The fluorescent dissolved oxygen sensor is based on the quenching principle of the specific fluorescence of the particular substance in physics. Blue light from a light-emitting diode (LED) illuminates the fluorescent material on the inner surface of the fluorescent cap, and the fluorescent material on the inner surface is excited to emit red light by detecting the phase difference between red and blue light and the internal calibration value. The ratio is calculated to calculate the concentration of oxygen molecules, and the final value is automatically compensated by temperature and pressure.