CS655 Soil Water Content Reflectometer 12 cm

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1. 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. 

2. How long will a CS650 or a CS655 last?

   The CS650-series sensors have the same rugged epoxy and stainless-steel rods that have been used for water content reflectometers since the CS615-L model was introduced in 1995. There are CS615-L and CS616 sensors in many locations that have been in continuous use for more than ten years with no reported problems. If a CS650 or CS655 remains undamaged by external forces such as lightning, harsh chemicals, or animal actions, the sensor is expected to continue working for decades. 

3. With the CS650 and the CS655, where is the water content measurement made?

   The volumetric water content reading is the average water content over the length of the sensor’s rods.

4. 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.  

5. Can the CS655 measure permittivity and electrical conductivity of sea water and sea ice?

   The electrical conductivity (EC) of sea water is approximately 48 dS/m. The CS655 can measure permittivity in water with EC between 0 and 8 dS/m. EC readings become extremely unstable at conductivities higher than 8 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 CS655 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 CS655 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.

6. 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.

7. Do the CS650 and the CS655 correct readings for temperature changes?

   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.  

8. How can the CS650 or the CS655 readings be corrected for temperature?

   The CS650/CS655 manual gives a temperature correction that works in coarse sand, but it should be used cautiously with other soil types. If a temperature correction is required, it is best to determine a soil-specific temperature correction. 

   When correcting for temperature, the following effects contribute to the sensor output:

   • The effect of temperature on the measurement electronics inside the sensor head. This is a relatively small effect compared to other temperature effects.
   • The change in the dielectric permittivity of water with temperature. At 0°C, the permittivity of water is approximately 88, at 20°C it is approximately 80, and at 70°C it is approximately 64. If the sensor is in a soil at any given water content, the changing permittivity of water will cause the period average at 0°C to be higher than it is at 20°C. The same soil will have a lower period average at 70°C than at 20°C. In other words, the sensor will overestimate water content at colder temperatures and underestimate it at warmer temperatures. However, that is only true if electrical conductivity is negligible.
   • The change in water content as bound water is captured and released. In soils with high clay content, some of the water is partially or fully immobilized by electrical charges on the surface of the clay minerals. The amount of bound water is temperature dependent and may have a small effect on the sensor readings.
   • The temperature effect of bulk electrical conductivity (EC) on period average. Bulk electrical conductivity increases with temperature; as it increases, it slows down the period average.

   The interaction of these effects may be complicated. For example, with increasing temperature, two things happen at the same time:  the falling permittivity of water is decreasing the period average, and the increasing EC is increasing the period average. The net result as to whether the period average goes up or down depends on how conductive the soil is and the contributions of the other temperature effects.

9. 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.

10. On the CS650 and the CS655, is it possible to disable the logic that causes NAN values?

    No. It is not possible to disable the logical tests in the firmware. If soil conditions cause frequent NAN values, it may be possible to perform a soil-specific calibration that will provide good results.

    If permittivity is reported but the volumetric water content value is NAN, Campbell Scientific recommends a soil-specific calibration that converts permittivity to water content. This will take advantage of the bulk electrical conductivity correction that occurs in the firmware.

    If both permittivity and volumetric water content have NAN values, it may be possible to perform a calibration that converts period average directly to volumetric water content.

    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. After a soil-specific equation is determined, it may be programmed into the data logger program or used in a spreadsheet to calculate the soil water content.