Methane sensors measure the concentration of dissolved methane in the seawater. These sensors are protected against sea water and pressure by a silicone membrane. Gas molecules diffuse through the membrane and into the detector chamber. Diffusion occurs along the partial pressure gradient that exists between the chamber and the ambient water. The concentration of methane in the chamber is then compared to the concentration in the surrounding water. Over time, contamination of the sensor membrane from oil droplets seeping out of the seafloor can obscure and bias measurements, so it is necessary to recover and maintain methane sensors periodically.