Time response of oxygen optodes on profiling platforms and its dependence on flow speed and temperature

The time response behavior of Aanderaa optodes model 3830, 4330, and 4330F, as well as a Sea-Bird SBE63 optode and a JFE Alec Co. Rinko dissolved oxygen sensor was analyzed both in the laboratory and in the field. The main factor for the time response is the dynamic regime, i.e., the water flow around the sensor that influences the boundary layer’s dynamics. Response times can be drastically reduced if the sensors are pumped. Laboratory experiments under different dynamic conditions showed a close to linear relation between response time and temperature. Application of a diffusion model including a stagnant boundary layer revealed that molecular diffusion determines the temperature behavior, and that the boundary layer thickness was temperature independent. Moreover, field experiments matched the laboratory findings, with the profiling speed and mode of attachment being of prime importance. The time response was characterized for typical deployments on shipboard CTDs, gliders, and floats, and tools are presented to predict the response time as well as to quantify the effect on the data for a given water mass profile. Finally, the problem of inverse filtering optode data to recover some of the information lost by their time response is addressed. *Corresponding author: E-mail: [email protected] Acknowledgments Full text appears at the end of the article. DOI 10.4319/lom.2014.12.617 Limnol. Oceanogr.: Methods 12, 2014, 617–636 © 2014, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS times τ well below 10 s are easily obtained if the sensors are pumped. Materials and procedures Optode descriptions The sensing principle of optical oxygen sensors is based on the dynamic or collisional quenching of luminescence by oxygen. After a luminophore has been excited with short-wavelength light, it may return to its ground state either through radiationless de-excitation or through luminescence, i.e., emission of light with longer wavelength. If oxygen is present, O2 can collide with the luminophore and absorb the excess energy of the excited state thereby quenching luminescence. Both the luminescence intensity I and the excited state lifetime Λ are reduced by collisional quenching, and their behavior is described by the Stern-Volmer equation (Eq. 1)