Frequently Asked Questions - Farmtasker by EllenEX

Ellenex sensors support all major LPWAN technologies like NB-IoT, LoRaWAN and Low Power Satellite. We also manufacture devices for Wirepas on request.

All our products are designed with industrial applications in mind. Our sensors can reliably measure and securely transmit information about pressure, level, temperature, distance, flow and water quality. We also offer sensor interfaces to connect legacy and specialized devices to the internet using LPWAN technologies.

We have a custom product division that will take care of manufacturing devices specifically for you. Please feel free to get in touch with our client growth team and they will assist you with the entire process.

Soil moisture is essential in horticulture and agriculture. Only with the right amount of water at the right time plants will grow and produce the desired yields.

Soil moisture is determined by precipitation, groundwater, irrigation and the consumption of water by plants. Some water is held in the soil against gravity and remains there. The soil thus has a water storage capacity that is needed by plants. The more precisely the water content can be determined, the better irrigation can be controlled and thus adapted specifically to the water requirements of the plants.

Soil moisture also plays a major role in other sectors. In civil engineering, the water content in the soil determines the load-bearing capacity or the stability of dike structures. Excessive soil moisture can even lead to landslides.

However, soil moisture sensors can also be used for other purposes, ranging from snow moisture measurement and concrete curing control to monitoring the water content in grain stores.

Electrical methods are particularly suitable for measuring soil moisture. This allows continuous monitoring of the water content in the soil and connection to irrigation control systems in a variety of ways.

Soil is a mixture of water, air and a mineral/organic soil matrix. Depending on the water content, the electrical characteristics change, whereby a distinction can be made between the electrical conductivity and the so-called dielectric characteristics.

The electrical conductivity depends not only on the water content, but largely on the dissolved salts. Measurement methods based on this principle are therefore often very unreliable and not suitable. Sensors on this basis, which are usually sold at very low prices, do show some dependence on the water content, but this changes over time. In addition, electrical conductivity measurement requires direct metallic contact with the soil and leads to corrosion of the electrodes. Therefore, reliable and permanent measurements are practically impossible.

Dielectric measurement is based on the special properties of the water molecule, which can align itself as a dipole in an electric field. This interaction can be detected and converted into a so-called volumetric water content. It can also be said that the soil between the electrodes is a capacitor whose capacitance changes with soil moisture. A great advantage is that this capacitance measurement does not require direct metallic contact of the electrodes. Therefore, a long-term stable sensor system is possible on this basis. In praxis, capacitance is measured using small AC voltages. Here the frequency selection is of decisive importance. Very low-cost capacitive sensors often use frequencies that are much too low, and there is considerable cross interference from electrical conductivity. High frequency capacitive sensors are more complex and expensive, but allow a reliable and accurate determination of the water content.

Material moisture measurement technology, as the upper field of soil moisture measurement technology, is an extensive research and development field. The standard scientific work in English on this subject is available free of charge:
Electromagnetic moisture measurement

The selection of a suitable sensor for soil moisture measurement depends on several aspects:

• Required measuring accuracy
• Soil / Substrate
• Measuring volume
• Interface

Of course, you want a sensor that is as accurate as possible. However, higher accuracy or additional special properties are usually associated with higher costs.

For scientific applications or very demanding irrigation tasks, the SMT100 is usually used, while the less expensive SMT50 is mainly found in simpler irrigation tasks. Both sensors are suitable for irrigation control, but differ in important details.

The SMT100 is designed for the full measuring range from 0 to 100 %. In addition, it enables a largely soil type-independent measurement with particularly high resolution. This is due, among other things, to its high internal measuring frequency, which effectively suppresses interference effects caused, for example, by variable electrical conductivity of the soil.

The measuring range of the SMT50 only goes up to 50% and the soil type has a stronger effect on the calibration curve, since a lower internal measuring frequency is used. The resolution is also lower than the SMT100, but is easily sufficient for many applications, especially when setting thresholds for irrigation control.

Moisture sensors for soils should provide measurement results that are influenced as little as possible by the soil type. Technically, this is a very big challenge, because the soil type influences the measured dielectric properties. One can reduce the disturbing influence by a clever choice of the measuring frequency. In general, it is better to use the highest possible measuring frequency in the range of a few hundred MHz. In the SMT100 this is realized, but it requires a high electrotechnical effort, which is reflected in the costs. The SMT50 operates at lower frequencies and is therefore less expensive. The disadvantage, however, is the greater dependence on the ground type.

Another important aspect when selecting a sensor is the measuring volume. The SMT100 and SMT50 belong to the class of point sensors, i.e. the water content is determined in a limited volume around the sensor. In some irrigation applications, a larger measurement volume is desired to better average over inhomogeneities of water distribution in the soil. The AquaFlex is particularly suitable for such applications. The measuring volume extends over the sensor length of 3 m and thus enables optimal averaging. With a sensor of this type, it is more difficult to achieve the same calibration and soil type independence as with a point sensor due to its design. For the practical application in irrigation technology, however, the large measuring volume is the decisive advantage.

The interface for control or data acquisition can be distinguished between analog and digital variants. With analog interfaces, a voltage value is usually output. The voltage range can be different depending on the sensor. For example, the SMT50 is designed for 0 - 3 V signal output. The analog SMT100 and the analog AquaFlex can be configured at the factory according to customer requirements. 0 - 10 V is the standard in automation technology, other voltage ranges such as 0 - 1 V, 0 - 3 V or 0 - 5 V are also possible on request. For the digital interfaces of the SMT100, the largest choice is either RS-485 with the software protocols TBUS, ASCII and Modbus or the SDI-12 standard, which is popular in environmental measurement technology. The AquaFlex is also available with RS-485 TBUS, ASCII and Modbus digital interface.

The advantage of the analog interface is the ease of use. The disadvantage is the somewhat lower resolution, the limited cable length and the higher hardware effort in the controller when using many sensors. Several digital sensors can be connected to a single cable (bus) and addressed by software via addresses. This reduces the hardware effort in the cabling, but the demands on the controller are higher and there is some configuration effort. Especially with RS-485, extended systems with long cable lengths can be realized with digital transmission. The transmission quality is excellent and the highest measured value resolution can be realized.