CS655-L 12 cm Soil Water Content Reflectometer

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1. Can the CS650-L and the CS655-L measure water content in compost?

   Campbell Scientific does not recommend using the CS650-L or the CS655-L to measure water content in compost. A compost pile is a very hostile environment for making dielectric measurements with soil water content sensors. All of the following combine to make it very difficult to determine a calibration function: high temperature, high and varying electrical conductivity, high organic matter content, heterogeneity of the material in the pile, changing particle size, and changing bulk density. The temperature and electrical conductivity values reported by the CS650-L or CS655-L may give some useful information about processes occurring in the compost pile, but these sensors will not be able to give useful readings for water content.  

2. Can the CS650-L and the CS655-L 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.

3. Can the CS655-L 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-L 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-L 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-L 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.

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

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

5. When a CS650-L or a CS655-L is put in water, why does the volumetric water content read NAN?

   The dielectric of water at room temperature is close to 80. The firmware for both the CS650-L and the CS655-L is programmed to change volumetric water content to NAN or 9999999 when the permittivity measurements are greater than 42. When testing in water, look at the permittivity reading rather than the water content reading. If a test is being done for functionality, pull the sensor about halfway out of the water to see both permittivity and volumetric water content readings.

6. How close to the soil surface can a CS650-L or a CS655-L be installed?

   Both the CS650-L and the CS655-L 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.

7. Using a CS650-L or a CS655-L, 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.

8. How are the CS650-L and the CS655-L different?

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

9. Can the CS650-L and the CS655-L measure water content in frozen 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. When liquid water freezes, its dielectric permittivity drops to 3.8, essentially making it look like soil particles to the sensor. A CS650-L or CS655-L installed in soil that freezes would show a rapid decline in its volumetric water content reading with corresponding temperature readings that are below 0°C. As the soil freezes down below the measurement range of the sensor, the water content values would stop changing and remain steady for as long as the soil remains frozen.  

10. Can the CS650-L and the CS655-L 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-L or CS655-L 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.