The CWS655-series sensors have several logical tests built into their firmware to ensure that the sensors do not report a number that is known to be erroneous. Erroneous readings are either outside the sensor’s operational limits or outside of published accuracy specifications.
A reported value of NAN does not necessarily mean that there is a problem with the sensor hardware. The conditions outlined below can lead to a value of NAN for permittivity and volumetric water content.
Calculated permittivity is less than 0 or greater than 88
The equation used to convert period average and electrical conductivity values to permittivity is a three-dimensional surface with two independent variables and eleven coefficients, plus an offset. Some rare combinations of period and electrical conductivity result in a permittivity calculation that is less than air (1) or greater than water at 0°C (88). These rare combinations are not expected when the sensor is in soil.
Bulk electrical conductivity (EC) is greater than 3.04 dS/m
When bulk electrical conductivity is greater than 3.04 dS/m, the solution EC is greater than 8 dS/m, which is the upper limit for accurate readings with the CWS655. When this occurs, the soil is considered out-of-bounds and will report a value of NAN for both permittivity and volumetric water content.
Calculated permittivity is less than 80% of the permittivity limit
A permittivity limit based on the bulk electrical conductivity (EC) reading is used to determine whether the bulk EC at saturation exceeds the sensor’s operational limit. That permittivity limit is calculated and compared to the permittivity reading. If the measured permittivity is more than 20% beyond the permittivity limit, both permittivity and volumetric water content are reported as NAN. This is the most common cause of NAN values with the CWS655-series sensors, and it occurs because of the soil properties and not because of a sensor malfunction.