An Optical Sensor System for Marine O2, pH and CO2 Measurements

Oxygen, pH and CO₂ are among the most important parameters for marine measurements. The state-of-the-art devices for measurement of these parameters are bulky and expensive, and a separate device is necessary for measurement of each parameter. Therefore we are developing a modular, compact and cost-effective measurement system for all three parameters. The system is composed of a waterproof and pressure resistant read-out device and exchangeable sensor membranes for oxygen, pH and CO₂.

The read-out device features an internal logger, a battery and a temperature sensor. It is small and lightweight with 150 g in water and is pressure resistant up to 500 meters. The internal logger and battery enable continuous measurements for one year with a logging interval of 5 minutes. Different sensor materials are mounted on the device using simple screw-caps. Thereby the system can be serviced extremely fast and it can be used for measurement of different analytes.

The sensor materials are based on stable and photostable near-infrared dyes embedded in polymeric matrices with minimized interference from autofluorescence. The temperature dependence of all three material types is well characterized and is compensated via the temperature probe of the measurement device. The dynamic range of the pH and CO₂ sensor materials is optimized for seawater measurements. The oxygen sensors are available in several configurations covering the dynamic range from ambient conditions to trace levels of the analyte.

The device was field-tested in combination with the different sensor materials during a field trip on the Baltic Sea in October 2015. They performed well and an example of a profiling measurement can be seen in figure 1. Further optimization (aiming at faster response times) is in progress.