Method for Online High-precision Seawater Dissolved Oxygen Measurement Based on Fast Digital Lock-in Algorithm

Online dissolved oxygen (DO) measurements play a significant role in modern ocean aquaculture and environmental monitoring. This paper proposes a novel optical method based on fast digital lock-in algorithm ( FDLIA) for achieving online high-precision DO measurement, where fluorescence lifetime detection is converted to phase detection. The factors affecting phase measurement accuracy are identified via error analysis and simulations; the proposed optimized FDLIA can achieve high-precision phase detection in simulations. In the development of DO probe, one 12-bit digital-to-analog converter is used to generate a 5 kHz sinusoidal signal as the excitation signal and two 12-bit analog-to-digital converters are used for synchronous sampling. The phase of the sampled fluorescence and excitation signals are then demodulated separately through the optimized FDLIA. Finally, the DO concentration was calculated by the phase difference. The actual measured values show that relative error less than +/- 1.5% and relative standard deviation below 2%. Moreover, the hardware is simplified and the performance requirements of the microprocessor is reduced significantly compared with the traditional phase detection method. The method presented here can be a workable reference for online DO measurement or other similar optical measurements.