Oxygen sensor

[1] The original sensing element is made with a thimble-shaped zirconia ceramic coated on both the exhaust and reference sides with a thin layer of platinum and comes in both heated and unheated forms.

The most common application is to measure the exhaust-gas concentration of oxygen for internal combustion engines in automobiles and other vehicles in order to calculate and, if required, dynamically adjust the air-fuel ratio so that catalytic converters can work optimally, and also determine whether the converter is performing properly or not.

These include technologies such as zirconia, electrochemical (also known as galvanic), infrared, ultrasonic, paramagnetic, and very recently, laser methods.

Unburnt fuel is pollution in the form of air-borne hydrocarbons, while oxides of nitrogen (NOx gases) are a result of combustion chamber temperatures exceeding 1300 kelvins, due to excess air in the fuel mixture thereby contributing to smog and acid rain.

This demand causes the voltage output to rise, due to transportation of oxygen ions through the sensor layer.

Modern spark-ignited combustion engines use oxygen sensors and catalytic converters in order to reduce exhaust emissions.

The ECU attempts to maintain, on average, a certain air-fuel ratio by interpreting the information gained from the oxygen sensor.

The primary goal is a compromise between power, fuel economy, and emissions, and in most cases is achieved by an air–fuel ratio close to stoichiometric.

For spark-ignition engines (such as those that burn gasoline or autogas / liquefied petroleum gas (LPG), as opposed to diesel), the three types of emissions modern systems are concerned with are: hydrocarbons (which are released when the fuel is not burnt completely, such as when misfiring or running rich), carbon monoxide (which is the result of running slightly rich) and NOx (which dominate when the mixture is lean).

Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental to emissions control and can even damage the vehicle.

When the engine is under low-load conditions (such as when accelerating very gently or maintaining a constant speed), it is operating in "closed-loop mode".

If modifications cause the engine to run rich, then there will be a slight increase in power to a point (after which the engine starts flooding from too much unburned fuel), but at the cost of decreased fuel efficiency, and an increase in unburned hydrocarbons in the exhaust, which causes overheating of the catalytic converter.

Where applicable, gasoline, propane, and natural gas engines are fitted with three-way catalysts to comply with on road vehicle emissions legislation.

[citation needed] The sensor element is a ceramic cylinder plated inside and outside with porous platinum electrodes; the whole assembly is protected by a metal gauze.

An output voltage of 0.2 V (200 mV) DC represents a "lean mixture" of fuel and oxygen, where the amount of oxygen entering the cylinder is sufficient to fully oxidize the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2).

An output voltage of 0.8 V (800 mV) DC represents a "rich mixture", which is high in unburned fuel and low in remaining oxygen.

This is where the quantities of air and fuel are in the optimal ratio, which is ~0.5% lean of the stoichiometric point, such that the exhaust output contains minimal carbon monoxide.

The wideband zirconia sensor is used in stratified fuel injection systems and can now also be used in diesel engines to satisfy the upcoming EURO and ULEV emission limits.

Pre- and post-catalyst signals are monitored to determine catalyst efficiency, and if the converter is not performing as expected, an alert gets reported to the user through on-board diagnostics systems by, for example, lighting up an indicator in the vehicle's dashboard.

For heated sensors, normal deposits are burned off during operation, and failure occurs due to catalyst depletion.

Another common cause of premature failure of lambda probes is contamination of fuel with silicones (used in some sealings and greases) or silicates (used as corrosion inhibitors in some antifreezes).

Leaks of oil into the engine may cover the probe tip with an oily black deposit, with associated loss of response.

These sensors are buried at various depths to monitor oxygen depletion over time, which is then used to predict soil respiration rates.

The traditional way of measuring oxygen concentration in a water sample has been to use wet chemistry techniques e.g. the Winkler titration method.

[12] Oxygen sensors play a critical role in the production of Active Pharmaceutical Ingredients made in a bioreactor by cell culture or fermentation.

Oxygen enters the sensor through a permeable membrane by diffusion and is reduced at the cathode, creating a measurable electric current.

The oxygen consumption of such a microsensor is so small that it is practically insensitive to stirring and can be used in stagnant media such as sediments or inside plant tissue.

In a given oxygen concentration there will be a specific number of O2 molecules colliding with the film at any given time, and the fluorescence properties will be stable.

These type of electrode sensors can be used for in situ and real-time monitoring of oxygen production in water-splitting reactions.

Based on the same principle than optode probes, a digital camera is used to capture fluorescence intensities over a specific area.