Development of a fiber optic sensor for measurement of pCO2 in sea water: design criteria and sea trials

Measurement of the partial pressure of CO₂ gas in sea water (pCO₂) is usually accomplished by gas chromatography or infrared spectrometry. Both techniques require large, complex and power-demanding apparatus. In this paper we explore the possibility of developing small, low-power sensors. We have developed and tested a prototype pCO₂ sensor for seawater based upon the fluorescence of a combination of dyes encapsulated within a gas permeable silicone membrane at the tip of a single optical fiber. The optical module (Douglas Instruments) delivers 30 Hz chopped white light to a filter and is passed through a dichroic mirror. This light is then focused on to a 220 μm optical fiber. The fiber, approximately 2 m long, was terminated with a standard coupler equipped with a small silicone nipple. The internal volume of the sensor tip (about 10 μl) was filled with a combination of a fluorescent indicator and two absorbing dyes so as to achieve the required sensitivity. HPTS (hydroxypyrenetrisulfonic acid) was chosen as the fluorescent species; Neutral Red and DNPA (2-(2,4-dinitrophenylazo)-1-naphthol-3,6-disulfonic acid) were selected as absorbers. Illumination at λ_ex = 450 nm yielded fluorescence at λ_ex = 530 nm, and fluoresced light was returned through the same fiber, reflected at 90° by the dichroic mirror, passed through an interference filter and focused on to a sensitive silicon photodiode. Experiments carried out both in the laboratory on standard solutions and at sea show a precision of 3% in the range 400–500 ppm pCO₂. To our knowledge, this is the first demonstration of an optical pCO₂ sensor for detecting oceanic signals. This technology is complementary to optical detection of pH and points the way towards full characterization of the CO₂ system within this measurement framework.