CS655 Soil Water Content Reflectometer 12 cm

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1. What is the warranty for the CS650 and the CS655?

   The CS650 and CS655 are 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 CS650/CS655 manual.

2. 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.

3. How close to the soil surface can a CS650 or a CS655 be installed?

   Both the CS650 and the CS655 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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. 

9. How close to each other can multiple CS650 or CS655 sensors be installed?

   CS650 and CS655 sensors are read one at a time using SDI-12 commands. Consequently, they are never active at the same time and do not interfere with each other electrically. When installing the sensors close together, a general guideline is to keep them at least 10 cm apart.

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

    Campbell Scientific does not recommend using the CS650 or the CS655 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 or CS655 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.