A novel optical sensor designed to measure methane bubble sizes in situ

This work presents a novel design for an optical bubble size sensor that is rugged, economical to build, and capable of accurately measuring methane bubble sizes in aquatic environments over long deployment periods. The sensor intercepts rising gas bubbles, elongates them in a thin glass tube, and routes elongated bubbles past an optical detector. The optical detector records information on bubble rise velocity and travel time, which can be combined with the flow path geometry to calculate bubble volume at flow rates up to 3 bubbles/second. The sensor circuitry is powered by 6-V C alkaline battery packs and is cased in a waterproof housing built from commercially available PVC pipe fittings. Laboratory testing indicates the sensor can accurately measure bubble volumes up to 1 mL in volume. We deployed the sensor in a lake with a history of methane ebullition and gathered data on bubble size distributions (most bubbles were between 0.025 mL and 0.2 mL) as well as the precise timing of bubbling events. The sensor also includes an optional gas collection system to allow for bulk gas sampling.