CS655 Sonde réflectométrique de teneur en eau du sol (12 cm)

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1. Can the CS650 and the CS655 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 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.

2. What are the major differences between the CWS655 and the CS655?

   The CWS655 is a wireless sensor with measurement electronics, radio, and power supply all integrated in a single device. The CWS655, however, requires the use of a CWB100 base station radio connected to a datalogger. Only the rods of the CWS655 should be buried in the soil; burying the body of the CWS655 will prevent the sensor from communicating with the CWB100.

   The CS655 is a cabled multiparameter smart sensor that sends data by RS-232 serial or SDI-12 communication through a direct connection to a datalogger. The CS655 is suitable for burial at any depth.

3. How are the CS650 and the CS655 different?

   The CS650 has rods that are 30 cm long, and the CS655 has rods that are 12 cm long. The difference in rod length causes some changes in specifications. For example, the CS650 is slightly more accurate in its permittivity and water content readings, but the CS655 works over a larger range of electrical conductivity. In addition, the CS650 handles a larger measurement volume and provides good accuracy in low EC (electrical conductivity) sand and sandy loam. The CS655 is typically more accurate in soil, works well over a wide range of soil textures and EC, and is easier to install because of its shorter rods.

4. With the CS650 and the CS655, 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 that distance at whatever depth the rods are placed.

5. Can the CS650 and the CS655 measure water content between 0 and 0.05?

   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.  

6. Can the cable of the CS650, CS655, CS616, or CS625 be spliced?

   Campbell Scientific does not recommend splicing sensor cables. Sensors may be ordered with custom cable lengths, and Campbell Scientific recommends purchasing the correct length for the application. If the sensor cable needs to be lengthened, a junction box (if practical) is a more favorable option than a splice.

   Note: A splice will void the sensor warranty, but a junction box does not modify the sensor and therefore does not void the warranty.

7. La CS650 et la CS655 peuvent-elles mesurer la teneur en eau dans des sols gelés ?

   Non. Le principe qui permet à ces capteurs de fonctionner est que l'eau liquide a une permittivité diélectrique de près de 80, tandis que les particules solides du sol ont une permittivité diélectrique d'environ 3 à 6. Lorsque l'eau liquide gèle, sa permittivité diélectrique baisse à 3,8, comme les particules du sol. Une CS650 ou une CS655 installée dans un sol qui gèle montrera un déclin rapide de sa teneur en eau volumétrique avec des lectures de température correspondantes inférieures à 0°C. Au fur et à mesure que le sol gèle sous la plage de mesure du capteur, les valeurs de la teneur en eau cesseront de changer et resteront stables tant que le sol reste gelé.

8. 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.

9. With regard to the CS650 and the CS655, is there a generic calibration equation for organic 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 for organic soils. In organic soils, the standard equations in the firmware will overestimate water content.

   When using a CS650 or a CS655 in organic 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.

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.