The electrochemical cell is a simple two electrode design and consists of a semi-permeable diffusion membrane, a reservoir of acid electrolyte, a sensing electrode and a counter electrode. CO diffusing into the cell reacts at the surface of the sensing elec- s trode producing, as a by-product, a number of ions (H+) and electrons (e-), The ions travel through the acid electrolyte to the counter electrode the negatively charged electrons travel to the counter electrode via the external circuit. Combining the electrons and ions at the counter electrode com pletes the reaction without any of the cells compo- nts being consumed. The amount of electrons produced by the reaction is directly proportional to the amount of CO present, so measuring current flowing through the exte circuit is a basic gas monitor