Look inside
a sensor.

Noor is setting up connected greenhouse irrigation. Before letting a soil probe control watering, she prepares soil samples with known amounts of water. Her capacitive probe uses two conductive electrodes to form a capacitor. Their electric field reaches into the surrounding soil. Water changes the soil's dielectric response, meaning how much electrical charge the probe holds at a given voltage. Wet soil lets it hold more charge than dry soil, increasing capacitance. The probe's electronics turn that change into an analog voltage, which the controller converts into a raw count. In the course example, sixty grams of water goes into two hundred grams of dry soil. Sixty divided by two hundred, multiplied by one hundred, gives thirty percent water content by dry mass. That sample produces two thousand two hundred fifty one counts, linking a measured soil condition to the example's watering threshold. Noor had assumed the raw count already meant a moisture percentage. It does not: she needs the relationship measured for that soil before the controller can interpret the number. She uses those samples to calibrate the probe before enabling irrigation. The takeaway: a capacitive soil probe senses an electrical change, and soil calibration gives that change a useful watering meaning.