Soil Sensor Accuracy

The accuracy stated by sensor manufacturers is not necessarily a measurement in a real application, but is a measurement of the dielectric permittivity of water based on a specified range. Accuracy is influenced by temperature, salinity, mineralogy, and more. But in a controlled environment, the stated accuracy measurement of sensors makes assumptions about the imaginary dielectric permittivity. It does not and cannot take into account all these real application variables that influence the imaginary permittivity. This is why stated accuracy may not align with stated (if disclosed) inter-sensor variability of such sensors. Manufacturers may also not clearly state the dielectric range of their stated accuracy.

Because a sensor’s claimed accuracy is based on the real dielectric permittivity in water only, making an assumption of the imaginary dielectric permittivity, and not taking take into account the variables of soil properties, most sensor manufacturers recommend calibration at each site, and often indicate that accuracy is based on such a site-specific calibration process.

Only if the real and imaginary dielectric permittivities are measured can a sensor automatically calibrate for most soil types and therefore provide an accuracy that matches that observed in a controlled environment and across all sensors produced. 

Most soil moisture sensor manufacturers don’t typically discuss inter-sensor variability, and instead only focus on accuracy as measured in the lab. But this inter-sensor variability impacts the confidence that climatologists, scientists, and international soil monitoring networks can have that a measurement at one place on earth can be compared to a measurement at another place on earth. Or that two or more sensors of the same make and model side-by-side will provide the same measurements.

The range of the real dielectric permittivity used by sensor manufacturers to state accuracy may not be the full dielectric range (that is, 0 to 80). This means that when the accuracy is based on a limited dielectric permittivity range such as 0 to 50, the accuracy of such sensors can be significantly compromised when the moisture is above 40% WFV. The calibration will be less applicable to different soils, with different moisture levels, than what was used for calibration

For most users, accuracy is hard to see. Actual soil moisture percentage using the gravitational method, while the most accurate, is time-consuming to calculate, prone to error, and a difficult process to determine. Since the accuracy of soil conditions cannot be seen like other environmental sensor such as water level and rainfall, most users just take what is stated in a sensor’s official technical specification as true without question. While that is the easiest approach, one must be careful to choose the right sensor measurement technology for their application.