A capacitive moisture [sensor](https://www.ampheo.com/c/sensors) measures how much the material under test (soil, wood, air, etc.) changes an electric field, and from that infers moisture content.


**Core idea (the physics in one breath)**

* The sensor is a small [capacitor](https://www.onzuu.com/category/capacitors): two electrodes separated by a dielectric.
* Capacitance 𝐶=𝜀𝑟𝜀0𝐴/𝑑 depends on the relative permittivity 𝜀𝑟 of what surrounds/fills the field.
* Water has a very high 𝜀𝑟 (~80 at room temp) compared to dry soil/wood/air (~2–10).
* When moisture increases, the fringing field “sees” more water → capacitance rises. You don’t measure water directly; you measure the capacitance change.

**Typical sensor structures**

* Soil probe (FR4 PCB): interdigitated “comb” electrodes. Most field is fringing into the soil around the board.
* Non-contact / through-housing: electrodes on one side of a plastic wall to sense moisture on the other side.
* Parallel plate (lab/industrial): sample inserted between plates for a more controlled geometry.

**How the electronics read it**

**1. RC time / charge–discharge**
Drive the sensor through a resistor and measure the time to a logic threshold:𝑡≈−𝑅𝐶ln⁣(1−𝑉𝑇/𝑉𝐷𝐷).
Capacitance ↑ → time ↑. Cheap, easy on [MCUs](https://www.ampheo.com/c/microcontrollers) (timer capture or analog comparator).

**2. Relaxation oscillator / frequency shift**
Put C in an RC (or LC) [oscillator](https://www.onzuu.com/category/oscillators) and measure frequency 𝑓∝1/𝑅𝐶. Capacitance ↑ → frequency ↓.

**3. Capacitance-to-digital converter (CDC)**
Dedicated IC (e.g., FDC/AD series) gives high resolution, built-in excitation, shielding, and drift handling.

**4. Bridge / impedance at fixed** 𝑓
AC-excite at e.g. 100 kHz–10 MHz, measure amplitude/phase to separate capacitive component from losses (salinity effects).

AC excitation is used (kHz–MHz). Unlike resistive (two-probe) sensors, capacitive types avoid DC electrolysis and last longer in soil.

**What the reading really tells you**

* The sensor responds to dielectric constant, which correlates with volumetric water content (VWC).
* You typically build a calibration curve: measure raw C (or frequency) at several known moistures and fit a line/curve (often quadratic).

**Real-world effects you must handle**

* Temperature: 𝜀𝑟 of water drops with temperature; provide temp compensation or co-locate a sensor.
* Salinity / ionic conductivity: raises dielectric loss and can skew low-frequency readings. Using higher excitation frequency reduces resistive artifacts.
* Soil type/packing: particle size and porosity shift the baseline—calibrate per soil type.
* Parasitics: cable and PCB stray capacitance add offset; keep leads short or use guard/shield driven at sensor potential (guard ring around electrodes).
* Drift & fouling: soil adhesion and aging change coupling; periodic re-zero or reference channel helps.

**Practical design tips (embedded)**

* Geometry: wider/longer interdigitated fingers increase sensitivity; solder mask or conformal coat protects copper without killing sensitivity.
* Shielding: add a ground plane behind the electrodes and a driven guard around them to push field outward into the medium.
* Excitation: 100 kHz–1 MHz is a good start for soil; too low → conductive losses; too high → cable/EMI issues.
* Averaging: filter readings (moving average) and rate-limit updates; moisture changes are slow.
* Power: duty-cycle the oscillator to save energy in battery-powered nodes.

**Example: simple RC-timing read (Arduino-style)**

const int SENSE_PIN = A0;     // analog comparator threshold via ARef, or use digital with Schmitt
const int DRIVE_PIN = 8;
const int R_SER = 10000;      // 10 kΩ in series with the sensor

unsigned long measure_us(){
  pinMode(DRIVE_PIN, OUTPUT);
  digitalWrite(DRIVE_PIN, LOW);
  delayMicroseconds(50);              // discharge sensor
  digitalWrite(DRIVE_PIN, HIGH);      // step input
  unsigned long t0 = micros();
  while (analogRead(SENSE_PIN) < 512); // wait until ~VDD/2 (rough)
  return micros() - t0;
}

void loop(){
  unsigned long t = measure_us();     // proportional to C
  // Convert t -> moisture via your calibration curve
}

(For precision: use an analog comparator/timer capture, keep wiring short, and average many samples.)

**Calibration sketch**

1. Pack soil samples to consistent density.
2. Measure raw output at oven-dry (0% VWC).
3. Increment moisture by adding a known mass of water to a known soil mass (gravimetric method) to get target VWC values (e.g., 5%, 10%, …).
4. Fit a model (linear or 2nd-order) mapping raw 𝐶 or 𝑡 to VWC. Store coefficients in MCU (EEPROM/flash).

**Capacitive vs. resistive soil sensors**

* Capacitive: durable, no DC corrosion, less sensitive to soil chemistry for the same frequency band; needs AC readout and calibration.
* Resistive: very cheap, but drifts quickly and electrodes corrode; strongly affected by salinity.

**TL;DR:** A capacitive moisture [sensor](https://www.ampheoelec.de/c/sensors) is just a capacitor whose electric field fringes into your material; more water → higher permittivity → higher measured capacitance. You excite it with AC and read the capacitance (time/frequency/CDC), then convert via a calibration curve, with care for temperature, salinity, and geometry.