CS650 Sonde réflectométrique de teneur en eau du sol (30 cm)

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1. Les tiges du CS650 peuvent-elles être raccourcies ?

   Campbell Scientific ne recommande pas expressement de raccourcir les tiges du capteur. L'électronique dans la tête du capteur a été optimisée pour fonctionner avec des tiges de 30 cm de long. Le raccourcissement de ces tiges changera la moyenne de la période. Par conséquent, les équations dans le firmware deviendront invalides et donneront des lectures inexactes.

2. Can the CS650 measure permittivity and electrical conductivity of sea water and sea ice?

   The electrical conductivity (EC) of sea water is approximately 48 dS/m. The CS650 can measure permittivity in water with EC between 0 and 3 dS/m. EC readings become extremely unstable at conductivities higher than 3 dS/m and are reported as NAN or 9999999. Because EC is part of the permittivity equation, an EC reading of NAN leads to a permittivity reading of NAN as well. Thus, the CS650 cannot provide good readings in sea water.

   With regard to sea ice, the electrical conductivity drops significantly when sea water freezes and the permittivity changes from approximately 88 down to approximately 4, as the water changes from a liquid to a solid state. With both EC and permittivity falling to levels that are within the CS650 measurement range, the sensor is expected to give valid readings in sea ice. The sensor is rugged and can withstand the cold temperatures. However, as the ice melts, there will be a point at which the electrical conductivity becomes too high to acquire a valid reading for either permittivity or electrical conductivity.

3. How was the general calibration performed for the CS650 and the CS655?

   Period average and electrical conductivity readings were taken with several sensors 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

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

4. 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.

5. 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.

6. 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.

7. Do the CS650 and the CS655 have surge protection?

   Yes. There is surge protection built into the sensor electronics. The sensor survives a surge of 2 kV at 42 ohm line-to-ground on digital I/O and 2 kV at 12 ohm line-to-ground on power. It also survives a surge of 2 kV at 2 ohm line-to-ground on the rods.

   If additional surge protection is required, consider using the SVP100 Surge Voltage Protector DIN Rail with Mounting Hardware. 

8. Les CS650 et CS655 peuvent-elles être réparées ?

   Les dommages sur l'électronique de la CS650 ou à l'électronique du CS655 ou aux tiges ne peuvent être réparées car ces composants sont placés dans l'époxy. Les dégâts sur les câbles, d'autre part, peuvent éventuellement être réparés. Pour plus d'informations, reportez-vous à la page Réparation et étalonnage.

9. Why is the permittivity reading NAN or 9999999 for the CS650?

   The CS650-series sensors have several logical tests built into their firmware to ensure that the sensors do not report a number that is known to be erroneous. Erroneous readings are either outside the sensor’s operational limits or outside of published accuracy specifications.

   A reported value of NAN or 9999999 does not necessarily mean that there is a problem with the sensor hardware. The conditions outlined below can lead to a value of NAN or 9999999 for permittivity and volumetric water content.

   SDI-12 communications issue

   If all of the following are true, there is likely an issue with the SDI-12 communications between the sensor and the datalogger: the sensor is being polled with an M1! SDI-12 command, the permittivity value reported is NAN, and subsequent values are all zeroes or never change. Possible causes include the following:

   • The sensor is not powered. Check to make sure that the red wire is well connected to a 12 Vdc source and that the black wire is connected to datalogger ground.
   • The sensor signal wire is not connected properly. Ensure that the green wire is well connected to the control port specified in the datalogger program.
   • The sensor has an SDI-12 address that does not match the datalogger program.
   • The sensor has cable or lightning damage.

   Calculated permittivity is less than 0 or greater than 88

   The equation used to convert period average and electrical conductivity values to permittivity is a three-dimensional surface with two independent variables and eleven coefficients, plus an offset. Some rare combinations of period and electrical conductivity result in a permittivity calculation that is less than air (1) or greater than water at 0°C (88). These rare combinations are not expected when the sensor is in soil. 

   Bulk electrical conductivity (EC) is greater than 1.14 dS/m

   When bulk electrical conductivity is greater than 1.14 dS/m, the solution EC is greater than 3 dS/m, which is the upper limit for accurate readings with the CS650. When this occurs, the soil is considered out-of-bounds and will report a value of NAN or 9999999 for both permittivity and volumetric water content.

   Calculated permittivity is less than 80% of the permittivity limit

   A permittivity limit based on the bulk electrical conductivity (EC) reading is used to determine whether the bulk EC at saturation exceeds the sensor’s operational limit. That permittivity limit is calculated and compared to the permittivity reading. If the measured permittivity is more than 20% beyond the permittivity limit, both permittivity and volumetric water content are reported as NAN or 9999999. This is the most common cause of NAN values with the CS650-series sensors, and it occurs because of the soil properties and not because of a sensor malfunction.

10. Quand est-il plus approprié d'utiliser un CS655 plutôt qu'un CS650 ?

    Dans un sol qui a une fraction importante de loam, terreau, loam limoneux, limono-argileux, loam argileux, argile, le CS655 est une option appropriée parce que ces sols ont tendance à être plus électriquement conducteur, et le CS655 fonctionne sur une plus grande plage de la conductivité électrique que le CS650. Dans les applications où un plus petit volume de mesure est souhaitée, tels que des pots de fleur de grande taille, les tiges de 12 cm de long du CS655 sont préférables aux tiges de 30 cm de long du CS650.