A high precision, fast response, and low power consumption in situ optical fiber chemical pCO2 sensor

A sensor system suitable for monitoring changes in partial pressure of carbon dioxide (pCO2) in surface seawater or in the atmosphere has been developed. Surface seawater samples are pumped into a PVC tube enclosing an inner Teflon AF tube, which served as a long pathlength gas-permeable liquid-core waveguide for spectrophotometry. The Teflon cell contains a pH-sensitive indicator-buffer solution consisting of bromothymol blue (BTB) and sodium carbonate. Carbon dioxide in the sample diffuses into the indicator-buffer solution to reach equilibrium, resulting in pH changes, which are detected by changes in the absorbance of BTB at wavelengths of 620 and 434 nm. The pCO2 in the sample is then derived from the pH change. The sensor has a response time of 2 min at the 95% equilibrium value and a measurement precision of 0.26–0.37% in the range 200–800 μatm pCO2. This chemical sensor takes advantage of a combination of long pathlength, multiple wavelength detection, indicator solution renewal, and in situ automatic control technology, and has the feature of low power consumption (the average being ∼4 W with a peak of ∼8 W).