CS655 12 cm Soil Water Content Reflectometer

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1. Can the CS650 and the CS655 be repaired?

   Damage to the CS650 or the CS655 electronics or rods cannot be repaired because these components are potted in epoxy. Cable damage, on the other hand, may possibly be repaired. For more information, refer to the Repair and Calibration page.

2. Can the cable for the CS650 or CS655 be shortened?

   Modifications to the CS650 or CS655, including shortening the cable, will void the warranty. However, shortening the cable will not affect the sensor’s performance. If a decision is made to shorten the cable, care should be taken to avoid damaging the cable jacket and exposing bare wire except at the ends that connect to the data logger or multiplexer terminals.

3. Can the CS650 and the CS655 measure water level?

   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.  

4. Can the CS650 and the CS655 measure water content in greenhouse pots?

   Yes, but the pots would have to be large. The CS650 and CS655 can detect water as far away as 10 cm (4 in.) from the rods. If the pot has a diameter smaller than 20 cm (8 in.), the sensor could potentially detect the air around the pot, which would underestimate the water content. In addition, potting soil is typically high in organic matter and clay, causing the probable need for a soil-specific calibration. 

5. Can the CS650 and the CS655 measure water content in gravel?

   Yes. Keeping the sensor rods parallel during installation is especially difficult in gravel, but it can be done. Gravel has large pore spaces that drain quickly, so the water content readings will likely show rapid changes between saturation and very dry. If small changes of water content at the dry end are of interest, a soil-specific calibration may need to be performed to convert period average directly to volumetric water content.

6. Can the CS650 and the CS655 measure gasoline or other hydrocarbons in soil?

   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. 

7. Can the CS650 and the CS655 detect stream bed erosion?

   The permittivity of saturated sediments in a stream bed is expected to read somewhere between 25 and 42, while the permittivity of water is close to 80. A CS650 or CS655 installed in saturated sediments could be used to monitor sediment erosion. If the permittivity continuously increases beyond the initial saturated reading, this is an indication that sediment around the sensor rods has eroded and been replaced with water. A calibration could be performed that relates permittivity to the depth of the rods still in the sediment. 

8. Using a CS650 or a CS655, 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.

9. How can it be determined if soil is out-of-bounds for a CS650 or a CS655?

   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:

   • Soil that is coarse textured (such as sand, loamy sand, or sandy loam) works well with a CS650 if the EC is low.
   • If the soil is located in an arid or semiarid region, it may have high EC.
   • If the soil is frequently fertilized or irrigated with water that has higher EC, it may have high EC.
   • If the climate provides enough rain to flush accumulated salts below the root zone, the EC is expected to be low and suitable for a CS650.

   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.

10. With regard to the CS650 and the CS655, is there a generic calibration equation for artificial soil?

    No. The equation used to determine volumetric water content in the firmware for the CS650 and the CS655 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 CS650 or a CS655 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.