User Manual
13
Table 8-1. Standard calibration coefficients for
linear and quadratic forms
Linear
Quadratic
C0
C1
C0
C1
C2
–0.4677
0.0283
–0.0663
–0.0063
0.0007
The linear equation is
VWC = -0.4677 + 0.0283 x period .
The quadratic equation is
VWC = –0.0663 – 0.0063
x
0.0007
x
period
2
Period is in microseconds. The result of both calibration equations is
volumetric water content on a fractional basis. Multiply by 100 to express
in percent volumetric water content.
Figure
8-3
shows the difference between the linear and the quadratic
calibration forms over the typical range. A CS616/CS625 output period of
16 microseconds is about 1.2% VWC and 32 microseconds is 44.9%. The
linear calibration is within ± 2.7% VWC of the quadratic. The linear
calibration underestimates water content at the wet and dry ends of the
range and overestimates it by up to about 2.6 % VWC at about 20% VWC.
Figure 8-3. Linear versus quadratic calibration differences
The linear and quadratic coefficients for the sandy clay loam data in
Figure 8-3 follow and can be used in similar soils.
