Soil Moisture logger sensor NUL-229

This logger sensor is based on measuring the partial vacuum pressure in a tensiometer. A tensiometer is a closed tube with a special ceramic part in its end. The tensiometer is filled with water and put in the soil. If the soil is dry, water goes out by diffusion through the ceramic holes and partial vacuum pressure is created in the tensiometer. When we wet the soil, the partial vacuum in the tensiometer pulls water into the tensiometer and the vacuum decreases. This is why soil moisture is measured in pressure level.