CWS655A 922 MHz Wireless Soil-Water Probe for Australia

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

   No. 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 for organic soils. In organic soils, the standard equations in the firmware will overestimate water content. 

   When using a CWS655 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.

2. 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.

3. 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.

4. 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. 

5. On the CWS655, 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.

6. How can it be determined if the NAN values are caused by soil conditions or a faulty CWS655?

   The best way to test whether the sensor is faulty is to connect an A205 interface to the 4-pin connector of the sensor and use DevConfig software to communicate directly with the sensor through a computer. This is described in the Wireless Sensor Network Instruction Manual. Select Connect in DevConfig, and press the Setup button on the back of the sensor.

   Select the Settings Editor tab and scroll down to Measurements. This will query the sensor every 5 seconds and display readings for volumetric water content (VWC), bulk electrical conductivity (EC), temperature (Ts), apparent bulk permittivity (Pe), voltage ratio (AR), period, internal sensor temperature (Ti), and battery voltage (BV).  Certain soil conditions may cause NAN values in the VWC, EC, and/or Pe fields, but a working sensor always reports Ts, AR, period, Ti, and BV.  If any of those fields report NAN, the sensor is damaged.  If only one, two, or three of the values are NAN, the NAN values are caused by soil conditions.

7. 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.

8. Can the body of the CWS655 be buried?

   Only the rods of the CWS655A should be buried. The body of the CWS655A was not designed for burial, and Campbell Scientific does not recommend burying it for the following reasons:

   • While the body of the CWS655A is underground, the radio signal is diminished, and the soil must be disturbed to replace the batteries. 
   • Eventually, corrosion of the pins occurs where the battery pack connects. Sometimes moisture gets into the sensor electronics and causes irreparable damage.

   If a wireless option is desired for fully buried water content sensors, consider using a CR200X-series datalogger with CS650-L or CS655-L cabled sensors.

9. Can the CWS655 measure water content in compost?

   Campbell Scientific does not recommend using the CWS655 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 electrical conductivity values reported by the CWS655 may give some useful information about processes occurring in the compost pile, but it will not be able to give useful readings for water content. In addition, the plastic housing of the CWS655 may likely be damaged by the high temperatures and acids formed during the composting process.

10. 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.