CWS655A 922 MHz Wireless Soil-Water Probe

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1. What is the warranty for a CS655?

   The CS655 is warranted by Campbell Scientific to be free from defects in materials and workmanship under normal use and service for 12 months from the date of shipment. For further details, see the “Warranty” section of the Wireless Sensor Network Instruction Manual.

2. How can it be determined if soil is out-of-bounds for a CWS655?

   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 CWS655. Without this information, an educated guess can be made based on soil texture, climate, and management: 

   • Mineral soils work well with a CWS655 if the EC at saturation is below 3.04 dS/m. 
   • 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 CWS655. 
   • Organic or artificial soils typically have high surface electrical conductivity and may become out-of-bounds if they receive significant fertilizer or water with high EC. 
3. Can the CWS655 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 CWS655 was inserted halfway into a saturated soil with a volumetric water content of 0.4, the probe 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.

4. With the CWS655, where is the soil temperature measurement made?

   A thermistor is encased in the epoxy head of the sensor next to one of the stainless-steel rods. This provides a point measurement of temperature at the soil surface. The temperature measurement is not averaged over the length of the sensor rods.

5. Using a CWS655, 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.

   Some users have obtained good results by applying a linear correction to the square root of reported permittivity before applying the Topp et al. (1980) equation. The linear correction is obtained by taking readings in saturated and dry soil and using volumetric water content measurements obtained from oven-dried soil samples to estimate actual permittivity. 

6. Can the CWS655 be repaired?

   Damage to the CWS655 electronics or rods cannot be repaired because these components are potted in epoxy. A faulty or damaged sensor needs to be replaced. For more information, refer to the Repair and Calibration page.

7. Can the CWS655 measure water content between 0 and 0.05?

   Probably not. The principle that makes the CWS655 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 CWS655 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 CWS655 period average and permittivity readings, but, to interpret those changes, a very careful calibration using temperature compensation would need to be performed. 

8. With regard to the CWS655, is there a generic calibration equation for artificial soil?

   The equation used to determine volumetric water content in the firmware for the CWS655 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 CWS655 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.

9. How important is it to install the CWS655 with the rods exactly parallel?

   The CWS655 works best when the rods are inserted into the soil as parallel to each other as possible. To make parallel pilot holes before installation, use the CS650G Rod Insertion Guide Tool. Minor deflection of a rod during insertion, such as when it contacts a small stone or root, may not affect the readings significantly. Major deflections, however, may cause the CWS655 to operate outside of published accuracy specifications, as well as to damage the sensor housing.

10. Can the CWS655 be partially inserted into the soil?

    Because the reported volumetric water content reading is an average taken along the entire length of the rods, the sensor should be fully inserted into the soil. Otherwise, the reading will be the average of both the air and the soil, which will lead to an underestimation of water content. If the sensor rods are too long to go all the way into the soil, Campbell Scientific recommends inserting the rods at an angle until they are fully covered by soil.