The sensor takes advantage of the permeability of silicone to gases but not to liquids. The probe was made of Detakta silicone rubber tubing (0.1m in length with an inner diameter of 0.25cm and an outer diameter of 0.35cm). A stream of helium (50mL/min) passed through the tubing, sweeping out the gases which diffused through the walls of the tubing from the fermentation broth.