MP306 Moisture Probe Operation Manual
2.4 Calibration
2.4.1 General
The results obtained from measurement of the absolute volumetric soil water content (VSW%) using the
MP306 are expected to be within ± 2–5% of the actual soil moisture as determined in the laboratory by
gravimetric and volumetric methods of determination. Recalibration is not expected to be necessary for most
applications in most commonly occurring agricultural soils. This is especially so when it is considered that
for practical end uses such as irrigation scheduling and irrigation control the change in VSW% is the most
important variable to be determined for management decision making. The change measured will be correct
in absolute VSW% units or mm of water applied as the relationship of voltage output to water content,
hence calibration slope remains constant, across all soil types.
Scientists or regulatory authorities may wish to calibrate the MP306 to verify the data measured. In these
cases it is simply necessary to compare the MP306 output in mV (0–1 mV) to the VSW% from the soil
samples, either prepared in the laboratory or obtained in the field. The resultant regression of these variables
will provide the new calibration of the MP306. As all MP306 respond to changes in water content of the soil
and the resultant changes in the dielectric constant in the same way the calibration would apply to all
MP306.
Data Loggers variously are able to store linear, polynomial or polynomial lookup tables depending on their
specifications. Then various forms of the MP306 soil calibration are supplied for these data loggers.
2.4.2 Linear Calibration
VSW% = a + b
χ
= INT SLOPE (
χ
)
= -0.1552 + 0.80829 (
χ
)
Where χ = MP306 output in volts
{Output Range of MP306
{Limits of VSW
0 ≤ MP
3
06 ≤ 1.20 V
0 ≤ VSW < 1.00
2.4.3 Polynomial Calibration
VSW% = a
o
+ a
1
χ
+ a
2
χ
2
+ a
3
χ
3
= -0.05334 + 1.19650
χ
– 2.5356
χ
2
+ 1.9961
χ
3
Where χ = MP306 output in volts
{Output Range of MP306
{Limits of VSW
0 ≤ MP
3
06 ≤ 1.20 V
0 ≤ VSW < 1.00