TDS: Total Dissolved Solids Water Quality Sensor

Working Principle

A TDS (Total Dissolved Solids) sensor measures the total concentration of dissolved ions in water by measuring electrical conductivity. Two parallel electrodes are immersed in the water; an AC voltage is applied (to prevent electrode polarisation), and the resulting current is proportional to the ion concentration. Higher TDS = more dissolved salts = higher conductivity.

TDS is expressed in parts per million (ppm), approximately equal to mg/L. A conversion factor (typically 0.5–0.7) relates conductivity (µS/cm) to TDS (ppm). Pure distilled water: <10 ppm. Tap water: 100–500 ppm. Seawater: ~35,000 ppm.

Electrical Characteristics

Parameter	Typical (DFRobot/Keyestudio TDS)
Measurement Range	0–1000 ppm
Accuracy	±10% (after temperature compensation)
Probe Type	2-electrode waterproof probe
Output	Analog voltage (0–2.3 V for 0–1000 ppm)
Supply Voltage	3.3–5.5 V
Excitation	AC signal (on-board oscillator, prevents polarisation)
Temperature Dependence	~2%/°C (must compensate)
Cable Length	~60 cm waterproof probe

Interfacing with an MCU

Connect VCC, GND, and analog output to an ADC pin. The probe connects to the module via a 2-pin header. For accurate readings, a temperature sensor (DS18B20 or NTC thermistor) must be used alongside the TDS probe for temperature compensation.

Calibration

• Reference solution: Use a 342 ppm (or 1000 ppm) NaCl calibration solution; adjust the potentiometer on the module until the reading matches
• Temperature compensation: Apply the formula: TDS₂₅ = TDS_raw / (1 + 0.02 × (T − 25)), where T is water temperature in °C
• Cross-check: Compare against a commercial TDS pen meter for verification

Code Example

/*
 * TDS Water Quality Sensor — Arduino
 * Module: DFRobot/Keyestudio TDS Meter V1.0
 * Wiring: VCC → 5V, GND → GND, Signal → A1
 */
#define TDS_PIN A1
#define VREF 5.0
#define TEMP 25.0   /* Replace with actual measurement */

float readTDS() {
    /* Average 20 samples */
    long sum = 0;
    for (int i = 0; i < 20; i++) {
        sum += analogRead(TDS_PIN);
        delay(10);
    }
    float voltage = (sum / 20.0) * VREF / 1024.0;

    /* Temperature compensation */
    float compCoeff = 1.0 + 0.02 * (TEMP - 25.0);
    float compVolt  = voltage / compCoeff;

    /* Voltage-to-TDS conversion (module-specific curve) */
    float tds = (133.42 * compVolt * compVolt * compVolt
               - 255.86 * compVolt * compVolt
               + 857.39 * compVolt) * 0.5;
    return tds;
}

void setup() {
    Serial.begin(9600);
    Serial.println("TDS Water Quality Sensor Ready");
}

void loop() {
    float tds = readTDS();
    Serial.print("TDS: ");
    Serial.print(tds, 0);
    Serial.println(" ppm");
    delay(2000);
}

Real-World Applications

Limitations

• Non-specific: Measures total ions, not individual contaminants; cannot distinguish safe minerals from harmful substances.
• Temperature dependent: Conductivity changes ~2%/°C; uncompensated readings are unreliable.
• Electrode fouling: Biofilm and mineral deposits on electrodes degrade accuracy over time.
• Limited range: Hobby modules max at ~1000 ppm; industrial/seawater needs higher-range probes.