The CS655 is a multiparameter smart sensor that uses innovative techniques to monitor soil volumetric-water content, bulk electrical conductivity, and temperature. It outputs an SDI-12 signal that many of our dataloggers can measure. It has shorter rods than the CS650, for use in problem soils.
The CS655 consists of two 12-cm-long stainless steel rods connected to a printed circuit board. The circuit board is encapsulated in epoxy and a shielded cable is attached to the circuit board for datalogger connection.
The CS655 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 loss-effect correction allows accurate water content measurements in soils with bulk EC ≤8 dS m-1 without performing a soil-specific calibration.
Soil bulk electrical conductivity is also calculated 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. Temperature measurement in other orientations will be that of the region near the rod entrance into the epoxy body.
External RF sources can affect the probe’s operation. Therefore, the probe should be located away from significant sources of RF such as ac power lines and motors.
Multiple CS655 probes can be installed within 4 inches of each other when using the standard datalogger SDI-12 “M” command. The SDI-12 “M” command allows only one probe to be enabled at a time.
The 14384 Pilot Tool has rods with similar diameters and the same spacing as the probe. The tool can be driven into the soil using force levels that might damage the probe. After removing the 14384, the probe is inserted into the established holes.
The 14383 Installation Tool can be used to help maintain the proper spacing and parallel orientation of rods during insertion. Use of the 14383 may reduce measurement errors by minimizing soil disturbance.
Number of FAQs related to CS655: 55
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No. The principle that makes these sensors 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. Gasoline and other hydrocarbons have dielectric permittivities in the same range as soil particles, which essentially make them invisible to the CS650 and the CS655.
The CS650 and CS655 are warranted by Campbell Scientific to be free from defects in materials and workmanship under normal use and service for 12 months from the date of shipment. For further details, see the “Warranty” section of the CS650/CS655 manual.
The CS650 and the CS655 are not ideal sensors for measuring water level. However, these sensors do respond to the abrupt change in permittivity at the air/water interface. A calibration could be performed to relate the period average or permittivity reading to the distance along the sensor rods where the air/water interface is located. From that, the water level can be determined. The permittivity of water is temperature dependent, so a temperature correction would be needed to acquire accurate results.
Campbell Scientific strongly discourages shortening the sensor’s rods. The electronics in the sensor head have been optimized to work with the 12 cm long rods. Shortening these rods will change the period average. Consequently, the equations in the firmware will become invalid and give inaccurate readings.
If a system has multiple CS650 or CS655 sensors, it will be necessary to connect many wires to a 12 V supply and many wires to ground. The DIN-Rail Mounting Kit is useful for attaching many wires to the same source in a clean and organized way. For more details, see the 5458 DIN-Rail Terminal Kit instruction manual.
Other methods of connecting several wires together, such as terminal strips or wire nuts, would also work.
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 CS650 or a CS655 was inserted halfway into a saturated soil with a volumetric water content of 0.4, the sensor 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.
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. If the CS650 or the CS655 is installed horizontally, which is the preferred method, then the sensor head will be at the same temperature as the soil, and the soil temperature value will be accurate. However, if the sensor is installed vertically, and/or with the sensor head above ground, the soil temperature reading will be less accurate. Because the sensor orientation is not known, no temperature correction was written into the firmware.
If information is available on soil texture, organic matter content, and electrical conductivity (EC) from soil surveys or lab testing of the soil, it should be possible to tell if the soil conditions fall outside the range of operation of the sensor. Without this information, an educated guess can be made based on soil texture, climate, and management:
When in doubt about soil texture and electrical conductivity, Campbell Scientific recommends using a CS655 because of the sensor’s wider range of operation in electrically conductive soils, as compared with the CS650.
Probably not. The principle that makes these sensors 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. Because the permittivity of water is over an order of magnitude higher than that of soil solids, water content has a significant impact on the overall bulk dielectric permittivity of the soil. When the soil becomes very dry, that impact is minimized, and it becomes difficult for the sensor to detect small amounts of water. In air dry soil, there is residual water that does not respond to an electric field in the same way as it does when there is enough water to flow among soil pores. Residual water content can range from approximately 0.03 in coarse soils to approximately 0.25 in clay. In the natural environment, water contents below 0.05 indicate that the soil is as dry as it is likely to get. Very small changes in water content will likely cause a change in the sensor period average and permittivity readings, but, to interpret those changes, a very careful calibration using temperature compensation would need to be performed.
Both the CS650 and the CS655 can detect water as far away as 10 cm in wet sand. That distance decreases as the soil dries down to approximately 4 cm in dry sand. In practice, a depth of 5 cm will give a water content reading that is within the sensor accuracy specification even if a small amount of air near the soil surface is detected and averaged into the reading.
Note: Campbell Scientific does not recommend installing the sensor in a depth shallower than 5 cm.