Electrochemical sensors are based on a chemical reaction between gases in the air and the electrode in a liquid inside a sensor. ... Optical sensors detect gases like carbon monoxide and carbon dioxide by measuring the absorption of infrared light. Electrochemical sensors are based on a chemical reaction between gases in the air and the electrode in a liquid inside a sensor. ... Optical sensors detect gases like carbon monoxide and carbon dioxide by measuring the absorption of infrared light. Electrochemical sensors are based on a chemical reaction between gases in the air and the electrode in a liquid inside a sensor. ... Optical sensors detect gases like carbon monoxide and carbon dioxide by measuring the absorption of infrared light.