Design of the Photoelectric Detection Circuit for a Multichannel Nutrient Sensor in Seawater

Existing nutrient sensors have shortcomings, such as their long measurement cycles (more than 1 h), low sensitivity, and infeasibility for use in vertical profiling systems. To deal with these drawbacks, in the current work, we devised a photoelectric detection scheme for a multichannel nutrient sensor in seawater[1]. The multi-channel detection method is adopted, that is, 5 independent light source driving circuits and 5 independent photoelectric detection circuits work at the same time, so as to realize the multi-channel simultaneous measurement of nutrient salts. The optical signals with stable wavelength and power emitted by 5 LEDs are irradiated to the corresponding circulation pool respectively, and the output optical signals are transmitted to the detection function module in the pressure resistant cabin through the optical water tight connector. All of the photoelectric signals are sent to a high-precision A/D (Analog-to-Digital Converter) unit for synchronous collection after I/V (Current to voltage conversion) and filtering. The concentration of nutrient salt corresponding to the absorbance is then obtained using the Beer-Lambert law. The obtained results indicate that the sensor based on the proposed scheme can reach a detection frequency of 1 Hz. This sensor is superior to the traditional nutrient sensor in terms of its operation in vertical profiling systems, allowing for decreases in sensing time of as much as 90% when compared with traditional instruments. The greatest advantage of the proposed sensor scheme is that it can be broadly used in vertical profiling systems for deep-sea investigation and research.