text: Free Ocean CO Enrichment (FOCE) is a technology facilitating studies of the consequences of ocean acidification for marine organisms and communities by enabling the precise control of CO enrichment within in situ, partially open, experimental enclosures. Current FOCE systems control experimental CO perturbations by real-time monitoring of differences in seawater pH between treatment (i.e. high-CO) and control (i.e. ambient) seawater within experimental enclosures.