The calibration of soil moisture sensors is crucial for accurate and reliable soil moisture measurements. Many sensors are supplied with a generic calibration table. Following these steps it is possible to generate a calibration table specific for your sensor and soil, greatly enhancing your soil moisture data.
Most soil moisture sensors are delivered with analogue output and a calibration table, or digital output (SDI-12 for example) with a % VWC output. These are typically derived from a generic soil calibration, which is likely to be different to the soil in your study area.
It’s also worth noting at this point that soil moisture sensors are likely to have different sensitivities, even within sensors of the same model. Individual calibration may be required in some situations.
With both the wet and dry weights now known the Gravimetric Soil Water content can be calculated as follows:
Water Content = (wet weight – dry weight) / (dry weight * 100)
To convert Gravimetric Soil Water content to Volumetric Water Content, you need the bulk density of the soil. This can be sampled and measured using a SEC 0200 Soil Core Sampler for example. Bulk density is weight by area of soil.
VWC = (Gravimetric Soil Water) x (Bulk Density)
(NOTES ON HOW TO ADD OFFSETS AND CALCULATIONS TO IOT NODES FOR CORRECTED MEASUREMENTS)