along with the relative water content, the HydroSense II calculates and displays
the amount of water required (in mm of rain) to return the soil to the “wet” level.
A negative deficit value indicates that that the water content is above the “wet”
value for that soil type.
Since the CS658 and CS659 measure average water content along the length of
their rods (20cm and 12cm respectively), their sample volumes are quite
different and this affects the water deficit value. For this reason it is important
that the current measurement and reference values are all made with the same
probe type. Some customers prefer to use different probes with different rod
lengths for different soils, however if the probe length used to set the reference
values is different to that used later to evaluate relative water content, the
HydroSense II may give erroneous readings.

HydroSense II Soil Moisture Measurement System
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By default the HydroSense II does not display the Water Deficit data set on the
main screen. To enable this mode, turn on Deficit Mode from the configuration
menus.
5.3.1 Soil types
The HydroSense II needs three different parameters to calculate relative water
content and deficit - “wet” and “dry” water content values and the probe length
used. Since these parameter groups represent soil specific attributes they are
referred to as “soil profiles”. The HydroSense II can hold up to 10 soil profiles
labelled “soil 1” to “soil 10”.
By default, some of the soil profiles in the HydroSense II are preset to typical
values. These values may be used or overwritten as desired, but it is important to
record what soil types were used for each profile stored in the display. The
default soil values are shown in the table below.
Soil
No
“Dry”
“Wet”
Probe
Length
Soil Description
1
7%
15%
20cm
Sand
2
10%
20%
20cm
Sandy Loam
3
18%
35%
12cm
Sandy Clay
4
15%
30%
12cm
Loam
5
20%
40%
12cm
Silty Clay
6
17%
35%
12cm
Clay Loam
5.3.2 Selecting a soil type and setting “wet” and “dry” values
Soil profiles can be changed from the main screen of the HydroSense II. To
select the soil profile, press the Right button to highlight the soil number as
shown above. To change soil profile, press the Read/OK button.
The soil profile list should be displayed. Use the Up and Down buttons to move
through the list to the desired soil type and press OK.

HydroSense II Soil Moisture Measurement System
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The soil profile details page will be displayed. This page shows the reference
values stored for this soil profile along with the probe length used. To select this
soil and return to the main screen, press the down button to highlight the
“SELECT THIS SOIL” and press OK.
From this screen, new reference values can also be set. To do so, ensure that the
CS658/9 sensor is inserted in the soil correctly and select “WET VWC” or
“DRY VWC” and press OK.
The screen above will be shown while the measurement is taken and after a
couple of seconds the soil profile details page will be shown with the reference
value changed. The same process can be used for both “wet” and “dry” values. If
desired, the soil profile can be cleared. To do so, select “CLEAR SOIL” and
press OK. Please note that this step cannot be reversed.
The probe length stored in a soil profile is automatically updated to match the
probe used whenever the “wet” and “dry” reference values are changed. If the
probe length used to change one of the references is different to that previously
used for the soil profile, the screen above will be shown. Selecting the “NEW
PROBE” option will overwrite the soil profile, clearing the other reference
value, whereas selecting the “OLD PROBE” option will discard the
measurement and leave the soil profile unchanged. Both reference values must
use the same probe length. This step cannot be reversed. Using the Up and Down
buttons, select the desired option and press OK or Back to discard the new
measurement and return to the soil profile screen.
Once the soil profile changes are complete, use the Up and Down buttons to
choose “Select this Soil” and press OK. This will return to the main screen and
use the new soil profile.

HydroSense II Soil Moisture Measurement System
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5.3.3 Storage
The HydroSense II does not store relative water content or deficit measurements
to flash. Only Water Content measurements are stored.
6User Interface
The following section contains a detailed description of the HydroSense II user
interface.
For the most part, the screenshots included in this section have been captured
using factory default settings (except where noted), however they may not reflect
the exact image seen on your screen because of configuration settings chosen or
operating system updates. These differences should be minor and are likely to be
cosmetic.
Throughout this section, we have used red highlights to mark areas of interest on
the screenshots. These red highlights are added in this manual for illustrative
purposes only and will not actually be visible on the screen of the device.
6.1 Buttons Below are a list of the buttons on the HydroSense II and their functions.
User Interface Buttons
The Menu/PWR button. When pressed in the main screen, the main menu will be displayed.
This key also doubles as the Power button. To turn the HydroSense II on or off, press and hold
this button for 3 seconds.
The Read/OK button. This key is used to initiate a new measurement in the main screen, and is
also used to select an item in a list or to answer “OK” to prompts throughout the HydroSense II
menu system.
The Store/Back button. From the main screen, this button is used to store the current reading to
disk. In the menu system it is used to move “Back” to the previous menu.
The Up button. This and the other direction keys are used to navigate lists.
The Down button. This button moves the cursor downward.
The Left Button. This button moves the cursor to the left.
The Right button. This button moves the cursor to the right.

HydroSense II Soil Moisture Measurement System
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6.2 Splash Screen
To turn on the HydroSense II, press and hold the Menu/PWR Button for 3
seconds. The splash screen shown below is displayed.
Please note that the Operating System version number is displayed along with
the device serial number. This splash screen is shown for 2.5 seconds.
6.3 Main Screen
After the splash screen disappears, the main screen is shown. The main screen
contains a number of different elements which are explained in the following
subsections.
6.3.1 GPS information
The top bar of the screen displays the current date and time information. This
time is synchronised with the GPS when available. The bottom bar shows the
current GPS coordinates as reported by the GPS. Both of these values are
updated automatically.
If the GPS has been turned off using the configuration menus, “GPS OFF” is
displayed in the bottom bar as shown above.

HydroSense II Soil Moisture Measurement System
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6.3.2 Status information
The zone name is shown in the upper left of the screen. When the current
position is not within any of the existing zones, “NOT IN A ZONE” is displayed
much like the top screenshot. When the HydroSense II moves within the
boundary of an existing zone (such as “Zone 00001” above) the zone name will
be displayed in a similar way to that shown in the bottom screenshot. This
change occurs automatically when the HydroSense II detects that it is within the
boundaries of a zone. For more information on zones, please see “Geo-tagging
and Zones”.
The upper left of the main screen shows a group of status icons. They are
detailed in the table below.
Status Icons
GPS. This icon is shown when the GPS has acquired synchronisation with the GPS satellite
constellation. This icon disappears when the GPS is turned off or GPS sync is lost.
Bluetooth Connection. This icon is displayed when a Bluetooth connection has been successfully
established. When the Bluetooth connection is closed, this icon disappears.
Bluetooth Active. This icon is shown whenever the Bluetooth radio is turned on and discoverable.
If the Bluetooth is turned off, this icon disappears.
Battery. This symbol indicates the state of charge of the battery pack. The icon changes from
to indicate that the battery is fully charged to when it is empty.
6.3.3 Measurement display
The centre left of the screen contains the measurement results.
VWC –This section shows the volumetric water content in percent.
This value is automatically compensated for varying soil conditions. If
the measurement is out of range the display will show “---” in this
position.

HydroSense II Soil Moisture Measurement System
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PER –The average period of the water content reflectometer
(CS658/9). The value is displayed in microseconds.
Below the sensor readings is displayed the model number and rod length of the
sensor used (in this case “CS658 20cm”). This is updated with each
measurement.
If the water content sensor is not properly connected or malfunctioning, the
display will show “SENSOR TIMEOUT” as shown above.
6.3.4 Deficit Display
When deficit mode is enabled, the water deficit section is displayed on the centre
right of the main screen. The top screenshot shows the main screen with deficit
mode disabled and the bottom screenshot is with deficit mode enabled.
The water deficit section contains the following information:
Soil type - This is soil number 1 to 10.
RWC - relative water content from 0% to 100% where 0% represents
“dry” (wilting point) and 100% is “wet” (field capacity).
DEF –water deficit expressed in mm of rain.

HydroSense II Soil Moisture Measurement System
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7Data Storage and Retrieval
The HydroSense II includes flash-based storage for data and configuration
settings. Being flash based, it is non-volatile and will be preserved when the
batteries are exhausted and changed.
7.1 Storing Data
To store data with the HydroSense II, Press the Store button from the main
screen. This will initiate the process of storing data. If the current location is not
within an existing zone, a new zone will be created, if a zone already exists, the
data will be attributed to that existing zone and it will be stored to flash.
N.B. the store function is intended to store volumetric water content values, so it
is normal to take a measurement before using the Store button.
7.2 What is stored?
The flash file system of the HydroSense II is employed to hold water content
measurements, the zone table and the table of configuration settings.
The most important among these is the water content data file. When the user
chooses to store a measurement by pressing the Store button, the most recent
measurement of volumetric water content (VWC) is stored along with the
measured period, sensor type, current date & time, latitude and longitude (if GPS
synchronisation has been achieved). If the GPS is switched off or does not have
a valid synchronisation, the time and date are still used, however the time and
date may not be accurate. When using the HydroSense II with the GPS disabled,
it is recommended to check and adjust the clock before storing data.
The water content data file is large enough to hold over 1000 data values. When
the file becomes full, the oldest data are overwritten and storage continues.
The zone data file holds a list of zones that have been created. Zones are
automatically created whenever a data point is stored in a new location. Each
zone record contains the centre position (latitude & longitude), radius in metres,
and the zone name. The zone file is large enough to hold over 100 zones,
however when the zone data file is full, new zones can no longer be created.
The configuration settings file is managed automatically by the HydroSense II.
These settings can be synchronised to the computer where they can be viewed,
modified, backed up and restored. For more information, please refer to the
HydroSense II Software User Guide
7.3 Software The HydroSense II ships with the latest version of the HydroSense II software on
CD. For the latest version, please contact your local agent, or refer to the
Campbell Scientific website.
For a complete guide to the use of this software, please refer to the HydroSense
II Software User Guide.

HydroSense II Soil Moisture Measurement System
18
7.4 Connecting via Bluetooth
The steps to forming a successful Bluetooth connection are as follows:
Start the HydroSense II software.
Turn on the HydroSense II display by holding the Power button for 3
seconds.
Using the software, click the Discover button to find Bluetooth devices
within range.
Once the HydroSense II has been discovered, click the “Connect”
button.
The first time that the HydroSense II is used with a new computer is necessary to
“pair” the computer and the HydroSense II before a connection can be made.
The pairing code is “1234”.
For more information, please refer to the HydroSense II Software User Guide
While a Bluetooth connection is open, the HydroSense II will not shut down. It is
important to disconnect the Bluetooth once data has been collected or settings
updated to avoid unduly shortening the battery life. Once the Bluetooth
connection is closed the device will power down as normal.
7.5 Collecting data
To collect data it is first necessary to establish a Bluetooth connection (see
above), however data can then be downloaded to the computer by clicking on the
“Synchronise” button in the HydroSense II software. The synchronisation
process will download any new water content data, update the zone table and
configuration settings.
For more information, please refer to the HydroSense II Software User Guide.
8GPS
The HydroSense II incorporates a receiver for the Global Positioning System
(GPS). This is a group of satellites in medium earth orbit (MEO) maintained by
the US Department of Defense for global navigation. The system provides
extremely accurate time and position information using a relatively inexpensive
radio receiver. In order to calculate position, the GPS module needs to receive
radio time signals from at least 4 satellites simultaneously, and it uses the slight
time shifts in these received signals to calculate the receiver’s position in space.
If the receiver detects signals from more than 4 satellites, it will use the data
from these extra satellites to generate a more accurate position.
8.1 GPS synchronisation
Since the GPS satellites are in orbit at an altitude of approximately 20000 km,
their radio signals are quite weak at the surface of the earth and they do not
penetrate buildings well. It is therefore generally necessary to be outdoors with a
clear view of the sky to reliably receive strong signals from the 4 or more
satellites required to calculate position. When the receiver has an adequate signal
from at least 4 satellites and has resolved the current position it is said that the
GPS has achieved “synchronisation”.
The GPS antenna of the HydroSense II is located inside the case above the LCD
facing upward toward the sky when held upright in a comfortable reading

HydroSense II Soil Moisture Measurement System
19
position. This is to maximise the received signal and thereby shorten the time
required to achieve synchronisation.
To synchronise the GPS, take the HydroSense II outside, away from any tall
buildings or large obstructions and turn it on. GPS synchronization will normally
be achieved in approximately 30 seconds, but can take up to a minute or so. The
HydroSense II indicates that valid GPS synchronisation has been achieved by
displaying the GPS sync icon on the main screen. When the GPS is
synchronized, the internal clock of the HydroSense II is adjusted to match the
GPS time to within a millisecond. This is the default behavior, however this
feature can be disabled if desired using the configuration menus.
8.2 Geo-tagging & Zones
When the GPS module is enabled, the HydroSense II is able to store water
content data that is tagged with position information (latitude & longitude).
Using this position information, data can be grouped by geographical location.
These geographical areas are referred to as “zones” and are characterised by a
centre coordinate and radius. Volumetric water content values are then grouped,
filtered and charted by the computer software based on the zone in which the
data was collected.
The HydroSense II keeps a table of up to 100 GPS zones in memory and
searches through this table every few seconds to determine if it is within the
boundary of a zone. When it has determined that the user is within a zone, it
displays the zone name on the main screen (shown above).
Zones are created when data are stored. When the Store key is pressed, and the
HydroSense II determines that the current location is not in an existing zone, this
message is shown on the screen. To create a new zone, select YES and press
OK. To exit without storing, press the Back button.

HydroSense II Soil Moisture Measurement System
20
When a new zone is created, the screen above is displayed. Use the Up and
Down keys to select the radius to use for this zone and press OK to save the new
zone details. Press Back to return to the main screen without creating a new zone
and without storing any data.
Once a zone is created, the water content data will be stored. By default the zone
name will be in the form “Zone xxxxx” where xxxxx is a number that
automatically increases each time a new zone is created, however the zones can
be renamed and updated at a later time using the HydroSense II software.
If the Store button is pressed when the GPS does not have a valid GPS sync the
screen above is displayed. This warning is intended to serve as a reminder that
these measurements will not be stored with an accurate date, time or position. To
continue and store data without GPS information, press OK, to return to the main
screen without storing, press Back.
The screens shown above are only displayed when a measurement is stored in a
new location. If the current position is within an existing zone, the Store button
simply causes data to be stored without any queries or warnings.
9Proper measurement Technique & Limitations
9.1 Measurement Principle
The HydroSense II uses the soil dielectric permittivity to estimate the volumetric
water content. The dielectric permittivity of water is much greater than the other
soil constituents making it possible to relate water content to measured dielectric
permittivity. Additionally, water and air are the only soil constituents that change
over a reasonable amount of time, without human intervention.
The electronics contained in the water content sensor generate the high
frequency electromagnetic energy necessary to polarize water molecules such
that their permittivity can be determined. The energy passes along a waveguide
formed by the two rods and reflects from the end of the rods and back into the
probe head where the reflected signal is detected and time of travel is measured.
The time of travel along the waveguide is predominantly dependent on the
dielectric permittivity and since the measured time is the net result of passing
down the length of the rods and back again, it reflects an average of the water
content over the volume of the waveguide. The probe electronics also detect
electrical conductivity (EC) between the rods and use this to correct the
permittivity measurement, allowing the probe to operate in a wider range of soil
EC. The calibration coefficients to convert this measured time of travel to
dielectric constant and ultimately water content are contained within the probe
itself and are the intellectual property of Campbell Scientific.