PlantCare has been able to develop a new type soil moisture sensor which eliminates the disadvantages of other sensor technologies. The PlantCare soil moisture sensor is based on a micro-thermal measurement and the result is - as with all other sensors - not an absolute value for the soil moisture, but a time, which is a measure of the soil moisture. A small tip is heated up by a heating element for a few seconds by about 1° C. After this heating period, the cooling curve is recorded electronically with great accuracy and the time required to reach a certain threshold is measured.
If the tip is immersed in water, the slightly heated tip cools rapidly as it cools slowly in the air. If this tip would be placed directly into the ground, the contact surface to the soil would not be defined, so that the readings would vary greatly. For this reason, the tip is surrounded by a felt. This hydrophilic felt encloses the tip completely and sucks the freely available water from the soil surrounding them. Therefore, the actual sensor does not feel the soil, but only the water-filled felt. Since the felt retains its properties for several years and also has a very low thermal conductivity compared to water, the sensor measures only the water present in felt.
The sensors measure the plant available water only.
The water contained in the roots is not measured. The measured values therefore do not change with the growth of plants and accordingly the increasing root volume.
The measuring range is from 0 to 100% which corresponds to 0-1500 hPa soil water potential.
The sensitivity of the sensors can be matched to the application. Thus, the sensors can be used in soil with a high clay content, in Hors Sol plantations, as well as in natural grown soil.
The measurement result is unaffected by salt or fertilizer content and soil temperature.
The sensors are easy to install and the embedding in the surrounding soil is not critical as it is in the case of tensiometers.
The sensors are small and can be simple inserted into pre-drilled holes.
The measurement can also be done close to the surface, which is not possible with capacitive measuring processes.
All components are hermetically sealed against moisture and the materials used will not rot even after long usage in the soil. Therefore, the sensors are completely maintenance free.
The energy consumption of the sensors is very small. A sensor can be powered by two 1.5 V AA-D-cell batteries for one year, provided that the time between two measurements is one hour.
In many cases, the actual sensor tip must be placed at locations where the sensor electronics can not be accepted. This is the case on lawns or the sensor electronics with the radio antenna must be mounted higher up to improve wireless connection. This is not a problem because the sensor tip can be connected to the sensor electronics by a virtually unlimited long cable.
In addition, the sensors measure soil temperature simultaneously to the soil moisture.
Reliable values through accurate calibration
To set two fixed points "dry" and "saturation" and also to eliminate all production-related variances, all sensors are calibrated before delivery. This is accomplished by an automatic calibration, in which several measurements are taken first in the absolutely dry state. The sensor calculates the mean value and saves it as a 0% value. Then this process is repeated with felts saturated with water and the 100% value is determined. This gives you two fixed points, 0% and 100% - soil moisture. A measurement therebetween thereafter takes place relatively, i. one measures for example 50% soil moisture, then this does not mean that there is still 50% water left in the soil. Rather, it means that the soil moisture has dropped to about halfway between saturation and total dryness.
Calibration of the lower dry value is done with a dry felt and constant room temperature and constant humidity by a 2-hour software routine.
The calibration of the upper value takes place by inserting the sensor tip into water and pushing out the air from the felt tip. Here the calibration process lasts 90 minutes.
We therefore determine the 100% reading when calibrating by placing the sensors in water, i. in full saturation.
This condition is achieved in practice when such high levels fall within such a short time and the water from the ground can not be absorbed quickly enough. The water seeps into the soil and excess water, which no longer finds any free pores, seeps deeper and deeper until it is absorbed by the free pores.
If the sensor measures e.g. in rain at 20cm depth before saturation occurs at this depth no 100% is reached.
If the sensor measures rain or irrigation in a certain period of time, then it will not measure 100%, but lower humidity will mean the field capacity, the water content that can be withheld by the earth against gravity.