An oxygen sensor is an electronic device that measures the proportion of oxygen in the gas being analyzed. The sensing element is usually made with a zirconium ceramic bulb coated on both sides with a thin layer of platinum and comes in both heated and unheated forms.

Automotive applications

In most modern automobiles, these sensors are attached to the engines exhaust manifold to measure the oxygen in the exhaust to determine whether the mixture of air and gasoline going into the engine is rich (too much fuel) or lean (too little fuel). This information is sent to the engine management ECU (Electronic Control Unit) computer, which adjusts the mixture to give the engine the best possible fuel economy and lowest possible exhaust emissions. It is often called a 'Lambda sensor' or even 'Lambda sond' in early cars.

Failure of these sensors, either through normal ageing or the use of leaded fuels, can lead to damage of an automobile's catalytic converter and expensive repairs.

Diving applications

A different type of oxygen sensor, which is sometimes called an oxygen analyser or ppO₂ meter, is used in SCUBA diving. They are used to measure the oxygen content of breathing gas mixes such as nitrox and trimix. They are also used within the oxygen control mechanisms of closed-circuit rebreathers to keep the partial pressure of oxygen within safe limits. This type of sensor operates by measuring the electricity generated by a small electro-galvanic fuel cell.