All wet optical sensors function on the principle of Severinghaus
carbon dioxide electrode, and therefore diffusion of CO2 from the
test medium (gaseous or aqueous) through a thin (ca. 10 mm) gas
permeable membrane, and a quick establishment of equilibrium
with the entrapped aqueous layer, which contains pH-sensitive
dye, usually occurs in a typical wet optical sensor for CO2 (Mills,
2009).