Because there are many different types of soil water content sensors available, choosing the best one for your application may seem difficult or even confusing. To help you make your selection, it’s important to understand what soil water content sensors actually measure, what makes a good soil water content sensor, and how to make sense of the manufacturer specifications. In this article, I’ll cover these topics to help you with your sensor selection process.
You may find it surprising that there are no commercially available soil water content sensors that measure water directly. Instead, what the sensors do is detect changes in some other soil property that is related to water content in a predictable way. Common soil properties that change with water content and are easy to measure include dielectric permittivity, thermal conductivity, and density of neutron flux. This article focuses on sensors that measure dielectric permittivity.
Dielectric permittivity sensors are the most common type of soil water content sensor on the market. These sensors use different technologies to measure the permittivity of the surrounding soil, including the following:
Regardless of the technology, the same principle is used: the bulk dielectric permittivity of soil changes with volumetric water content.
The simplest way to think of permittivity is as stored electrical energy. The sensor generates an electric field in the soil and, because the water molecule is polar, unbound water molecules in the soil rotate to line up with the electric field lines.
That rotation of unbound water molecules requires energy, which is stored as potential energy in the aligned water molecules. The more water there is in the soil, the more energy gets stored, and the higher the bulk permittivity of the soil.
The other constituents of soil—mineral and organic solids, and air—store electrical energy too, but water can store more than ten times as much as the other parts of soil. Because of that, water movement in and out of the sensor’s measurement volume is the major contributor to changes in permittivity.
The soil water content sensor is designed to have an electrical signal that changes with permittivity and, therefore, with water content. Some sensors determine permittivity and then convert that to water content, while others convert the sensor’s electrical output to volumetric water content in a single step. No matter which method is used, the water in the soil affects the bulk dielectric permittivity, which affects the sensor’s electrical output. This is important to remember when you are comparing accuracy specifications.
An ideal high-performance soil water content sensor has all of the following properties:
As you might imagine, these properties can be in competition with one another. For example, a durable, high-accuracy sensor may be more expensive when compared with a less durable sensor with lower accuracy. Sensor manufacturers try to find the right balance between these competing factors. To decide which sensor is best for your application, determine which of these factors are most important to you, and then look for the sensor that best matches your highest priorities.
To really understand the specifications for soil water content sensors, it is helpful to think about the relationship between water content, dielectric permittivity, and the electrical signal that varies with permittivity. The relationship between those three things will determine the accuracy and resolution of the water content value and the operational range of the sensor.
The accuracy of the volumetric water content measurement depends on several things:
If all soil water content sensors provided an accuracy specification for measuring permittivity, we’d have a much easier time comparing the performance of different sensors. Unfortunately, many users do not fully understand the relationship between permittivity and volumetric water content, and the different conditions that can add error to the final estimate of water content.
Because there is no single calibration function that will work for all soils, manufacturers usually select one or a few “representative” soils and provide a water content accuracy specification based on that selection. So, for example, what does an accuracy specification such as “±1% volumetric water content” really mean? This specification means ±1% under the conditions where the calibration work took place. Those conditions were likely indoors with minimal temperature variation and only one or a few representative soil types.
Another important consideration is that the accuracy of dielectric soil water content sensors decreases as the soil gets very dry. When the amount of water in the soil is very low, its contribution to the overall bulk permittivity of the soil is smaller and may drop below the sensor’s ability to detect changes. In dry soil, temperature often affects the sensor output more than changes in water content.
Tip: Be very cautious with specifications that promise high accuracy at low water content levels.
Note: Accuracy may be specified as absolute accuracy (% water/% dry soil) or as percentage of reading. To convert percentage of reading to absolute accuracy, multiply by the upper and lower limits of the operational range.
For example: Campbell Scientific’s CS655 accuracy specification for dielectric permittivity in the soil range is ±(3% of reading + 0.8) from 1 to 40 for solution electrical conductivity ≤ 8 dS/m. If the sensor permittivity reads 1 (in air), the accuracy of that reading is ±(1 x 0.03 + 0.8) = ±0.83. If the sensor permittivity reads 40, the accuracy of that reading is ±(40 x 0.03 + 0.8) = ±2.
A final word about accuracy: Manufacturers write their specifications to cover a wider range of conditions than the sensor experiences after installation. You can usually get better accuracy than the specifications indicate if you perform your own soil-specific calibration.
The resolution specification describes how much the soil water content needs to change before the sensor can detect that change. It is mostly determined by the quality of the measurement of the sensor’s electrical output. (This measurement may happen inside the sensor electronics or in an external device such as a datalogger.) The resolution may also be affected by rounding errors coming from mathematical processing and the format of the digital data. The highest-performing soil water content sensors will have both high accuracy and high resolution.
When you compare the operational range of different soil water content sensors, it is important to understand how the range is defined and the limitations of the dielectric method.
Volumetric water content range can be expressed in different ways:
Note: Volumetric water content is sometimes expressed as a fractional number rather than a percent. To convert a fraction (volume water/volume dry soil) to percent, just multiply by 100%.
For example: Delta-T’s ML3 ThetaProbe specification for water content range is 0 to 0.5 m3/m3. Multiply both ends of that range by 100% to get a range of 0 to 50%.
If the sensor’s measurement range works from air dry to saturated conditions in your soil, that is good enough. Most mineral soils have a maximum water content of 40% to 50%. Some clays and organic soils can have water contents as high as 60% to 70%. Make sure you choose a sensor with a range suited to your soil type.
Unfortunately, the truth is that there is no ideal soil water content sensor. They all have their advantages and disadvantages. The best sensor for your application is the one that gives you what you care about most. There are a lot of good sensors available, and if you understand what the sensor is really measuring, then it becomes easier to compare them and make the best choice for your application.
Finally, when in doubt, you can always contact Campbell Scientific to find a trusted advisor that will help you understand your options to make the best measurement possible.
If you have any comments or questions regarding dielectric permittivity sensors, post them below.