The CWS655 is a wireless version of our CS655 soil water reflectometer. It has 12 cm rods and monitors soil volumetric water content, bulk electrical conductivity, and temperature. This reflectometer has an internal 900 MHz spread-spectrum radio that transmits data to a CWB100 Wireless Base Station or to another wireless sensor. The internal radio's frequency is commonly used in the US and Canada.Read More
The CWS655 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.
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 data logger does not have enough available channels left for attaching additional sensor cables.
|Measurements Made||Soil electrical conductivity (EC), relative dielectric permittivity, volumetric water content, soil temperature|
|Water Content Accuracy||±3% VWC typical in mineral soils, where solution EC ±10 dS/m|
|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 Range||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 Diameter||3.2 mm (0.13 in.)|
|Rod Length||12 cm (4.7 in.)|
|Dimensions||14.5 x 6 x 4.5 cm (5.7 x 2.4 x 1.77 in.)|
|Weight||216 g (7.6 oz)|
|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||
|Bulk Electrical Conductivity||±(5% of reading + 0.05 dS/m)|
Internal 25 mW FHSS Radio
|Frequency||902 to 918 MHz|
|Where Used||US and Canada|
|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, data scrambling, RSSI reporting|
Note: The following shows notable compatibility information. It is not a comprehensive list of all compatible or incompatible products.
Latest firmware for the CWS655.
The Wireless Sensor Planner is a tool for use with Campbell Scientific wireless sensors. It assists in designing and configuring wireless sensor networks.
Number of FAQs related to CWS655: 35
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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.
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.
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.
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.
The volumetric water content reading is the average water content over the length of the sensor’s rods.
The maximum recommended number in a Campbell Scientific wireless sensor network is 50.
The CWS655 can detect water as far away as 10 cm in saturated sand. As the soil dries down, that distance decreases to approximately 4 cm in dry sand.
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.
To get accurate water content readings, a soil-specific calibration is probably required if any of the following are true:
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.