Besides, according to
the value of RMSE (0.0079∼0.0195) of β model, the sensor
accuracy reaches the ‘fair’ rank. In spite, the quadratic is not
inferior to β model, to fit the quadratic model with reasonable
accuracy, a sufficient number of data points are required.
On the other hand, it is reasonable to assume that the β model
is applicable to most soil and soilless substrate through
observing a similarity of calibrations for a wide range of soil
and soilless substrate types respectively. If this is the case,
only two steps (one in the dry media, the other is optional) are
needed to establish β model that is accurate as the quadratic
model. Note that the convex curve of perlite may be explained
by its swelling property, which causes bigger porosity.