Capacitance Probes

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.

There is a similar instrument manufactured in the States. Hope this is a simple enough explanation.

by Tony M Meissner

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.

by Rick Gatto

Regarding commercial application of the enviroscan: We have used it in vineyards with trickle and flood irrigation. Can set refill points by assessing the point of inflection on the soil moisture depletion curve. Can assess depth of water extraction = root zone depth. Can assess water use as vines come out of dormancy, extraction patterns and root development.

Most importantly it gives a real time assessment of the irrigation practice, depth of wetting, water past the root zone and refill. It also helps to deal with summer rainfall, was the rain effective?, to what depth.

the cost of 32 probes + logger + software is minimal compared to capital investment in a vineyard With current prices the cost would be returned with a 2 - 6 % increase in yield in a 40 ha vineyard.

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.

by Evan Christen

I provide a water management service to irrigation managers in Western Australia to a range of horticulture crops, turf and effluent disposal systems. 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.

by Peter Moller

Due to the recent discussion about the Sentek EnviroScan* capacitance probe system, I thought it would be appropriate to discuss the Water Management Research 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 rumor 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 leveled 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.

by Richard Mead

*The USDA-ARS does not endorse the product name, but only uses it for descriptive terminology for the benefit of the reader.

PRODUCT DESCRIPTION
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How does EnviroSCAN work?
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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
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Who Uses EnviroSCAN?
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EnviroSCAN is currently in use in the following crop types:
Almonds
Apples
Apricots
Artichokes
Asparagus
Avocados
Bananas
Berries
Broccoli
Brussell Sprouts
Capsicums
Carrots
Cashews
Celery
Cherries
Citrus - Orange, Lemon, Lime, Mandarin
Cotton
Cucumber
Eggplant
Flowers
Grapes - Nursery, Table, Wine
Hops
Lettuce
Lychees
Macadamias
Mangos
Onions
Pasture
Peaches
Pears
Peas
Potatoes
Pistachio Nuts
Plums
Rockmelons
Snow Peas
Strawberries
Sugarcane
Sweetcorn
Tomatoes - Field, Hydroponic
Turf
Watermelon
Woodlots

Waste Water Applications
Mining Applications
Hydroponics and Nurseries

Growers, Grower Groups, Corporate Growers
Government Departments (CSIRO, DPI, etc)
Researchers, Consultants

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
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The following are distributors of the EnviroSCAN and their contact details.

AUSTRALIAN DISTRIBUTORS
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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

Northern Territory
Darwin Irrigation Supplies
Ask for Brian Hood
Phone: 089 411 955
Fax: 089 811 161

CALIFORNIA - USA
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Handley Irrigation
Ask for Dale Handley
Phone: 209 798 2184
Fax: 209 798 2184
E-Mail: 71242.2636@compuserve.com

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
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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
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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

by Rick Gatto