VH400 Piecewise Curve

Most curves can be approximated with linear segments of the form:

y= m*x-b,

where m is the slope of the line

The VH400's Voltage to VWC curve can be approximated with 4 segents of the form:

VWC= m*V-b

where V is voltage.

m= (VWC2 - VWC1)/(V2-V1)

where V1 and V2 are voltages recorded at the respective VWC levels of VWC1 and VWC2.

After m is determined, the y-axis intercept coefficient b can be found by inserting one of the end points into the equation:

b= m*v-VWC

Voltage Range Equation
0 to 1.1V VWC= 10*V-1
1.1V to 1.3V VWC= 25*V- 17.5
1.3V to 1.82V VWC= 48.08*V- 47.5
1.82V to 2.2V VWC= 26.32*V- 7.89

Most soils have a holding capacity of less that 50%, so the curves stop at 2.2V which represents 50% VWC. Above 50% you can use an approximation as follows:

2.2V - 3.0V VWC= 62.5*V - 87.5