CS650 and CS655 Water Content Reflectometers
18
M4!
command reports the sensor calculated values even if those values are
likely to be erroneous.
The
M3!
command uses several logical tests built into the operating system to
ensure that the sensors do not report a number that is likely to be erroneous.
Erroneous readings are either outside the sensor operational limits or outside of
published accuracy specifications. When one of the following conditions
occurs, the logical test replaces the calculated value with another value such as
9999999 or NAN.
Permittivity < 0 or > 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. Combinations of period and electrical
conductivity result in a permittivity calculation that is less than zero or greater
than 88. These rare combinations are not expected when the sensor is in soil,
but if they do occur, the
M3!
command reports NAN for permittivity.
Permittivity < 1
If the CS650 calculates a permittivity value greater than zero but less than 1,
the
M3!
command reports a permittivity value of 1.
Permittivity too low for the Topp et al equation
The Topp et al (1980) equation used by the CS650 to estimate volumetric
water content works well for most mineral soils. However, at low water
contents, the Topp equation may report a negative value for volumetric water
content. When the estimated permittivity is greater than 1 but less than 1.881,
the
M3!
command reports a volumetric water content value of zero.
Permittivity more than 42
The Topp et al (1980) equation used by the CS650 to estimate volumetric
water content works well for most mineral soils up to a maximum water
content of about 0.45. If the CS650 estimates the soil permittivity to be more
than 42, which calculates to a volumetric water content of 0.52, then the
M3!
command reports 9999999 or NAN for 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
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, the
M3!
command reports NAN or 9999999 for both
permittivity and volumetric water content. This is the most common cause of
NAN values with the CS650-series sensors, and it occurs because of soil
properties and not because of a sensor malfunction.
Bulk electrical conductivity (EC) is too high
When bulk electrical conductivity is greater than 1.14 dS/m, the solution EC is
greater than 3 dS/m, which is the upper limit for accurate readings with the
