Dissolved Oxygen Sensor

I. Working Principle

The HD-DJ8 optical dissolved oxygen sensor is based on the quenching of fluorescence emitted by a specific substance when excited. Excitation light illuminates the fluorescent material on the surface of the sensing film; the material is excited and emits fluorescence, and the decay time of that fluorescence is governed by the concentration of oxygen molecules at the film surface. By measuring the luminescence lifetime of the fluorescence the oxygen concentration is calculated, and the final value is output after temperature, barometric pressure and salinity compensation.

The sensor is maintenance-free in normal use and is suitable for the continuous monitoring of dissolved oxygen in rivers, lakes, aquaculture, wastewater treatment and industrial processes.

II. Key Features

1. Fluorescence lifetime method - no electrolyte, no membrane replacement and no flow dependence.

2. Measuring range 0–20 mg/L with a resolution of 0.01 mg/L and an accuracy of ±2%.

3. Automatic temperature compensation, with additional barometric pressure and salinity compensation.

4. RS485 (Modbus/RTU) and 4–20 mA outputs for easy integration into monitoring systems.

5. Compact body with 3/4 NPT thread for immersion mounting in tanks, channels and pipelines.

6. Low power consumption (0.2 W at 12 V) and IP68 protection for long-term field deployment.

7. Two-point calibration for reliable long-term accuracy.

III. Technical Parameters

IV. Installation and Wiring

The cable is a 4-core shielded cable: red - power supply, blue - ground (GND), green - RS485-B, yellow - RS485-A. Check the wiring order carefully before powering on, to avoid unnecessary losses caused by incorrect connection. Cable colours may differ slightly between production batches; the colour print on the cable always prevails.

Check the wiring order before powering on, and seal the connection point against water.

V. Maintenance and Care

Sensor body: rinse the outer surface with clean water; if dirt remains, wipe with a soft damp cloth. For stubborn deposits, add a little household detergent to the water.

Fluorescent film surface: rinse with clean water or wipe gently with a soft cloth. Apply light pressure only, so that the measuring area is not scratched, which would affect accuracy.

If the reading is too high or too low, clean the fluorescent film and stir the sensor during measurement to prevent material from attaching to the film.

VI. Calibration

Zero-point calibration: add 5 g of sodium sulfite to 95 mL of water and stir well. Immerse the sensor in the solution, wait about 3 minutes until the value is stable, then perform zero-point calibration according to the communication instructions.

Slope calibration: place the sensor in air-saturated water, wait about 3 minutes until the value is stable, then perform slope calibration according to the communication instructions.

Do not expose the inner surface of the fluorescent film to sunlight or strong light, do not touch the film with hands or sharp objects, and avoid air bubbles adhering to the film surface during use.