sensor-soil-moisture-dat
board
- STH1052-dat - soil moisture sensor board
working principles
The principle of detecting soil moisture is mainly based on measuring how the presence of water affects the electrical or physical properties of the soil. The most common types are as follows:
🌱 1. Resistive (Conductivity-Based) Principle
Principle:
The more water in the soil, the higher its conductivity (lower resistance) because water contains electrolytes. When the soil is dry, resistance increases.
How it works:
- Two metal probes are inserted into the soil.
- A small voltage is applied across them.
- The resulting current or resistance is measured and converted to moisture content.
Advantages: Simple, inexpensive, fast response.
Disadvantages: Electrodes corrode easily, affected by soil salinity, limited long-term stability.
🌾 2. Capacitive Principle
Principle:
The dielectric constant of water (~80) is much higher than that of dry soil (~4) or air (~1).
As soil moisture increases, the dielectric constant of the soil rises, and the sensor’s capacitance increases.
How it works:
- The sensor forms a capacitor (with metal probes or plates).
- The capacitance change is measured and calibrated to indicate moisture level.
Advantages:
- No direct electrical contact with soil (non-corrosive).
- High stability, suitable for long-term monitoring.
Disadvantages:
- Slightly higher cost.
- Requires high-frequency measurement circuitry.
🌿 3. Time Domain Reflectometry (TDR)
Principle:
The propagation speed of an electromagnetic pulse in soil depends on the soil’s dielectric constant, which varies with moisture content.
More water → higher dielectric constant → slower signal propagation.
How it works:
- A high-frequency pulse is sent along probes.
- The reflection time or waveform change is measured.
- The dielectric constant is calculated and converted into volumetric water content.
Advantages: Very accurate, measures volumetric water content.
Disadvantages: Expensive and complex equipment.
🍂 4. Neutron Scattering Method (Scientific Use)
Principle:
Fast neutrons are slowed down when they collide with hydrogen atoms (mainly from water molecules).
The number of slow neutrons detected indicates the soil water content.
Advantages: Extremely accurate.
Disadvantages: Very expensive, requires radioactive sources, strict safety requirements.
✅ Comparison Table
| Type | Measurement Basis | Accuracy | Cost | Stability | Characteristics |
|---|---|---|---|---|---|
| Resistive | Conductivity | ★★ | Low | ★ | Simple but corrodes easily |
| Capacitive | Dielectric constant | ★★★ | Medium | ★★★ | Stable, most commonly used |
| TDR | Electromagnetic wave velocity | ★★★★★ | High | ★★★★★ | High precision, research use |
| Neutron | Hydrogen atom count | ★★★★★ | Very High | ★★★★ | Laboratory / scientific use |