CS655 12 cm Soil Water Content Reflectometer

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1. 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.

2. Can the CS650 and the CS655 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. 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.

4. Can the CS655 rods be shortened?

   Campbell Scientific strongly discourages shortening the sensor’s rods. The electronics in the sensor head have been optimized to work with the 12 cm long rods. Shortening these rods will change the period average. Consequently, the equations in the firmware will become invalid and give inaccurate readings.

5. Can the cable for the CS650 or CS655 be shortened?

   Modifications to the CS650 or CS655, including shortening the cable, will void the warranty. However, shortening the cable will not affect the sensor’s performance. If a decision is made to shorten the cable, care should be taken to avoid damaging the cable jacket and exposing bare wire except at the ends that connect to the data logger or multiplexer terminals.

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. When is it more suitable to use a CS655 rather than a CS650?

   In soil that has a significant fraction of fines (loam, silt loam, silty clay loam, clay loam, clay), the CS655 is a suitable option because these soils tend to be more electrically conductive, and the CS655 operates over a larger range of electrical conductivity than the CS650. In applications where a smaller measurement volume is desired, such as larger greenhouse pots, the 12 cm long rods of the CS655 are preferable to the 30 cm long rods of the CS650.

8. Is it necessary to purchase a DIN-Rail Mounting Kit with a CS650 or a CS655?

   If a system has multiple CS650 or CS655 sensors, it will be necessary to connect many wires to a 12 V supply and many wires to ground. The DIN-Rail Mounting Kit is useful for attaching many wires to the same source in a clean and organized way. For more details, see the 5458 DIN-Rail Terminal Kit instruction manual. 

   Other methods of connecting several wires together, such as terminal strips or wire nuts, would also work.

9. Can the cable length for a CS650 or a CS655 really be up to 2,000 ft?

   The cable properties and power requirements of the CS650 and the CS655 are such that communication with a data logger may work for cable lengths greater than 2,000 ft. If multiple sensors are communicating through the same universal or control terminal, the total length of all of those sensors must not exceed 2,000 ft. 

   In practice, it is less expensive to purchase a new data logger than to buy a CS650 or CS655 with 2,000 ft of cable. If the cable is run through conduit, or if a 2,000 ft long trench needs to be excavated, then the installation cost becomes more expensive than buying another data acquisition system and sensors with shorter cables.

10. Can the CS650 and the CS655 measure water content in greenhouse pots?

    Yes, but the pots would have to be large. The CS650 and CS655 can detect water as far away as 10 cm (4 in.) from the rods. If the pot has a diameter smaller than 20 cm (8 in.), the sensor could potentially detect the air around the pot, which would underestimate the water content. In addition, potting soil is typically high in organic matter and clay, causing the probable need for a soil-specific calibration.