ISFF sampling of CS615 soil moisture probes

Campbell CS615 Manual in PDF form

Sampling CS615 Square Wave Period

The CS615 probes are sampled with CR10 data loggers, using the P27 Period Average function, as shown in the Sample Program 1 in section 10 of the Campbell CS615 manual. The P27 function provides a measurement of the period in milliseconds of the square wave propagating in the probe's wave guide. This raw measurement of the wave period is reported as the variable Qsoilraw or Qtsoil.

The sensor output is essentially unaffected by modest misalignment of the two, nominally parallel, rods. The two rods are separated by 3 cm at their base, and Campbell reports that the measurements are not significantly different for the tips being separated by 2-4 cm. This range represents an acceptable 'misalignment' caused by insertion during the installation procedure. Within this range, the principal consequence of misalignment is that the soil is no longer sampled uniformly along the length of the rods.

Conversion to Approximate Volumetric Water Fraction

The wave period in milliseconds is then converted to an approximate volumetric water fraction, Qsoil, using the polynomial conversion shown in the manual for soil electrical conductivities less than 1.0 dS/m: 

	Qsoil = A0 + (A1 * Qtsoil) + (A2 * Qtsoil2)
where
	A0 = -0.187
	A1 =  0.037
	A2 =  0.335

Correction of CS615 Water Fraction

The above polynomial conversion to volumetric water fraction does not account for any differences between probes or for any soil variability. The results have differed from other measurements by as much as a factor of 2. Other independent measurements of soil moisture near the CS615 probes should be used to correct the raw volumetric water fractions. This procedure is discussed in the individual project reports.

Zero calibration values