Insert the Profile Probe
User Manual for the
Profile Probe
type PR2
PR2-UM-3.0
Delta-T Devices Ltd
Notices
Copyright
All rights reserved. Under the copyright laws, this manual may
not be copied, in whole or in part, without the written consent of
Delta-T Devices Ltd. Under the law, copying includes translation
into another language.
Copyright © 2004, Delta-T Devices Ltd.
Patent pending
The Profile Probe has been developed by Delta-T Devices and
uses novel measurement techniques, patent pending.
CE conformity
The Profile Probe type PR2 conforms to EC regulations
regarding electromagnetic emissions and susceptibility when
used according to the instructions contained within this user
manual, and is CE marked by Delta-T Devices Ltd.
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: PR2-UM-3.0 Jan 2008
Delta-T Devices Ltd.
Tel:
+44 1638 742922
130, Low Road
Fax:
+44 1638 743155
Burwell
E-mail: [email protected]
CAMBRIDGE CB25 0EJ
Web:
UK
www.delta-t.co.uk
Contents
Introduction
Description
Parts list
Care and safety instructions
Routine maintenance
PR2 Cleaning and Chemical Avoidance
Instructions
How the Profile Probe works
Operation
Preparation for reading
Insert the Profile Probe
Portable monitoring
Record readings with a data logger
Calibration
Conversion to soil moisture
Reading accuracy
Troubleshooting
Problems
4
4
6
7
7
8
9
10
10
11
12
13
17
19
22
24
24
Technical reference
26
Specifications
Performance
Definitions
References
26
27
29
31
Technical Support
32
Soil-specific calibration
34
Laboratory calibration non-clay soils
Laboratory calibration for clay soils
Field calibration
Index
35
37
40
43
Introduction
Description
The Profile Probe measures soil moisture content at different
depths within the soil profile. It consists of a sealed
polycarbonate rod, ~25mm diameter, with electronic sensors
(seen as pairs of stainless steel rings) arranged at fixed intervals
along its length.
When taking a reading, the probe is inserted into an access
tube. The access tubes are specially constructed thin-wall
tubes, which maximise the penetration of the electromagnetic
field into the surrounding soil.
The output from each sensor is a simple analogue dc voltage.
These outputs are easily converted into soil moisture using the
supplied general soil calibrations or the probe can be calibrated
for specific soils.
Advantages
„
The Profile Probe is dual-purpose
each probe can be used both for portable readings from
many access tubes and for installation within one access
tube for long-term monitoring.
„
„
„
„
„
„
Fully sealed and robust.
4 z Introduction
High accuracy: ±4%.
Easy installation with minimal soil disturbance.
Large sampling volume ~ 1.0 litres at each profile depth.
Simple analogue output, 0 to ~1.0 Volts
Works reliably even in saline soils.
Profile Probe User Manual 3.0a
PR2/6 and
PR2/4 in soil
Sensor 1
Pair of
sensor rings
1 metre
Electromagnetic
fields extend into
the soil and detect
soil moisture
Profile Probe User Manual 3.0a
Introduction z 5
Parts list
Your consignment may have the following parts:
Part
Sales Code
Description
Profile Probe
PR2/4
PR2/4 with 4 sensors (as
shown) or PR2/6 with 6
sensors. Both supplied in
protective tube, with spare orings and centring springs
or
PR2/6
Access tube spacer
SPA1
Spares kit
PR2-SP
Cleaning kit
AT-CR1-
Cleans access tubes
Bag
PR-CB2
Carrying bag for PR2
Access tubes
ATS1
ATL1
Short or long fibreglass tubes
suitable for PR2/4 or PR2/6,
including cap, bung and
collar.
Augering equipment
-
See Augering Manual
Insertion equipment
-
See Augering Manual
Extraction equipment
-
See Augering Manual
Meter
HH2
Moisture meter plus
accessories
Data logger
DL6
DL2e
6-channel data logger
optimised for PR2,
or
General purpose multichannel logger adaptable for
many logging needs.
PRC/d-HH2
1.5m to HH2
PRC/M12-05
5m to DL6
PRC/w-05
5m to DL2e or other loggers
EXT/M12-05
5m, 10m and 25m extension
cables (PRC/M12-05 and
EXT/M12-10 are identical)
or
or
Cables
EXT/M12-10
EXT/M12-25
6 z Introduction
Corrects PR2 depths when
access tubes are mounted
flush with soil surface
Profile Probe User Manual 3.0a
Care and safety instructions
„
Keep your PR2 in its protection tube and fit the
connector cap when the probe is not in use. The Profile
Probe should be stored in a dry environment (definitely
non-condensing), and protected from sharp blows
„
Earth yourself on the metal connector before touching
the detector rings to avoid any possibility of damage by
electrostatic discharge.
„
Don’t lay the PR2 in a puddle because water may creep
under the rings – if you suspect this has happened warm
gently (< 50°C) for 24 hours.
„
Lay as much of the cable as possible along the
surface of the soil when taking a reading in order to
minimise any electrical interference with other equipment.
Routine maintenance
„
Periodically examine the o-rings and centring springs.
They should be kept clean, and if they show any signs of
damage, replace them. Pay attention particularly to the
lowest centring spring when inserting the PR2 into an
access tube – a gentle twisting action helps.
„
The Profile Probe should be periodically recalibrated.
You should run a simple annual check on the calibration
(see the Troubleshooting section) and contact your local
Delta-T representative if there is a problem. Otherwise the
PR2 should be returned for routine re-calibration every 5
years.
Profile Probe User Manual 3.0a
Introduction z 7
PR2 Cleaning and Chemical Avoidance
Instructions
The PR2 shaft is made of polycarbonate plastic which is an
exceptionally strong material, and it can withstand bending
forces far in excess of anything likely to be encountered in
practice. However, polycarbonate can develop stress cracking
when exposed to certain chemicals. Such stress cracking greatly
weakens the polycarbonate and may lead to brittle fracture of the
shaft, even at very low stresses.
It is important to follow these guidelines. Failure to observe these
precautions can damage the probe and may invalidate the
warranty.
„
Clean the probe in use if necessary by wiping with damp
plain paper towels.
„
Use only clean water to damp the paper. Do not use
chemicals or cleaning agents of any sort in the water.
„
Never use any chemical solvents or cleaners on the probe,
or near to it. Avoid strong chemical vapours, especially
during probe storage.
„
Do not immerse the probe in water. If this happens, allow
the probe to dry in warm air for at least 24 hours.
„
Make sure the probe is thoroughly dry before storing it in
the protection tube.
8 z Introduction
Profile Probe User Manual 3.0a
How the Profile Probe works
Before you rush out and hammer your access tubes into the soil,
it will help to understand a little about how the Profile Probe
works:
When power is applied to the Profile Probe...
...it creates a 100MHz signal (similar to FM
radio).
The signal is applied to pairs of stainless steel
rings....
... which transmit an electromagnetic field
extending about 100mm into the soil.
The field passes easily through the access tube
walls, but less easily through any air gaps.
The water content of the soil surrounding the
rings...
...dominates its permittivity.
ε
(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
Soil Moisture
22 %
…resulting in a stable voltage output that…
...acts as a simple, sensitive measure of soil
moisture content.
Profile Probe User Manual 3.0a
Introduction z 9
Operation
Preparation for reading
Install access tubes
The Profile Probe must be used within an access tube. The
process of augering holes and installing access tubes is
described in the Augering Manual.
Equipment
You may require the following equipment for a site visit:
PR2 in protective tube
Access tube spacer
Spare
„ collars
„ caps
„ centring
springs
If setting up
logging:
Data logger (DL6 or
DL2e) and cable
For portable
reading: HH2
meter and
cable
Roll of paper
towels
Cleaning rod
10 z Operation
Profile Probe User Manual 3.0a
Insert the Profile Probe
„ Remove the tube cap and check for damp
If the access tube has been left empty for several weeks, check
for condensation by threading paper towel into the slot in the
cleaning rod and pushing this to the bottom of the tube. If there
is any water present, you will need to dry the tube thoroughly.
„ Check the centring springs
centring spring
Remove the PR2 from its protective tube.
probe
The Profile Probe is fitted with centring springs so
rod
that the probe is correctly centred within an access
tube. They must be fitted and working properly for
the probe to take accurate readings. Each centring spring
(coiled spring) sits on top of an o-ring (see illustration).
o-ring
„ Fit spacer (if required)
If your access tube has been installed flush with the soil surface,
you will need to fit the access tube spacer (SPA1). Slide the
spacer over the tip of the probe and push all the way up past the
top o-ring.
„ Insert the Profile Probe
Take care as the first centring spring is pushed into the tube not
to pinch the spring unevenly against the side of the tube. A
slight twisting motion as the spring goes in will help protect it.
„ Align the probe
The probe should be aligned consistently each time it is inserted,
using the alignment marks on the access tube and the label on
probe handle.
If you want to maximise the sampling at each location, we
suggest that you take the average of three readings at each
location, with the tube rotated through 120° each time – the
three small screw heads can be used for this purpose.
Ensure that the Profile Probe is pushed all the way down over
the top o-ring.
The PR2 is then fully sealed in its access tube and ready either
for immediate reading or for attaching to a logger for extended
monitoring.
Profile Probe User Manual 3.0a
Operation z 11
Portable monitoring
„ Set up the HH2 meter
Connect the Profile Probe to your HH2
using the supplied PRC/d-HH2 cable
Press Esc to turn the meter on, and if
necessary press again until it displays:
Delta-T Devices
ΔTMoisture Meter
Make sure the meter is set to read from a PR2:
„ Press Set and scroll down to the Device option.
„ Press Set again and scroll down to:
„
Device:
v PR2
Press Set to confirm this choice.
If you intend to store readings, you may find it useful to define
each reading by setting a plot label and sample number –
accessed by pressing Set and scrolling to the Data option.
See Calibration section for advice on setting Soil Type and
Soil Set-Up.
For other options, refer to the HH2 User Manual.
„ Taking readings
Insert the Profile Probe into an access tube.
Press Read to take a reading - it takes about 3 seconds.
Press the arrow keys to view
readings from other depths. You
can choose different units from
the Display option.
PR2
Store?
22.7%vol v100mm
Press Store to save or Esc to discard the reading.
If you want to maximise the sampling volume, take 2 further
readings with the probe rotated through 120° each time.
Remove the PR2, replace the access tube cap and move on to a
new site….
„ Viewing stored readings
If you have saved data, connect the HH2 to your
PC and run HH2Read to retrieve the readings.
12 z Operation
Profile Probe User Manual 3.0a
Record readings with a data logger
The Profile Probe has been designed to make its use with data
loggers straightforward. It is particularly simple to use with the
DL6 data logger as they have been designed to work together.
DL6 connection and configuration
You will need access either to a PC with DL6 Control Panel
software installed, or to a Pocket PC with Pocket DeltaLINK.
„ Connect the Profile Probe
The PR2 can be connected directly to the DL6 with the supplied
cable. Extension cables can be added as required up to 100m.
„ Configure the DL6
Using Pocket DeltaLINK or DL6 Control Panel:
„ Click on Connect to Logger and for the Programs
window, then in the Sensors tab set Channel 1 either to
PR2/4 or PR2/6.
„
„
Set the Recording Interval in the Main tab…
„
When finished, click on
logger.
… there are many other options - refer to the DL6 user
manual for details.
Profile Probe User Manual 3.0a
to install the program in the
Operation z 13
To check the connections, select the Sensors tab and click on
to see the PR2 readings.
„ Start logging
When ready click on
to start logging…
to select the Logger window and Start
„ Collect the data
…later when you want to collect the data, connect to the DL6
and click on
to select the Dataset window from which you
can retrieve and display all stored readings.
See the DL6 User Manual for configuration options.
DL2e connection and configuration
The Profile Probe is fitted with a screened 8-way connector.
When used with a DL2e this should be connected using the
PRC/w-05 cable, which provides the following connections.
Cable
Colour
Function
Red
Power V+
Black
Power 0V
5-15V DC, PR2/4 80mA, PR2/6 120 mA.
Power 0V is cable screen.
Green
Signal COM
Common signal output
Yellow
Vout 1
Top sensor, 100 mm depth
Grey
Vout 2
Brown
Vout 3
White
Vout 4
Blue
Vout 5
Pink
Vout 6
Notes
For PR2/4, Vout 5 and 6 are not
connected.
Notes:
1. These connections are not the same as the PR1.
2. The cable screen serves as the power return and is given black
insulation.
3. Do not connect the Power 0V and Signal common at the
logger. This will create reading errors.
14 z Operation
Profile Probe User Manual 3.0a
Link the green
wire to the
minus –
terminal on
every channel
used by the
PR2
This diagram shows the connections for Profile Probe sensor 1
connected to channel 1 of a DL2e in differential mode, and
powered through the logger’s internal power supply.
Further details can be found in your Ls2Win installation. The
DL2 Program Editor contains on-line help and an application
note on each sensor type.
Four DL2e sensor codes are supplied for the PR2:
P2M provides a conversion from mVolts to soil water content (in
3
-3
m .m ) suitable for general mineral soils.
P2O converts to m3.m-3 for general organic soils.
P2C converts to %vol for general mineral soils.
P2D converts to %vol for general organic soils.
Note: All DL2e data in m3.m-3 units is restricted to 0.01 m3.m-3
resolution. The P2C and P2D codes using %vol show much
better resolution, and are preferred.
Power supply
Profile Probes types PR2 require 5.5 to 15 Vdc power. Power
can be applied continuously, or via a warm-up relay for greater
economy of power consumption.
You can power Profile Probes directly using DL2e internal
batteries. However, if several probes are to be used, or if the
data logger has to supply significant power to other sensors or
accessories, we recommend powering the data logger and
sensors from an external power supply.
The DL2e has two warm-up relay-controlled outputs. Each relay
can typically power up to 12 PR2/4 or 8 PR2/6 Profile Probes.
Note: For best economy the Profile Probe should be powered
up using a 1 second warm-up time.
Profile Probe User Manual 3.0a
Operation z 15
Other data loggers
The connections and power requirements will usually be the
same as for the DL2e.
If you simply want to log the sensor voltages directly, they can
be treated as differential voltage sources of range 0 – 1.1 V DC,
and the data logger should be configured accordingly.
Warning: we recommend connecting Profile Probes as
differential voltage sensors because they are powered sensors.
Although you can measure Profile Probes single-ended, this will
introduce an additional measurement error that depends mainly
on the length of your cable. It may also have undesirable side
effects on the apparent reading from other sensors attached to
the data logger.
You can either convert the data to soil moisture units after
logging, or program your data logger to convert the output
automatically before logging the data, using the information
supplied in the Conversion to Soil Moisture section.
16 z Operation
Profile Probe User Manual 3.0a
Calibration
The Profile Probe detects soil moisture by responding to the
permittivity (ε′) of the damp soil (see illustration on page 8) – or
more accurately to the refractive index of the damp soil, which is
~ equivalent to √ε.
As a result, the performance of the Profile Probe is best
understood if it is split into these two stages:
Soil calibration: soil moisture (θ) determines √ε
Profile Probe response: √ε determines PR2 output (Volts)
Soil calibration
Soil calibration
6
√ε
damp soil
5
4
Slope (a1)
3
2
1
Soil offset (a0)
0
0
10
20
30
40
50
Water content of soil (%vol)
This method of detection is very sensitive and accurate, but of
course soils can be enormously different one from another.
The soil offset and the slope of the line in the graph above both
depend slightly on soil type, varying with density, clay content,
organic matter etc.
This can be usefully summed up in a simple equation describing
the relationship between √ε and the soil water content, θ, which
contains two parameters (a0 and a1) that reflect the influence of
the soil:
ε = a0 + a1 × θ
[1]
The accuracy of your Profile Probe readings can be improved if
you choose appropriate values for a0 and a1. This is usually very
simple…
Profile Probe User Manual 3.0a
Operation z 17
Generalised calibrations
Most soils can be characterised simply by choosing one of the
two generalised calibrations we supply, one for mineral soils
(predominantly sand, silt or clay) and one for organic soils (with
a very high organic matter content).
a0
a1
Mineral soils
1.6
8.4
Organic soils
1.3
7.7
These values have been used to generate the slope and offset
conversions and linearisation tables in the Conversion to soil
moisture section.
Soil-specific calibration
If it is important to work to higher accuracy, you may choose to
carry out a soil-specific calibration, but please bear this in mind:
For normal agricultural soils, if you use one of the generalised
3
-3
calibrations, you can expect typical errors of ~ ±0.06 m .m ,
including installation and sampling errors.
If instead you use a soil-specific calibration, you can expect
3
-3
typical errors of ~ ±0.05 m .m .
As a guideline, we suggest that you only need to do a soilspecific calibration if one of the following applies:
4
Your soil is heavy clay, highly organic, or in some respect
“extreme”.
4
You are working to high levels of accuracy, or you need a
controlled error figure rather than a “typical” error figure.
and the following do not apply
8
Your soil is very stony (insertion errors are likely to outweigh
the calibration errors)
8
your soil cracks when it dries (again measurement errors
are likely to be higher than calibration errors)
The procedure for carrying out a soil-specific calibration is
detailed in Appendix A.
18 z Operation
Profile Probe User Manual 3.0a
Profile Probe response
All Profile Probes have the same dielectric performance:
Profile Probe output
7
6
PR2 readings
5
fitted curve
√ε
4
3
2
1
0
0.0
0.2
0.4
0.6
0.8
1.0
PR2 output (Volts)
This relationship can be fitted very precisely up to ~ 0.5 m3.m-3
by the following polynomial:
ε = 1.125 − 5.53V + 67 .17V 2 − 234 .42V 3 + 413 .56V 4 − 356 .68V 5 + 121 .53V 6
[2]
and can be approximated by the following linear relationship:
ε = 0.37 + 4.43V
3
up to 0.3 m .m
-3
[3]
Conversion to soil moisture
Profile Probes can either be used to give instantaneous readings
of soil moisture using a hand-held meter, or they can be
connected to a data logger to record moisture data over time.
In either case you will probably want to configure the meter or
data logger to convert the Profile Probe output to soil moisture
content. Three data conversion methods can be used:
♦
polynomial conversion
♦
linear conversion (slope and offset)
♦
linearisation table conversion
Profile Probe User Manual 3.0a
Operation z 19
Polynomial conversion
Combining the Soil calibration and Profile Probe response steps,
the conversion equation becomes:
θV =
3
[1.125 − 5.53V + 67.17V 2 − 234.42V 3 + 413.56V 4 − 356.68V 5 + 121.53V 6 ] − a0 m .m
a1
-3
where a0 and a1 are the calibration coefficients above.
For a generalised mineral soil this becomes:
3
-3
θ min = −0.057 − 0.66V + 8.00V 2 − 27.91V 3 + 49.23V 4 − 42.46V 5 + 14.47V 6 m .m ,
And for an organic soil:
3
-3
θ org = −0.023 − 0.72V + 8.72V 2 − 30 .44V 3 + 53 .71V 4 − 46.32V 5 + 15.78V 6 m .m .
Slope and offset conversion
Combining the Soil calibration and linear Profile Probe response
equations:
θV ≈
[0.37 + 4.43V ] − a0
a1
3
-3
3
-3
m .m , up to 0.3 m .m .
Using the values of a0 and a1 for generalised mineral and organic
soils:
From probe V to:
Slope
m3.m-3 / V
Offset
m3.m-3
3
-3
0.528
-0.146
3
-3
0.575
-0.121
m .m , Mineral soil
m .m , Organic soil
To convert data readings from volts, multiply by the slope and
add the offset. This gives readings in m3.m-3.
Linearisation table conversion
The following table of values is used for the DL2e logger sensor
codes P2M, P2O, P2C, P2D.
20 z Operation
Profile Probe User Manual 3.0a
soil moisture
3
-3
m .m
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
%vol
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
mineral
soil
Volt
organic
soil
Volt
0.257
0.379
0.497
0.595
0.677
0.749
0.810
0.860
0.899
0.930
0.956
0.977
0.995
1.011
1.026
1.038
1.050
1.060
1.070
1.079
1.088
0.177
0.280
0.394
0.501
0.590
0.667
0.734
0.793
0.843
0.882
0.914
0.940
0.962
0.981
0.997
1.012
1.025
1.037
1.048
1.057
1.067
DL2e slope and offset conversion
For DL2e data loggers, you can create sensor codes using slope
and offset, if you cannot use the linearisation table codes above.
These will be accurate only for the restricted range up to 30%vol.
The sensor code ‘Conversion Factor’ is the reciprocal of the
‘Slope’ figures above. Base units are mV and Engineering units
are %vol.
From probe mV to:
Conversion
Factor
mV/%vol
Offset
%vol
%volume, Mineral soil
18.96
-14.6
%volume, Organic soil
17.38
-12.1
Profile Probe User Manual 3.0a
Operation z 21
Reading accuracy
The Profile Probe is accurate and reliable.
However this doesn’t guarantee that the readings you take with a
PR2 are an accurate measure of the soil moisture. There are
three particular sources of error that you need to consider when
measuring soil moisture with the Profile Probe:
♦ Installation problems
♦
Soil type and Sampling errors
♦
Salinity
Installation problems
An ideal installation would avoid creating either air gaps or soil
compaction around the access tube – and then the soil would
not shrink or swell as it dried out or rewetted. It’s possible to get
remarkably close to this ideal in some deeply cultivated soils,
and close to impossible in some stony soils or hard clay.
We obviously can’t quantify your potential installation errors, but
experience suggests that a loose, gappy, access tube
3
-3
installation could lead to errors of ± 10% (± 0.1 m .m ), so...
„
„
Take as much care as you can over the installation
Remember to fit a collar to your access tube.
Soil type and sampling errors
Again, it’s not really possible to quantify the potential errors
associated with soil type, but be aware of the following:
„
Almost all measurement problems are worst in heavy clay
soils.
„
If your soil cracks badly in dry conditions, the readings from
your Profile Probe may be more indicative of crack size than
soil moisture content!
„
The linear relationship in equation [1] is less applicable to
3
-3
heavy clay soils at low soil moisture levels (< 0.1 m .m ).
See ref. [7].
„
Soil moisture content may vary significantly even within a
small volume of soil. When you rotate the Profile Probe
within its access tube the reading changes you observe
reflect real soil moisture variability.
22 z Operation
Profile Probe User Manual 3.0a
Salinity
Changes in soil salinity cause a change in reading, which will
appear as a change in soil moisture. Typical effects on Profile
3
-3
Probe readings are an apparent change of < 0.005 m .m soil
-1
moisture for a change of 100 mS.m soil salinity.
In most situations this sensitivity is of little significance because
-1
a change of 100 mS.m is very unlikely - but it may need to be
considered particularly when irrigating with saline irrigation
water.
See Salinity Performance in the Technical Reference section.
Profile Probe User Manual 3.0a
Operation z 23
Troubleshooting
Problems
When getting problems from a probe or sensor always try to
identify which part of the measurement system is the source of
the difficulty. For the Profile Probe 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
•
A Delta-T DL6 logger
•
A Delta-T DL2e logger
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 the soil calibration being used is appropriate for your
soil, and that the correct conversion method is being used – see
Calibration section.
The probe itself
Try to isolate the problem into one of the following areas
•
The Probe or the connecting cable
Then try to narrow down the area further
•
Mechanical problems faults, or damage
•
Electrical or electronic problems or faults
Calibration check
We recommend that you check the calibration of your PR2 at
least once a year by taking an air reading and a water reading as
follows:
Air reading
Keep the PR2 in its protection tube and hold it away from any
other objects. Take a reading using an HH2 meter, or other
meter or logger. The reading should be 75 ±20mV.
24 z Troubleshooting
Profile Probe User Manual 3.0a
Water reading
Insert the PR2 fully into an access tube and immerse the tube
into a large body of water at 20 to 25°C. The water container
should be sufficiently large so that the PR2 is >100mm from any
edge. Take a reading using an HH2 meter, or other meter or
logger. Although this reading is outside the PR2’s specified
accuracy range, the reading should lie between 1040 and
1100mV.
Centring springs
Check that the centring springs are all fitted, clean and
undamaged. Immediately replace any that do become
damaged.
Installation problems
Augering and access tube insertion
Most PR2 errors are caused by inserting an access tube into the
wrong size of augered hole.
If the hole is too large, gaps around the tube will result in
generally low readings and poor response to soil moisture
changes – unless the gaps fill with rainwater.
If the augered hole is too small, the effort necessary to hammer
the access tube into the soil will often result in gaps forming
around the tube at the top and compaction of the soil lower down
the tube.
Refer to the Augering Manual for advice on augering holes of the
correct size.
Profile Probe User Manual 3.0a
Troubleshooting z 25
Technical reference
Specifications
Technical Specifications for PR2/4 and PR2/6
3
-3
Measurement
Volumetric soil moisture content, θV (m .m or %vol.).
Range
Accuracy specified from 0 to 0.4 m .m ,
3
-3
Full range is from 0.0 to 1.0 m .m
3
Accuracy
3
-3
3
-3
± 0.04 m .m , 0 to 40°C
± 0.06 m .m , 0 to 40°C
Salinity errors
Soil sampling
volume
Environment
Stabilisation
Power
requirement
Outputs
-3
after soil specific
calibration
with generalised soil
calibration in 'normal' soils
Included in above figures (50 to 400 mS.m-1)
Vertically: ~95% sensitivity within ±50mm of upper ring
of each pair.
Horizontally: ~95% sensitivity within a cylinder of radius
100mm.
0 to 40°C for full accuracy specification,
–20 to 60°C full operating range.
IP67 rated
Full accuracy achieved within 1s from power-up.
Minimum: 5.5V DC with 2m cable, 7.0V with 100m.*
Maximum: 15V DC
PR2/4 consumption: < 80 mA
PR2/6 consumption: < 120 mA
4 (PR2/4) or 6 (PR2/6) analogue voltage outputs:
3 -3
~0 to 1.0V DC corresponding to 0 - 0.6 m m (mineral
calibration)
Cable
8-core screened. Maximum length: 100m
Construction
material
25.4mm polycarbonate tube with pairs of stainless steel
rings
PR2/4 length: 750mm
Weight: 0.6kg
PR2/6 length: 1350mm
Weight: 0.95kg
Size / weight
*using cables supplied by ΔT
26 z Technical reference
Profile Probe User Manual 3.0a
Performance
Field sensitivity
The signal is applied to the upper ring of each pair, so the
electromagnetic field is stronger around the upper ring.
Although this field extends a considerable distance into the soil
(~100mm), it is strongest close to the rings, and so the soil close
to the rings contributes most to the output.
Normalised sensitivity versus sample radius
1
0.6
Profile Probe
Output (V out /Vmax)
0.8
0.4
Damp Soil
0.2
0
0
20
40
60
80
100
120
140
160
Radius of sample cylinder (mm)
Salinity
The Profile Probe output has been tested as follows:
PR2 conductivity response
60.0
PR2 output (%vol)
50.0
40.0
30.0
20.0
PR2 in wet soil
ideal response in wet soil
10.0
PR2 in damp soil
ideal response in damp soil
0.0
0
100
200
300
400
500
600
Pore Water Conductivity (mS.m-1)
Profile Probe User Manual 3.0a
Technical reference z 27
Temperature
The Profile Probe has a very low intrinsic sensitivity to changes
in temperature, as in this example:
PR2 output versus temperature
50
PR2 output (%vol)
40
30
20
10
0
-40
-20
0
20
40
60
80
Temperature (°C)
This relationship is dependent on soil composition (particularly
clay content) and the soil moisture level, see ref. [7].
Electromagnetic Compatibility (EMC)
Europe
The Profile Probe has been assessed for compatibility under the
European Union EMC Directive 89/336/EEC and conforms to the
appropriate standards, provided the probe body and moisture
measuring rings are completely inserted into the access tube
within the soil or other material being measured. The cable
connecting the Profile Probe to its associated instrumentation
should be routed along the surface of the soil.
If the probe is not installed in this way, some interference may be
experienced on nearby radio equipment. Under most conditions,
moving the equipment further from Profile Probe (typically 1-2
metres) will stop the interference.
Profile Probes installed near to each other will not malfunction
due to interference.
North America
This device complies with Part 18 of the FCC Rules.
Operation is subject to the following two conditions:
(1) this device may not cause harmful interference, and
(2) this device must accept any interference received, including
interference that may cause undesired operation
28 z Technical reference
Profile Probe User Manual 3.0a
Definitions
Volumetric Soil Moisture Content is defined as
θV =
VW
VS
where Vw is the volume of water contained in
the sample
and Vs is the total volume of the soil sample.
3
-3
The preferred units for this ratio are m .m , though %vol is
frequently used.
3
-3
Soil Moisture Content varies from approx. 0.02 m .m for sandy
3
-3
soils at the permanent wilting point, through approx. 0.4 m .m
for clay soils at their field capacity, up to values as high as 0.85
3
-3
m .m in saturated peat soils.
Gravimetric Soil Moisture Content is defined as
θG =
MW
MS
-1
g.g
sample.
where MW is the mass of water in
the sample,
and M S is the total mass of the dry
To convert from volumetric to gravimetric water content, use the
equation
θ G = θV *
ρW
ρS
where ρW is the density of water (= 1),
and ρ S is the bulk density of the
sample ( =
MS
).
VS
Organic and Mineral definitions:
The generalised 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:
-3
(g.cm )
use for
bulk
densities:
-3
(g.cm )
Mineral
~ 1 %C
< 7 %C
1.25 - 1.5
> 1.0
Organic ~ 40 %C
> 7 %C
0.2 - 0.7
< 1.0
Profile Probe User Manual 3.0a
Technical reference z 29
Salinity
-1
The preferred SI units for ionic conductivity are mS.m (where S
is Siemens, the unit of electric conductance = ohm-1).
The following conversions apply:
1 mS.m
-1
= 0.01 dS.m-1
-1
= 0.01 mS.cm
-1
= 0.01 mmho.cm
-1
= 10 µS.cm
Soil salinity is also partitioned into the following descriptive
categories:
non-saline
-1
0 - 200
mS.m
slightly saline
200 - 400
mS.m
moderately saline
400 - 800
mS.m
strongly saline
800 - 1600
mS.m
extremely saline
> 1600
mS.m
30 z Technical reference
-1
-1
-1
-1
Profile Probe User Manual 3.0a
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
8.
Whalley, W.R., R.E. Cope, C.J. Nicholl, and A.P.Whitmore,
2004
In-field calibration of a dielectric soil moisture meter
designed for use in an access tube.
Soil Use and Management, 20, 203-206
Profile Probe User Manual 3.0a
Technical reference z 31
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.
32 z Technical Support
Profile Probe User Manual 3.0a
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:
Tech Support Team
Delta-T Devices Ltd
130 Low Road, Burwell, Cambridge CB25 0EJ, U.K.
email: [email protected]
email: [email protected]
web: www.delta-t.co.uk
Tel: +44 (0) 1638 742922
Fax: +44 (0) 1638 743155
Profile Probe User Manual 3.0a
Technical Support z 33
Appendix A
Soil-specific calibration
This note provides details of 3 techniques for generating soil-specific
calibrations:
1.
Laboratory calibration for substrates* and non-clay soils
2.
Laboratory calibration for clay soils
3.
Field calibration
* 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
ThetaProbe and Profile Probe (see
Calibration section).
The goal of calibration is to
generate two coefficients (a0, a1)
which can be used in a linear
equation to convert probe readings into soil moisture:
ε = a 0 + a1 × θ
Using the ThetaProbe to calibrate the Profile Probe
Soil calibrations using the ThetaProbe and Profile Probe are very similar because they measure the same fundamental dielectric property (√ε) at the
same frequency (100MHz). However both their calibrations are influenced
by their slight sensitivity to conductivity - and they differ in how this
sensitivity changes with water content. The ThetaProbe (and methods 1. or
2. below) can be used effectively for creating soil-specific Profile Probe
calibrations at low water contents and/or low conductivities. At high
conductivity and high water content it is far better to generate Profile Probe
calibrations using the field calibration technique (3.).
34 z Soil-specific calibration
Profile Probe User Manual 3.0a
Appendix A
Laboratory calibration 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:
„ ThetaProbe and meter
„ Soil corer (if doing a calibration for a cohesive soil rather than sand
or a substrate)
„ Heat-resistant beaker (≥ 500ml)
„ 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 ≥ 500ml, 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
Profile Probe User Manual 3.0a
Soil-specific calibration z 35
Appendix A
Insert ThetaProbe into the sample and record its
output in Volts.
Vw = 0.672V
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 ThetaProbe into the dry sample and
record this reading.
V0 = 0.110V
Calculate a0
For the ML2,
ε = 1.07 + 6.4V − 6.4V 2 + 4.7V 3
In the dry soil V = V0 = 0.110 Volts, and substituting
this value into the above equation gives ε 0 = 1.70 .
Since θ0 = 0, this is the value needed for a0
a0 = 1.70
Calculate θw
The water content of the wet soil, θw, can be
calculated from the weight of water lost during
36 z Soil-specific calibration
Profile Probe User Manual 3.0a
Appendix A
drying, (Ww – W0) and its volume, Ls:
θ w = (Ww − W0 ) Ls = (743.3 − 627.2 ) 463.5 = 0.25
θw = 0.25
Calculate a1
In the wet soil V = Vw = 0.672 Volts and substituting
gives ε w = 3.91
Finally
a1 = ε w − ε 0
(
) (θ
w
− θ 0 ) = (3.91 − 1.70) (0.25 − 0) = 8.80
a1 = 8.80
Laboratory calibration for clay soils
This technique is adapted to avoid the near-impossibility of inserting the
ThetaProbe into a completely dry clay soil. It requires taking measurements
at 2 significantly different, but still damp, moisture levels.
Equipment you will need:
„ ThetaProbe 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 insitu 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
Profile Probe User Manual 3.0a
Soil-specific calibration z 37
Appendix A
Weigh the wet sample, including the beaker.
Ww = 743.3g
Insert ThetaProbe into the wet sample and
record its output in Volts.
Vw = 0.672V
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 ThetaProbe.
Vm = 0.416V
38 z Soil-specific calibration
Profile Probe User Manual 3.0a
Appendix A
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).
Weigh the dry sample in the beaker.
W0 = 627.2g
Calculations
Substituting in the ML2 equation
ε = 1.07 + 6.4V − 6.4V 2 + 4.7V 3 provides two
dielectric values, √εw and √εm, at two known
water contents, θw and θm:
For the wet soil
Substituting Vw = 0.672 gives
ε w = 3.91 = a0 + a1 ⋅ θ w
for θ w = (743.3 − 627.2 ) 463.5 = 0.25
For the moist soil
Substituting Vm = 0.416 gives
ε m = 2.96 = a 0 + a1 ⋅ θ m
For
θ m = (693.2 − 627.2) 463.5 = 0.14
Calculate a1
Then a1 =
(
εw − εm
) (θ
w
− θ m ) = 8.73
a1 = 8.73
Calculate a0
and a 0 = ε w − (a1 ⋅ θ w ) = 1.72
a0 = 1.72
Profile Probe User Manual 3.0a
Soil-specific calibration z 39
Appendix A
Field calibration
Field calibration is the surest method of calibration. We strongly
recommend it for Profile Probe installations featuring high water content
(usually high-clay-content) and high conductivity, as it is the only technique
likely to give good results. However it is typically far more time consuming
and requires access to considerably more equipment than laboratory
calibration.
General principle
Install access tubes and take Profile Probe measurements (as voltages)
over a period of time when the soil moisture content is changing. Over the
same period, measure the water content at appropriate depths and spacing
around the access tubes either by gravimetric sampling or using a Neutron
Probe or using ThetaProbes. These comparison readings can then be used
to construct a calibration for the Profile Probe.
For best results this approach requires comparison readings over a
significant range of soil moisture contents. If the changes in water content
over the measurement period are small, the calibration becomes very
sensitive to any measurement errors. The extreme case of this occurs
when readings are only available at a single soil moisture content. It is still
possible to calibrate the Profile Probe in these cases - by assuming a
default value for the intercept coefficient, a0.
Equipment you will need:
ƒ
Installed Profile Probe access tubes, and Profile Probe with either
meter or data logger
ƒ
Either installed ThetaProbes, ~150mm from the access tubes at
the appropriate depth
Or Neutron Probe access tubes installed ~300mm from the Profile
Probe tubes
Or gravimetric sampling equipment (see previous methods)
Or a portable ThetaProbe attached to a suitable length extension
rod and a suitable auger for sampling at depth
The gravimetric and portable ThetaProbe methods both require essentially
destructive measurements, which limit their re-use at the same site, so they
may require a number of similar sites. But see below for fixed intercept
calibration.
40 z Soil-specific calibration
Profile Probe User Manual 3.0a
Appendix A
Notes and example
Process
Take Profile Probe readings (as voltages)
over a period of time as the soil moisture
content changes. Ideally this would include
3 or more distinct soil moisture levels
covering a change > 0.1m3.m-3.
At the same time, take several independent
soil moisture readings spaced around the
Profile Probe access tube. These could be
taken either with ThetaProbes or a Neutron
Probe or by gravimetric sampling.
The number of samples required depends
on the uniformity of the soil and the size of
the sampling volume.
If it is difficult to take readings over a range
of moisture levels, it is still possible to
calibrate the Profile Probe using a single
soil moisture comparison using the fixed
intercept method below.
150mm
V
→ √ε
0.462
0.577
2.31
2.78
Convert the Profile Probe measurements
into √ε using its calibration equation [2].
Graph these √ε readings against the soil
moisture measurements.
(This illustration and the following
procedures are taken from Excel, but the
principles can also be applied within other
graphing or spreadsheet programs)
Profile Probe User Manual 3.0a
Soil-specific calibration z 41
Appendix A
Variable Intercept
Fit a linear trendline to the data, and in the
Options tab choose to display the equation.
You may need to adjust the number format
for the equation to 3 decimal places.
The calibration coefficients can then be
read off directly. In the example shown, a0
= 1.537 and a1 = 8.656.
Fixed Intercept
Fit a linear trendline as above, but in the
Options also choose “Set intercept =”.
We suggest you use the following default
intercept values:
Organic soil
1.4
Mineral soil
1.6
Heavy clay
1.8
In this example the intercept has been set
to a0 = 1.8, and the calculated value for a1 =
7.794.
42 z
Profile Probe User Manual 3.0a
Index
A
I
Access tube, 4, 6, 7, 9, 10, 11,
12, 20, 22, 26, 28, 31, 40,
41
bung, 6
cap, 6, 7, 11, 12
Accuracy, 22, 26
Air gaps, 9, 22
Installation, 4, 10, 15, 18, 22
C
Calibration, 12, 17, 18, 20, 26,
31, 34, 35, 37, 40, 41, 42
soil-specific, 18, 34
Cleaning and Chemical
Avoidance Instructions, 8
Connections, 14, 15, 16
Conversion factor, 21
Conversions, 16, 18, 19, 21
linearisation table, 18, 19, 21
polynomial, 19
slope and offset, 17, 18, 19,
20, 21
D
Data logger, 6, 11, 13, 14, 15,
16, 19, 20, 40
DL2e, 6, 14, 15, 16, 20, 21
DL6, 6, 13, 14
Definitions, 29
Dielectric performance, 19, 31,
34, 39
M
Moisture content, 4, 9, 19, 22,
26, 31, 34, 40, 41
P
Power supply, 9, 14, 15, 16, 26
R
Range, 26
References, 31
S
Salinity, 22, 23, 26, 27, 30
Sampling
volume, 4, 12, 26, 41
Soil
clay, 17, 18, 22, 28, 29, 34, 35,
37, 40, 42
composition, 28
dry, 22, 35
mineral, 15, 18, 20, 21, 26, 36,
39
organic, 15, 17, 18, 20, 21, 29,
36
type, 12, 22
Specifications, 26
F
Field sensitivity, 27
Profile Probe User Manual 3.0a
Index z 43
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