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CWS655E 868 MHz Wireless Soil-Water Probe
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Überblick

Der CWS655E ist eine kabellose Variante unseres CS655 Bodenfeuchtesensors. Er bestimmt volumetrischen Wassergehalt, elektrische Leitfähigkeit des Bodens und Bodentemperatur. der Sensor hat ein internes 868 MHz Funkmodem, dass die Daten an eine CWB100-Basis oder einen anderen kabellosen Sensor überträgt. Die 868 MHz-Frequenz wird in Europa vielfach verwendet.

The CWS655 is an soil-water-content reflectometer with a 900-MHz radio built in. The sensor has probes that are inserted into the soil and the sensor derives the water content of the soil. It is battery-powered and contains a spread-spectrum radio transceiver, so it has no cables to limit its placement. As part of a Campbell Scientific wireless network, it sends data to a CWB100 Wireless Base Station, which then can forward the data to a computer. The 900-MHz frequency is used in the USA and Canada.
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Funktionen und Vorteile

  • Vielseitiger Sensor, misst elektrische Leitfähigkeit, volumetrischen Wassergehalt, Dielektrizitätskonstante und Temperatur
  • Messwerte werden korrigiert zur Vermeidung von Effekten durch verschiedene Bodenarten und elektrische Leitfähigkeiten
  • Breitbandfunkmodem mit internen Frequenzsprüngen ermöglicht Übertragung über größere Strecken und mit weniger Inteferenzen
  • Eine zuverlässige, wartungsarme und stromsparende Methode zum Messen wo Sensorkabel unpraktisch sind oder aus anderem Grund unerwünscht
  • Übertagungen können von bis zu 3 anderen kabellosen Sensoren weitergegeben werden
  • Battery powered
  • Kompatibel mit CR300, CR800, CR850, CR1000, CR6 und CR3000 Datenlogger

Bilder

Technische Beschreibung

The CWS655E has 12-cm rods that insert into the soil. It measures propagation time, signal attenuation, and temperature. Dielectric permittivity, volumetric water content, and bulk electrical conductivity are then derived from these raw values.

Measured signal attenuation is used to correct for the loss effect on reflection detection and thus propagation time measurement. This allows accurate water content measurements in soils with bulk ≤3.7 dS m-1 without performing a soil-specific calibration.

Soil bulk electrical conductivity is also derived from the attenuation measurement. A thermistor in thermal contact with a probe rod near the epoxy surface measures temperature. Horizontal installation of the sensor provides accurate soil temperature measurement at the same depth as the water content measurement. For other orientations, the temperature measurement will be that of the region near the rod entrance into the epoxy body.

Why Wireless?

There are situations when it is desirable to make measurements in locations where the use of cabled sensors is problematic. Protecting cables by running them through conduit or burying them in trenches is time consuming, labor intensive, and sometimes not possible. Local fire codes may preclude the use of certain types of sensor cabling inside of buildings. In some applications measurements need to be made at distances where long cables decrease the quality of the measurement or are too expensive. There are also times when it is important to increase the number of measurements being made but the datalogger does not have enough available channels left for attaching additional sensor cables.

Spezifikationen

Weather Resistance IP67 rating for sensor and battery pack (Battery pack must be properly installed. Each sensor is leak tested.)
Operating Temperature Range -25° to +50°C
Operating Relative Humidity 0 to 100%
Power Source 2 AA batteries with a battery life of 1 year assuming sensor samples taken every 10 minutes. (Optional solar charging available.)
Average Current Drain 300 μA (with 15-minute polling)
Rod Length 12 cm (4.7 in.)
Body Dimensions 14.5 x 6 x 4.5 cm (5.7 x 2.4 x 1.77 in.)
Weight 216 g (7.6 oz)

Measurement Accuracies
Volumetric Water Content ±3% VWC typical in mineral soils that have solution electrical conductivity ≤10 dS/m. Uses Topps Equation (m3/m3).
Relative Dielectric Permittivity
  • ±(3% of reading + 0.8) for solution EC ≤ 8 dS/m (1 to 40 dielectric permittivity range)
  • ±2 for solution EC ≤ 2.8 dS/m (40 to 81 dielectric permittivity range)
Bulk Electrical Conductivity ±(5% of reading + 0.05 dS/m)
Soil Temperature ±0.5°C (for probe body buried in soil)

Internal 25 mW FHSS Radio
Frequency 868 MHz
Where Used Europe
FHSS Channel 16
Transmitter Power Output 25 mW (+14 dBm)
Receiver Sensitivity -110 dBm (0.1% frame error rate)
Standby Typical Current Drain 3 μA
Receive Typical Current Drain 18 mA (full run)
Transmit Typical Current Drain 45 mA
Average Operating Current 15 μA (with 1-second access time)
Quality of Service Management RSSI
Additional Features GFSK modulation, data interleaving, forward error correction, BCH (31,21), data scrambling

Kompatibel mit

Please note: The following shows notable compatibility information. It is not a comprehensive list of all compatible products.

Datenlogger

Product Compatible Note
CR1000 (retired)
CR200X (retired)
CR216X (retired)
CR3000
CR5000 (retired)
CR6 The CR6 datalogger must have data logger OS version 4.0 or higher.
CR800 (retired)
CR850 (retired)
CR9000X (retired)

Downloads

Wireless Sensor Planner v.1.7 (30.5 MB) 08-08-2013

The Wireless Sensor Planner is a tool for use with Campbell Scientific wireless sensors.  It assists in designing and configuring wireless sensor networks.

FAQs für

Number of FAQs related to CWS655E: 33

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  1. With regard to the CWS655, is there a generic calibration equation for artificial soil?

    The equation used to determine volumetric water content in the firmware for the CWS655 is the Topp et al. (1980) equation, which works for a wide range of mineral soils but not necessarily for artificial soils that typically have high organic matter content and high clay content. In this type of soil, the standard equations in the firmware will overestimate water content. 

    When using a CWS655 in artificial soil, it is best to perform a soil-specific calibration. For details on performing a soil-specific calibration, refer to “The Water Content Reflectometer Method for Measuring Volumetric Water Content” section in the CS650/CS655 manual. A linear or quadratic equation that relates period average to volumetric water content will work well.

  2. Can the CWS655 be repaired?

    Damage to the CWS655 electronics or rods cannot be repaired because these components are potted in epoxy. A faulty or damaged sensor needs to be replaced. For more information, refer to the Repair and Calibration page.

  3. Does the CWS655 correct readings for temperature changes?

    No. The temperature sensor is located inside the sensor’s epoxy head next to one of the sensor rods. The stainless-steel rods are not thermally conductive, so the reported soil temperature reading is actually the temperature of the sensor head near the soil surface.

    Because the sensor is installed vertically with the sensor head above ground, the soil temperature reading is not representative of the temperature over the length of the 12 cm rods, but the reading is closer to the temperature of the soil surface. Because the temperature reading is not representative of the entire thickness of soil measured for water content, no attempt was made to correct the water content readings for temperature changes.

  4. How important is it to install the CWS655 with the rods exactly parallel?

    The CWS655 works best when the rods are inserted into the soil as parallel to each other as possible. To make parallel pilot holes before installation, use the CS650G Rod Insertion Guide Tool. Minor deflection of a rod during insertion, such as when it contacts a small stone or root, may not affect the readings significantly. Major deflections, however, may cause the CWS655 to operate outside of published accuracy specifications, as well as to damage the sensor housing.

  5. Can the CWS655 measure water content in frozen soil?

    No. The principle that makes the CWS655 work is that liquid water has a dielectric permittivity of close to 80, while soil solid particles have a dielectric permittivity of approximately 3 to 6. When liquid water freezes, its dielectric permittivity drops to 3.8, essentially making it look like soil particles to the CWS655. A CWS655 installed in soil that freezes would show a rapid decline in its volumetric water content reading with corresponding temperature readings that are below 0°C.  As the soil freezes down below the measurement range of the sensor, the water content values would stop changing and remain steady for as long as the soil remains frozen. 

  6. How was the general calibration performed for the CWS655?

    Period average and electrical conductivity readings were taken with several CWS655 probes in solutions of varying permittivity and varying electrical conductivity at constant temperature. Coefficients were determined for a best fit of the data.  The equation is of the form

    Ka(σ,τ) = C0*σ3*τ2 + C1*σ2*τ2 + C2*σ*τ2 + C3*τ2 + C4*σ3*τ + C5*σ2*τ + C6*σ*τ + C7*τ + C8*σ3 + C9*σ2 + C10*σ + C11

    where Ka is apparent dielectric permittivity, σ is bulk electrical conductivity (dS/m), τ is period average (μS), and C1 to C11 are constants.

  7. Can the CWS655 be calibrated by incremental insertion into saturated soil?

    No. The abrupt permittivity change at the interface of air and saturated soil causes a different period average response than would occur with the more gradual permittivity change found when the sensor rods are completely inserted in the soil. 

    For example, if a CWS655 was inserted halfway into a saturated soil with a volumetric water content of 0.4, the probe would provide a different period average and permittivity reading than if the probe was fully inserted into the same soil when it had a volumetric water content of 0.2.

  8. Using a CWS655, when should a soil-specific calibration be performed?

    To get accurate water content readings, a soil-specific calibration is probably required if any of the following are true:

    • The soil has more than 5% organic matter content.
    • The soil has more than 20% clay content.
    • The soil is derived from volcanic parent material.
    • The soil has porosity greater than 0.5.

    For details on performing a soil-specific calibration, refer to “The Water Content Reflectometer Method for Measuring Volumetric Water Content” section in the CS650/CS655 manual.

    Some users have obtained good results by applying a linear correction to the square root of reported permittivity before applying the Topp et al. (1980) equation. The linear correction is obtained by taking readings in saturated and dry soil and using volumetric water content measurements obtained from oven-dried soil samples to estimate actual permittivity. 

  9. On the CWS655, is it possible to disable the logic that causes NAN values?

    No. It is not possible to disable the logical tests in the firmware. If soil conditions cause frequent NAN values, it may be possible to perform a soil-specific calibration that will provide good results. 

    If permittivity is reported but the volumetric water content value is NAN, Campbell Scientific recommends a soil-specific calibration that converts permittivity to water content. This will take advantage of the bulk electrical conductivity correction that occurs in the firmware. 

    If both permittivity and volumetric water content have NAN values, it may be possible to perform a calibration that converts period average directly to volumetric water content. 

    For details on performing a soil-specific calibration, refer to “The Water Content Reflectometer Method for Measuring Volumetric Water Content” section in the CS650/CS655 manual. After a soil-specific equation is determined, it may be programmed into the data logger program or used in a spreadsheet to calculate the soil water content.

  10. With the CWS655, where is the electrical conductivity measurement made?

    The bulk electrical conductivity (EC) measurement is made along the sensor rods, and it is an average reading of EC over the top 12 cm of soil.

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