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Volumetric water content—defined
To evaluate the performance of any water content sensor, you need to first understand its technology. In order to do this, it’s necessary to understand how volumetric water content (VWC) is measured. Volumetric water content is the volume of water divided by the volume of soil (Equation 1) which gives the percentage of water in a soil sample.
So, for instance, if a volume of soil (Figure 1) was made up the following constituents: 50% soil minerals, 35% water, and 15% air, that soil would have a 35% volumetric water content.
The percentage of water by mass (wm) can be measured directly using the gravimetric method, which involves subtracting the oven-dry soil mass (md) from the mass of moist soil (giving the mass of water, mw) and dividing by md (Equation 2).
The resulting gravimetric water content can be converted to volumetric by multiplying by the dry bulk density of the soil (⍴b) (Equation 3).
Why capacitance technology works
Volumetric water content can also be measured indirectly: meaning a parameter related to VWC is measured, and a calibration is used to convert that amount to VWC. All METER soil moisture sensors use an indirect method called capacitance technology. In simple terms, capacitance technology uses two metal electrodes (probes or needles) to measure the charge-storing capacity (or apparent dielectric permittivity) of whatever is between them.
Table 1 illustrates that every common soil constituent has a different charge-storing capacity. In a soil, the volume of most of these constituents will stay constant over time, but the volume of air and water will fluctuate.
Apparent Dielectric Permittivity
3 - 16
2 - 5
Table 1. Charge storing capacity (apparent dielectric permittivity) of common soil constituents
Since air stores almost no charge and water stores a large charge, it is possible to measure the change in the charge-storing ability of a soil and relate it to the amount of water (or VWC) in that soil. (For a more detailed explanation of capacitance technology watch Soil Moisture 201).
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Have a question about measuring soil moisture in your unique application? METER scientists have over 100 years combined experience measuring soil hydraulic and physical properties.
When capacitance technology was first used to measure soil moisture in the 1970s, scientists soon realized that how quickly the electromagnetic field was charged and discharged was critical to success. Low frequencies led to large soil salinity effects on the readings. Over time, this new understanding, combined with advances in the speed of electronics, enabled the original capacitance approach to be adjusted for success. Modern capacitance sensors, such as METER sensors, use high frequencies (70 MHz) to minimize effects of soil salinity on readings.
The circuitry in capacitance sensors can be designed to resolve extremely small changes in volumetric water content, so much so, that NASA used METER’s capacitance technology to measure water content on Mars. Capacitance soil moisture sensors are easy to install and tend to have low power requirements. They may last for years in the field powered by a small battery pack in a data logger.
TEROS and ECH20: same trusted technology
Both TEROS and ECH20 soil moisture sensors use the same trusted, high-frequency (70 MHz) capacitance technology that is published in thousands of peer-reviewed papers. Figure 3 shows the calibration data for the ECH20 5TE and TEROS 12.
The new TEROS line, however, leverages advances in calibration techniques, an installation tool, and better raw materials to produce sensors that are more durable, accurate, easier and faster to install, more consistent, and linked to a powerful, intuitive near-real-time data logging and visualization system (Figure 4).
Here are some of the changes you will see in the new TEROS water content sensor line:
Minimum sensor to sensor variability: TEROS 11/12 sensors use a completely new calibration procedure that maximizes accuracy and minimizes sensor-to-sensor variability while keeping the sensor cost reasonable. So you can be confident that every sensor you install is going to read exactly like the next one.
Large volume of influence: The TEROS 11/12 sensors deliver a one-liter volume of influence (versus the 200 mL typical for most sensors).
Reliable, long-life sensor performance: Improved sharpened, high-quality stainless steel needles slip easily into even hardened soils, and a durable epoxy fill means the sensor lasts up to 10 years in the field. In the TEROS 12, we’ve positioned a temperature sensor perfectly inside the middle needle so the needles are robust, yet extremely sensitive to soil temperature change.
Reduced installation error: The new TEROS Borehole Installation Tool mistake-proofs installation and delivers consistent, flawless insertion into any soil type (even hard clay) while minimizing site disturbance. Sensors are installed perfectly perpendicular to the sidewall with uniform pressure then gently released to prevent air gaps.
Verification standard: TEROS sensor repeatability can be checked with an accuracy verification standard. No other soil moisture sensor has this ability. Just slide the verification clip onto a sensor and plug it into a logger. If it reads within the right range, your sensor is good to go.
Seamless data collection: For easy and reliable data collection, combine TEROS sensors with the new ZL6, where all data are delivered in near-real time through the cloud.
Why TEROS wins
We created the new TEROS sensor line to eliminate barriers to good accuracy such as installation inconsistency, sensor-to-sensor variability, and sensor verification. TEROS soil moisture sensors use the same dependable ECH20 technology, but go beyond the ECH20 line to optimize the accuracy of the whole data set. They combine consistent, flawless installation, extremely robust construction, minimal sensor-to-sensor variability, a large volume of influence, and advanced data logging to deliver the best performance, accuracy, ease-of-use, and reliability at a price you can afford.
Want more details? In the video below, soil moisture expert Leo Rivera explains why we’ve spent 20 years creating the new TEROS sensor line.
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In 1999, soil moisture measurement systems were expensive. One WSU student envisioned a low cost, low power, connected sensor that growers could use to manage irrigation. He asked Dr. Gaylon S. Campbell to develop the technology, and the resulting sensor measured the capacitance of soil at 6 MHz, initiating an entirely new class of affordable soil moisture sensing technology. The ECH2O probe’s overmolded circuitry and electrodes sealed in circuit board material made it inexpensive to manufacture and allowed it to be buried in the soil. Experience showed the sensor worked well in natural soils with low salinity but lost accuracy at medium to high salinity levels.
We received many requests to shorten the ECH2O, especially from greenhouse growers, who wanted a sensor short enough to insert into their potting containers. The new ECH2O 10 was useful in these new applications, yet challenges with the nutrient-rich irrigation water used in greenhouses and nurseries pushed our scientists to find ways to minimize the influence of electrical conductivity.
In 2003, we began experimenting with higher measurement frequencies, finally settling on 70 MHz, which minimized sensitivity to salinity and improved overall sensor performance, making the new EC-5 accurate in almost any soil or soilless media. Prongs made it much easier to install, and the combination of low sensor cost and minimal power consumption made it ideal to use in large networks. It became one of our most popular water content sensors.
Once the EC-5 was released, we were inundated with requests to add temperature and electrical conductivity (EC) to the sensor from growers who use the EC of their soil or soilless media as a surrogate for nutrients that are available to the plant. We built the ECH2O TE, which measured the EC with gold electrodes on the surface of the circuit board material.
Soon after releasing the ECH2O TE, we engineered a companion sensor, the ECH2O TM, which measured only water content and temperature. This was an important sensor for many research applications, as soil temperature is often combined with water content when measuring in a soil profile.
Despite the EC-5’s popularity, some customers missed the length of the retired ECH2O 10 and 20 sensors. The 10HS was introduced with 10 cm prongs to increase its sphere of influence and include more soil volume in the VWC measurement (1.3 liters versus the EC-5 which measured 0.24 liters).
Although the ECH2O TE measured EC accurately, small pinholes in the gold measurement circuit allowed water to reach the copper underneath and corrode the surface. The 5TE changed the long gold surface to small, stainless steel screw electrodes which would be impervious to corrosion and could last for several years in the soil.
The 5TM was introduced as a companion to the 5TE so they could both be updated to the new, more robust design.
In 2013, we combined steel needles with an epoxy over-molding process, increasing sensor lifetime. We automated the epoxy over-molding process, making the robust GS3 sensor extremely affordable. The steel needles gave this sensor an extended surface area to optimize EC measurements while minimizing substrate disturbance during insertion. Temperature was measured with an onboard thermistor, and electrical conductivity was measured using a stainless steel electrode array.
Our commercial agriculture customers wanted a no-frills, bulletproof water content sensor with a large volume of influence that could measure in harsh environments. So after making advances in the epoxy over-molding process, we designed a hard shell and filled it with epoxy, introducing our most ruggedized volumetric water content sensor.
The new TEROS line of soil moisture sensors combines METER’s well-published high-frequency capacitance technology with an ultra-rugged form, an installation tool, and a new calibration procedure to deliver our most accurate easy-to-use sensor with an excellent price-to-performance ratio (see Figure 4).