M3! and M4! commands

Typically, the M4! command is used to report volumetric water content, electrical conductivity, temperature, permittivity, period, and voltage ratio. The 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 CS650. For the CS655, the upper limit for bulk EC is 3.04 dS/m, corresponding to a solution EC of 8 dS/m. When this bulk EC condition occurs, the soil is considered out-of-bounds and the M3! command reports NAN or 9999999 for both permittivity and volumetric water content.

Voltage ratio is too high

When the voltage ratio is greater than 17, bulk electrical conductivity readings become unstable. When this bulk EC condition occurs, the M3! command reports NAN or 9999999 for both permittivity, volumetric water content, and bulk EC.