Why moisture sensor get under 25 as a value, while the soil is dry?
This is a very common and frustrating issue for anyone starting with plant monitoring or smart gardening. A moisture sensor reading below 25 when the soil is visibly dry is almost always due to one of three main reasons: Sensor-Specific Issues (Most Common) Soil-Specific Issues Electrical/Code Issues Here’s a detailed breakdown of each cause and how to fix it. 1. Sensor-Specific Issues (The #1 Culprit) The most common moisture sensors (like the YL-69 or FC-28 ) are resistive sensors . This design is the primary source of your problem. How They Work: They work by passing a small electrical current between the two exposed "prongs" and measuring the electrical resistance of the soil. Water conducts electricity, so: Wet Soil = Low Resistance = High Reading (e.g., 60-90% on a 10-bit ADC) Dry Soil = High Resistance = Low Reading (e.g., 10-30% on a 10-bit ADC) The Fatal Flaw: Electrolysis & Corrosion Because they run a constant DC current th...