This is the archive of the second mailing of the SoWaCS group
and consists of the Introductory covering message,
and the discussion document (below)
******** ********
Welcome to the second message of the
Soil Water Content Sensor discussion group!
********** (SoWaCS group ) ************
TDR and FDR S E N S O R S
(Time and Frequency Domain Reflectometry respectively.)
& NEW VOLTAMMETRIC SENSORS
********** ************
Receiving this confirms that you are on the circulation list.
Run by Bruce Metelerkamp bruce@icfr.unp.ac.za
***###***
The full transcripts will be placed on WWW page, URL:
index.html
(FOR EASIER READING, GO STRAIGHT TO THIS TEXT PAGE)
* * * # # # * * *
23 April 1996
Following the discussion on another soil water sensor, (the
CS615 TDR like sensor marketed by Campbell Scientific), a
number of questions on related sensors have arisen.
(There was not an excessive amount of comment - which means, I
can only presume that either there was not much interest in
this new sensor, or, that the document had all that any one
could want to know about it (which I doubt!) ).
The general principles of TDR are not well known, so
addressing this has been a target for this message:
First and foremost the differences and similarities of the TDR
and FDR sensors and how they work will be discussed.
Then specific comments of users (as related to me and to the
Trickle-L archive) are added.
The attached file then discusses mainly the capacitance
sensor, and as an example of this, the EnviroSCAN soil water
measuring system is extensively covered.
However, there are many other manufacturers; and we are
looking for feedback on all types in use.
I have for example had a report of a user of a Didcott system
who says that for his soils, (very low bulk density), the
sensor "did not perform" and he has moved to TDR and neutron
probe. Any feedback - even if negative - is welcomed.
Future discussions will centre around follow-ups to this
message, as well as the following sensors:
Comparison of various TDR and FDR systems : Tektronix 1502B,
Trase, Watermatic (Cuming), DeltaT, Troxler Sentry 200 AP,
MCM100 (IMAG-DLO), Didcott, Aquaflex, Dataflow (Wesdata),
SENTEK EnviroSCAN etc
The Watermark sensor
The Heat Dissipation (HDS) or Granular Matrix sensor.
3 Probe heat pulse
DRW microlink heat pulse
Irrometer
PLEASE SEND FEEDBACK ON ANY OF THESE LISTED ABOVE.
As a last note, I have found that there have been and probably
will continue to be a lot of researchers and users in my
position. And that is that they do not know what sensors and
technologies are out there, but feel hesitant to ask for it on
a widely read-email list, or know it must exist yet do not
know how to access or ask for it - or have only heard of "the net"!
For these reasons, and the fact that commercial details and
practical user experience are not generally adequately
reported on in the literature, that archives of this
information on the Web are so important. If I leave my current
occupation, the wealth of knowledge I have collected will not
go with me if I have put it on the Web. Inhouse technical
papers never seem to get finished, and if they are, can't be
found.
It does take a little, nay - a lot, of effort to get this done,
but the reduction in the amount of "re-inventing of the wheel"
is well worth it.
.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
Bruce Metelerkamp SOIL WATER RESEARCH OFFICER
Institute for Commercial Forestry Research,
University of Natal, PO Box 100281
Scottsville, ZA3209
Rep. of South Africa Voice:27 331 62314
E-mail: bruce@icfr.unp.ac.za FAX:27 331 68905
URL http://www.icfrnet.unp.ac.za/
/index.html
/RR.html
!!!!!!! DISCUSSION DOCUMENT PROPER !!!!!!!!
******** ********
Welcome to the second message of the
Soil Water Content Sensor discussion group!
********** (SoWaCS group ) ************
TDR and FDR S E N S O R S
(Time and Frequency Domain Reflectometry respectively.)
********** ************
Receiving this confirms that you are on the circulation list.
Run by Bruce Metelerkamp bruce@icfr.unp.ac.za
***###***
The full transcripts will be placed on WWW page, URL:
index.html
(FOR EASIER READING, GO STRAIGHT TO THIS TEXT PAGE)
* * * # # # * * *
23 April 1996
Following the discussion on another soil water sensor, (the
CS615 TDR like sensor marketed by Campbell Scientific), a
number of questions on related sensors have arisen.
First and foremost the differences and similarities of the TDR
and FDR sensors and how they work will be discussed.
These are directly measuring sensors based on similar
principles. The latter are also termed capacitance sensors.
-----------------------
Sensor Description
-----------------------
Both types of probes (TDR and FDR) consist of metal electrodes
(2 or more spikes) that are placed in the soil (or other
substrate to be measured). It is possible to use coaxial
electrodes (concentric rings), but this is normally used only
for specific applications.
These sensors can either be left permanently buried in the
field, or can be periodically pushed into the soil where and
when required. For deeper soil levels, holes can be augered
down which the probes can be used as at ground level.
However, for both types of sensors, good contact (lack of air
voids) between wave guides and the soil is ESSENTIAL for good
results.
They can be multiplexed (more or less easily), and some
capacitance sensors can be lowered down access tube much like
neutron probes.
There are sensors that can be lowered down an access tube -
and so are configured to operate when not in contact with the
soil.
---------------------------
Principle of Operation
---------------------------
TDR
A stepped pulse of high frequency electromagnetic energy is
propagated down the wave guide, and the time taken for the
signal to travel forth and back along the rods gives a measure
of the soil water content (SWC).
FDR
The soil-water-air matrix around the sensor forms the
dielectric of a capacitor which then completes an oscillating
circuit. Changes in soil water content cause a shift in
frequency.
At higher frequencies, energy is stored in the "soil
capacitor" as polar water molecules line up in the imposed
electric field, and thus the soil water content can be
detected. At lower frequencies, because of the time required
for a dipole to equilibrate with the applied electric field,
energy is lost that is proportional to the ionic conductivity
of the medium, and so the conductance of the medium can be
measured.
(Wasn't that the briefest introduction to TDR and FDR you have
ever seen?)
------------------------
PROs AND CONs.
------------------------
TDR
the probes must be rods, and are better when longer.
an indication of conductance can be obtained from the
attenuation of the reflected signal (but is not always
accurate).
stratification of SWC can be determined.
can integrates SWC over greater distances.
wave guide separation is not critical.
BUT:
graphical interpretation of signal trace is not always
unambiguous when done by algorithm.
high salinity renders readings inaccurate (greater than 20
dSm-1 renders one type of probe unstable).
accuracy and resolution increase with increasing SWC.
power consumption is high compared to FDR.
cost of equipment traditionally high.
FDR
the electrodes do not have to be rods, and can be shorter than
15cm. (Some can be lowered down access tubes). Most, however,
must have good soil sensor contact to eliminate air gaps.
both SWC and conductivity can be measured.
interpretation of data is more direct than TDR.
sphere of influence (measurement) is much smaller than TDR.
BUT:
wave guide separation is critical.
accuracy and resolution decrease with increasing SWC.
susceptible to noise (main problem: phase errors in
electronics of wiring).
high price, failure rate relatively high.
=====================================================
FDR
=====================================================
From the monthly digest of TRICKLE-L for SEPTEMBER 1995, I
found a lot of very useful discussion and information
regarding capacitance sensors, in particular, on a system that
uses them, the EnviroSCAN system.
I have taken out most of the irrelevant detail (as well as the
contributors) to make reading easier, but for the full
transcript, go to:
http://sunsite.unc.edu/pub/academic/agriculture/agronomy/agron
omy-topics/trickle-l/log9509.trickle-l.html
The next log (log9510) at this archive has more on the system.
THE DETAILS ARE ALL AS TAKEN FROM THE ARCHIVE.
APOLOGIES FOR ANY OUT OF DATE INFO.
THE CONTACT ADDRESSES ARE SUPPLIED ONLY TO REDUCE THE AMOUNT
OF REQUESTS FOR THIS INFORMATION, AND AS SUCH SHOULD NOT BE
SEEN AS ADVERTISING!
(To get your products listed as extensively - join the
discussion by sending me info on the sensors and experience
you have).
*********************
Example of discussion on one type of CAPACITANCE SENSOR - the
SENTEK EnviroSCAN soil water measuring system
This is an entire measurement system rather than a sensor
itself.
It uses capacitance type sensors.
According to CSA (Campbell Scientific Australia):
The Enviroscan system has been around for quite a while now
and is
marketed within AUS by Sentek. They have a well structured
distribution system handled mainly by ag consultants. Although
they are relatively expensive, there are quite a few of these
systems is use. From what we can determine (and after having
spoken to a number of researchers who have evaluated them),
they
appear to be quite a good tool for qualitative analysis but
have
limitations when used for quantitative analysis as one would
expect
with capacitance sensors.
Peter Moller of Agrilink (one of the above-mentioned
agricultural consultants), has advised me that the
manufacturer of the EnviroSCAN (Sentek Pty Ltd) does not have
a home-page itself, but that Agrilink (a distributor of
EnviroSCAN), has provided a description as a service.
This can be found at the URL
http://www.iinet.net.au/~agrilink/scan.html,
however, most of the info there is contained below.
Quoting directly from this homepage:
"EnviroSCAN measures the dielectric constant of the soil and
consequently, its water content. This property of soils has
been known for some time, but recent advances in electronics
have made the technique both simple and practical enough to
use as the basis of a field management technique for
scheduling irrigations."
Ron Brase of Crop Care Services, Inc. comments:
The current "high-tech" devices, TDR and FDR, with continuous
data recording offer some very in results. We have become fairly
familiar with a capacitance (FDR) instrument manufactured by EnviroSCAN in
Australia. We worked a little with and EnviroSCAN system in 1995 and
expect to work with 3 or 4 in 1996.
***************************************************
From the Trickle-l archive log9509
****************************************************
Subject: Re: Capacitance probe discussion
Due to the recent discussion about the Sentek EnviroScan*
capacitance probe system, I thought it would be appropriate to discuss the
Water Management esearch Lab's observations with Sentek* and Troxler*
capacitance probes.
Both probes use a capacitance technique known as FDR
(frequency domain reflectometry).
Let me briefly discuss how a capacitance probe works (if
you're rusty in basic electronics, get out your physics book). A capacitance
sensor consists of a pair of electrodes (circular metal rings) connected to an oscillator.
When the probe is inserted into a PVC access tube installed in
the field and activated (using radio frequencies), the soil-water-air matrix
around the PVC tube forms the dielectric of a capacitor which
then completes an oscillating circuit. Changes in soil water
content cause a shift in frequency.
Our lab first experimented with the Troxler Sentry* 200 AP
capacitance probe.
It is a single probe/sensor portable unit, which hooks up to a
multiplexer or directly to a reading unit. Using field and
boxed soil conditions we observed the following:
a) It is extremely critical to have good sensor-tube-soil
contact for reliable estimation of soil moisture. When the soil was
repacked in a boxed condition (~ 1 meter cube.....~35 cubic ft.), better
correlations were obtained.
b) A linear calibration equation comparing frequencies vs.
true volumetric soil moisture status gave better results than factory or
derived exponential calibration equations.
c) True zone of influence readings did not begin until the
probe was 30 cm (1 ft.) below the soil surface.
The Troxler Sentry 200 AP unit did not at that time (circa mid
to late 1993) have software for continuous datalogging. Hence, it was
strictly a portable unit, similar to a neutron probe whereby it is carried
from probe tube to probe tube for raw count readings.
If you want detailed information about the Troxler* probe,
contact Troxler.*
I have heard through the rumour mill that the company has
updated and improved their system.
If you want detailed information concerning our results, send
for the WMRL paper entitled "Capacitance Probe Use in Soil Moisture
Measurements" (Mead, Paltineanu, Ayars and Liu). E-mail Paula Lynch at
PLYNCH@CATI.CSUFRESNO.EDU
for paper requests.
About the time we were winding down our research with the
Troxler* probe, we obtained an Sentek EnviroScan* RT5 system. The Sentek
Enviroscan* RT5 system is not portable but is installed as an array of probes
connected to a self contained datalogger. Each probe can attach up to 8
sensors in 10 cm (4 in.) intervals. Our initial and eventual goal was to install
these probes in our monolith lysimeters which focus on shallow saline water table
contributions to the crop root system. While the lysimeters
were being constructed, we evaluated the Sentek* probes for measurement
in different soil textures, bulk densities and soil water salinity. The
Sentek* system uses a unique calibration setup system whereby raw readings
are taken in air (Fa) and in a water basket (Fw). Sentek* defines this initial setup
as 'Universal Frequency' (UF) as [Fa-F/Fa-Fw]. It is basically a ratio of the extremes
of air and water sensor readings with an actual soil
reading thrown in the numerator.
Making a long story short, we found the following:
a) There was a positive correlation between the UF and true
soil volumetric water content in three soil types and 4 bulk densities, albeit significant
differences occurred between the soil types and densities.
b) There was some curvature in the calibration equations for
clay and sandy loam soils, but only when quadratic regressions were derived.
c) As soil salinity increased, sensor moisture values were positively skewed
(looked wetter). After salinity levels of 15.4 ECe, the skewed values
levelled off. We found that using saline water in the Fw (water basket) setup
procedure seemed to lower the skewed values.
Our studies imply that there is good volumetric soil moisture
measurement using this system, yet site specific calibrations will have to
be performed for the most precise measurements. "Wetter" than normal
readings might occur from the probes when installed in saline environments.
There is concern about shrink/swell soils (2:1 expanding
clays) creating air gaps near or cracks adjacent to the access tube. According to Sentek*,
there is a slurry recipe that involves a mixture of cement, kaolinite and water in certain
ratios that is poured into the augured hole prior to access just tube installation.
We have yet to try this technique.
If you want more detailed information about the EnviroScan*
RT5 system, contact Sentek.* If you want detailed information concerning
our results, send for the WMRL paper entitled "Evaluating the Influence of
Soil Texture, Bulk Density and Soil water salinity on a Capacitance Probe
Calibration" (Mead, Ayars and Liu). Again, contact Paula at
PLYNCH@CATI.CSUFRESNO.EDU .
The obvious advantages of 'both' FDR capacitance probes are:
1) Relinquished radioactive hassles (both regulation and
exposure).
2) Lack of random counting error and quick response in
readings.
3) Relatively light weight.
4) Good accuracy as long as there is good soil-tube contact
and relatively low saline conditions.
5) The potential to continuously monitor a soil profile
(permanent installation).
6) Access tube installation time is equivalent to neutron
probe tube installation.
Richard Mead
The Environscan is a soil moisture measuring instrument
developed in Australia.
It works on the capacitance principle ie it measures the
change in capacitance of the soil depending on the moisture
level, as there is a large difference in the dielectric
constant of soil, air and water. The Environscan measures the
change in frequency response of the soil's capacitance due to
its soil moisture status. The calibration curve is supposed to
be universal, but there is some evidence that this is not so.
Care has to be taken in installation as air gaps can
dramatically alter the response.
Subject: EnviroSCAN - Calibration
The calibration equation used in the software of the
EnviroSCAN soil moisture monitoring system is deemed
"universal" as it has been generated using calibration data
obtained from varying soil types. This data was generated by
the CSIRO and Department of Agriculture in Australia and the
United States Department of Agriculture in California. This
equation is supplied as the default equation in the software
and is more than adequate for the majority of users that are
using the data for irrigations scheduling purposes. There
never will be a soil moisture sensing system that would not
require a calibration process (destructive sampling) to be
performed if the user requires accurate absolute values on
their particular soil type. The software provided with the
EnviroSCAN has the ability for the user to enter their own
calibration constants/equations.
The main drawback is a: maximum 8 sites per logger, finding
representative sites in vineyard. however the cost of the access tube is
minimal thus many can be installed and the probes moved around. The problem
of tube installation appears to be largely solved. Soils that display
a large degree of swell/shrink could be a problem if allowed to dry down to
shrinkage limits. We have such soils but high watertables rarely allow
excessive cracking.
I phased out the neutron probe 4 years ago and now utilise the
Enviroscan Soil Moisture Monitoring System. It provides a
richer picture of what is happening in the soil profile both in the root zone
& below due to its ability to continuously log soil moisture. Irrigation
agronomists Australia wide have installed over 300 systems of the latest
version (RT5) over the last 18 months and have plenty of commercial
experience demonstrating that the economic benefits far out way the
capital cost. We have done cost analysis to compare the three year cost of a
range of soil measuring devices & find that the Enviroscan is in the same
cost bracket.
My business has changed since providing a neutron probe
service in 1990 with a small group of customers limited to one region, to a
business that utilises the Enviroscan with a broad range of customers spread
1000km north of Perth to 500km south of Perth. I utilise remote downloading
& modem for data transfer & support whilst the soil moisture content is
collected every day (60 min intervals) on site.
How does EnviroSCAN work?
-------------------------
EnviroSCAN combines state of the art electronics and software
in a system that is simple to learn and practical to use.
A solar powered, central logging facility is connected by
cable to probes at each monitoring site.
Probes can be located by cable runs up to 500 metres from the
logger.
Standard length probes of 0.5 metres are used for shallow
rooted crops or turf while 1, 1.5 or 2 metre probes are used
for tree crops, vines and broadacre crops such as cotton.
In research, waste water management, environmental and mining
applications probes of longer lengths such as 5, 10 or 15 metres are used.
By contrast, in hydroponic and nursery applications, probes as
short as 10 or 20cm are used.
Each probe can be fitted with multiple sensors. The sensors
snap on to the probe with slots provided every 10 centimetres allowing the
user to specify the sensor depths.
Up to 32 sensors are supported by 1 logger on 2 cable runs of
up to 16 sensors with up to 4 probes on each cable run in any
configuration.
The sensors utilise electrical capacitance to measure soil
moisture. The capacitance field generated between the two metal plates of
the sensor extends beyond the PVC access tube into the
surrounding soil. The capacitance field is affected by the
dielectric constant of the material the field passes through.
The dielectric of soil, regardless of whether it is sand, loam
or clay, is relatively constant ranging from 3 to 6 while air is 1 and
water is 80.
Soil moisture is determined by the change in the amount of air
and water in the soil structure. This changes with rainfall,
irrigation, drainage, evaporation and crop water use.
Using capacitance the changing ratio of air and water at each
soil depth can be measured very quickly and accurately.
The sensor readings are converted to volumetric soil water
content using a default calibration equation in the software that takes into
account the varying soil types. This calibration was
established using gravimetric sampling which is the
international standard used to calibrate methods such as the
Neutron Probe.
Data can be logged at intervals as short as every one minute
and is downloaded to a computer for display. In sandy soils
and vegetable crops it is usual to log at 10 minute intervals
while in heavy soils and tree crops and vines it is usual to
log at 15 to 30 minute intervals. Data is displayed in easy to
read graphs allowing growers to make quick decisions about
when to irrigate and how long to irrigate.
EnviroSCAN has been extensively field tested with over 500
systems installed Australia wide.
Independent testing by CSIRO, Department of Agriculture and
Research groups have confirmed the accuracy and reliability of
the information provided by EnviroSCAN.
MARKET INFORMATION
******************
EnviroSCAN is suitable for use in all common soil types from
course sand to cracking clay and will work in all soil moisture ranges from
saturated to powder dry.
DISTRIBUTION
************
The following are distributors of the EnviroSCAN and their
contact details.
Ed's note:
(FOR UP TO DATE INFO ACCESS:
http://www.iinet.net.au/~agrilink/contacts.html
)
AUSTRALIAN DISTRIBUTORS
-----------------------
Northern Queensland
Piccone Horticultural Ask for Marie Piccone
Phone: 071 254 955 Fax: 071 254 961
Southern Queensland
Crop Tech Research Ask for John Hall
Phone: 071 597 433 Fax: 071 597 470
Northern Rivers
Hortech Services Ask for Peter Broomhall
Phone: 07 204 4521 Fax: 07 886 0389
Central Northern NSW
McGregor Gourlay Ask for Paul Castor
Phone: 067 524 122 Fax: 067 525 025
Central NSW/Hunter Valley
Hacom Pty Ltd Ask for Warwick Harrison
Phone: 063 640 257 Fax: 063 640 268
MIA / Griffith
Quiprite Pty Ltd Ask for Andrew McLennan
Phone: 069 623 766 Fax: 069 624 458
Northern Victoria
Soil Moisture Monitoring Svcs Ask for Adrian Orloff
Phone: 058 219 069 Fax: 058 219 079
Southern Victoria
Irritech Management Svcs Ask for Graham Palmer
Phone: 059 896 216 Fax: 059 896 086
Tasmania
Serve-Ag Pty Ltd Ask for Peter Rand
Phone: 004 270 800 Fax: 004 270 801
South Australia / Sunraysia
Yandilla Park Services Ask for Trevor Sluggett
Phone: 085 861 200 Fax: 085 851 394
Western Australia
Agrilink Technology Ask for Peter Moller
Phone: 09 275 9990 Fax: 09 275 9991
Email: agrilink@iinet.net.au
http://www.iinet.net.au/~agrilink
Northern Territory
Darwin Irrigation Supplies Ask for Brian Hood
Phone: 089 411 955 Fax: 089 811 161
CALIFORNIA - USA
----------------
Handley Irrigation Ask for Dale Handley
Phone: 209 798 2184 Fax: 209 798 2184
E-Mail: 71242.2636@compuserve.com
Sentek is seeking distributors in other countries including
Israel, South Africa, South America, New Zealand and Europe. Distributors
are typically agronomic consultancy companies that provide an irrigation
scheduling service (usually utilising the Neutron Probe) and
maintain high credibility in their geographic area. The
infrequency of data combined with the high cost of operating
the Neutron Probe are the main reasons these companies are
seeking alternate technologies to enable them to provide a
service which is more economically viable at the same time
providing real time agronomically credible data.
PRICING
*******
Pricing on the EnviroSCAN systems varies greatly dependent
upon property size, crop type, soil type, etc. For general
information purposes a basic system comprising 2 x 0.5 metre
probes and 8 x sensors would cost approximately US$6,000
whereas as an 8 x 1.5 metre probe, 32 sensor system would cost
approximately US$15,000. For a detailed quotation for your
property/application please contact your nearest distributor
or Sentek.
GENERAL
*******
For further information please contact:
Manager/Product Development - Rick Gatto
Marketing/Distribution/Pricing - Nigel Robinson
Software - Gabriel Levy
Agronomy - Peter Buss
Sentek Pty Ltd, 69 King William Street, Kent Town SA 5067,
Australia
Phone +61 8 363 0839
Fax +61 8 362 8400
E-Mail 100237.1224@compuserve.com.
==========================
A lot of discussion last month concerned the Sentek Enviroscan
capacitance probes and capacitance probes in general. Two pictures are now
available in the Web page photo listing which display the Troxler 200 AP
probe and the Sentek system.
All photos can be viewed at the URL address:
http://asset.arsusda.gov/wmrl/photo.html
END
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
Bruce Metelerkamp SOIL WATER RESEARCH OFFICER
Institute for Commercial Forestry Research,
University of Natal, PO Box 100281
Scottsville, ZA3209
Rep. of South Africa Voice:27 331 62314
E-mail: bruce@icfr.unp.ac.za FAX:27 331 68905
URL http://www.icfrnet.unp.ac.za/
/index.html
/RR.html