Oxygen sensor

An oxygen sensor is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analyzed. It was developed by Robert Bosch GmbH during the late 1960s under supervision by Dr. Günter Bauman. 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 planar-style sensor entered the market in 1998 (also pioneered by Robert Bosch GmbH) and significantly reduced the mass of the ceramic sensing element as well as incorporating the heater within the ceramic structure. This resulted in a sensor that both starting operating sooner and responded faster. The most common application is to measure the exhaust gas concentration of oxygen for internal combustion engines in automobiles and other v/ehicles. Divers also use a similar device to measure the partial pressure of oxygen in their breathing gas.

Scientists use oxygen sensors to measure respiration or production of oxygen and use a different approach. Oxygen sensors are used in oxygen analyzers which find a lot of use in medical applications such as anesthesia monitors, respirators and oxygen concentrators.

There are many different ways of measuring oxygen and these include technologies such as zirconia, electrochemical (also known as Galvanic), infrared, ultrasonic and very recently laser. Each method has its own advantages and disadvantages.

Automotive applications
Automotive oxygen sensors, colloquially known as an O2 sensors or lambda sensors make modern electronic fuel injection and emission control possible. They determine if the air/fuel ratio exiting a gas-combustion engine is rich(unburnt fuel vapor) or lean(excess oxygen). Closed loop feedback controlled fuel injection means that the fuel injector output is varied according to real-time sensor data rather than operating with a predetermined fuel map (open loop). In addition to improving overall engine operation, they reduce the amounts of both unburnt fuel and oxides of nitrogen from entering the atmosphere. Unburnt fuel is pollution in the form of air born hydrocarbons, while oxides of nitrogen(NOX gases) are a result of excess air in the fuel mixture resulting in smog and acid rain producing compounds. Volvo was the first automobile manufacturer to employ this technology, along with the 3-way catalyst.

This information is sent to the engine management ECU computer, which adjusts the mixture to give the engine the best possible fuel economy and lowest possible exhaust emissions. Failure of these sensors, either through normal aging, the use of leaded fuels, or due to fuel contamination with eg. silicones or silicates, can lead to damage of an automobile's catalytic converter and expensive repairs.

Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental. When the engine is under low-load conditions (such as when accelerating very gently, or maintaining a constant speed), the engine is operating under 'closed-loop mode'. This refers to a feedback loop between the fuel injectors, and the oxygen sensor, to maintain stoichiometric ratio. If modifications cause the mixture to run lean, there will be a slight increase in fuel economy, but with massive nitrogen oxide emissions, and the risk of damaging the engine due to detonation and excessively high exhaust gas temperatures. If modifications cause the mixture to run rich, then there will be a slight increase in power, again at the risk of overheating and igniting the catalytic converter, while decreasing fuel economy and increasing hydrocarbon emissions.

When an internal combustion engine is under high load (such as when using wide-open throttle) the oxygen sensor no longer operates (it works, but the signal isn't used to make adjustments in relation to fuel trim/control), and the engine automatically enriches the mixture to protect the engine. Any changes in the sensor output will be ignored in this state, while changes from the air flow meter can lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.

Function of a lambda probe
Lambda probes are used to reduce vehicle emissions, by ensuring that engines burn their fuel efficiently and cleanly. Robert Bosch GmbH introduced the first automotive lambda probe in 1976, and it was first used by Volvo and Saab in that year. The sensors were introduced in the US from about 1980, and were required on all models of cars in many countries in Europe in 1993.

By measuring the proportion of oxygen in the remaining exhaust gas, and by knowing the volume and temperature of the air entering the cylinders amongst other things, an ECU can use look-up tables to determine the amount of fuel required to burn at the stoichiometric ratio (14.7:1 air:fuel by mass for gasoline) to ensure complete combustion.

The probe
The sensor element is a ceramic cylinder plated inside and out with porous platinum electrodes; the whole assembly is protected by a metal gauze. It operates by measuring the difference in oxygen between the exhaust gas and the external air, and generates a voltage or changes its resistance depending on the difference between the two. The sensors only work effectively when heated to approximately 300°C, so most newer lambda probes have heating elements encased in the ceramic to bring the ceramic tip up to temperature quickly when the exhaust is cold. The probe typically has four wires attached to it: two for the lambda output, and two for the heater power, although some automakers use a common ground for the sensor element and heaters, resulting in three wires. Earlier non-electrically-heater sensors had one or two wires.

Zirconia sensor
The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. An output voltage of 0.2 V (200 mV) DC represents a lean mixture. That is one 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). A reading of 0.8 V (800 mV) DC represents a rich mixture, one which is high in unburned fuel and low in remaining oxygen. The ideal point is 0.45 V (450 mV) DC; this is where the quantities of air and fuel are in the optimum ratio, called the stoichiometric point, and the exhaust output will mainly consist of fully oxidized CO2.

The voltage produced by the sensor is so nonlinear with respect to oxygen concentration that it is impractical for the electronic control unit (ECU) to measure intermediate values - it merely registers "lean" or "rich", and adjusts the fuel/air mixture to keep the output of the sensor alternating equally between these two values.

This type of sensor is called 'narrow band', referring to the narrow range of fuel/air ratios to which the sensor responds. The main disadvantage of narrow band sensors is their slow response: the control unit determines the exhaust gas composition by averaging the high and low swings in the sensor's output, and this process creates an inevitable delay (this statement may be misleading - see discussion).

Wideband zirconia sensor
A variation on the zirconia sensor, called the 'wideband' sensor, was introduced by Robert Bosch in 1994 but is (as of 2006) used in only a few vehicles. It is based on a planar zirconia element, but also incorporates an electrochemical gas pump. An electronic circuit containing a feedback loop controls the gas pump current to keep the output of the electrochemical cell constant, so that the pump current directly indicates the oxygen content of the exhaust gas. This sensor eliminates the averaging delay inherent in narrow band sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more rapidly. In the automotive industry this sensor is also called a UEGO (for Universal Exhaust Gas Oxygen) sensor. UEGO sensors are also commonly used in aftermarket dyno tuning and high performance driver A/F display equipment. Wideband zirconia sensor is used for Stratified Fuel Injection systems and for the first time can be used for diesel engines as essential ECU's feedback sensor for oxygen content in exhaust gas in the next EURO and ULEV emission stages.

Titania sensor
A less common type of narrow band lambda sensor has a ceramic element made of titanium dioxide (titania). This type does not generate its own voltage, but changes its electrical resistance in response to the oxygen concentration. The resistance of the titania is a function of the oxygen partial pressure and the temperature. Thus, for some sensors there is a gas temperature reading to compensate for the resistance change due to temperature. The resistance value at any temperature is ~1/1000th the change in oxygen concentration. Luckily, at lambda = 1, there is a large change of oxygen, so the resistance change is tyically 1000 times between rich and lean, depending on the temperature. Being an N-type semi-conductor with a structure TiO2-x, the x defects in the crystal lattice conduct the charge. So, for fuel rich exhaust, the resistance is low, and for fuel lean exhaust the resistance is high. The control unit feeds the sensor with a low-current five volt supply and measures the resulting voltage across the sensor. Like the zirconia sensor, this type is so nonlinear that in practice it is used simply as a binary "rich or lean" indicator. Titania sensors are more expensive than zirconia sensors, but they also respond faster. In automotive applications the Titania sensor, unlike the zirconia sensor does not require a reference sample of atmospheric air to operate properly. This makes the sensor assembly easier to design against water contamination. While most automotive sensors are submersible, zirconia-based sensors require a very small supply of reference air from the atmosphere. In theory, the sensor wire harness and connector are sealed. Air that leaches through the wire harness to the sensor is assumed to come from an open point in the harness - usually the ECU which is housed in an enclosed space like the trunk or vehicle interior.

Location of the probe in a system
The probe is typically screwed into a tapped hole in the exhaust, located after the branch manifold of the exhaust system combines, and before the catalytic converter. New vehicles are required to have a sensor before and after the exhaust catalyst to meet U.S. regulations requiring that all emissions components be monitored for failure. Pre and post-catalyst signals are monitored to determine catalyst efficiency. Additionally, some catalyst systems require brief cycles of lean (oxygen-containing) gas to load the catalyst and promote additional oxidation reduction of undesirable exhaust components.

Sensor surveillance
The air-fuel ratio and naturally, the status of the sensor, can be monitored by means of using an air-fuel ratio meter that displays the read output voltage of the sensor.

Sensor failures
Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles. Heated sensor lifetime is typically 100,000 miles. The failure for unheated sensors is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burned off during operation and failure occurs due to catalyst depletion, similar to the reason a battery stops producing current. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the mileage worsens.

Leaded gasoline will contaminate the oxygen sensors and catalytic converters. Most oxygen sensor are rated for some service life in the presence of leaded gasoline but sensor life will be shortened to as little as 15,000 miles depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty.

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). In this case, the deposits on the sensor are colored between shiny white and grainy light gray.

Leaks of oil into the engine may lead to probe tip covered with oily black deposit, with associated loss of response.

An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel rationing system.

Applying external voltage on the zirconia sensors, eg. by checking them with an ohmmeter, may damage them. 

Diving applications


The diving 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 concentration 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.

A company based out of the United Kingdom performed a 6 year study of oxygen sensors and their performance in the diving industry related to safety and reliability. Some interesting facts for the diving rebreather market: http://www.deeplife.co.uk/or_files/DV_O2_cell_study_070329.pdf

Scientific applications
In marine biology or limnology oxygen measurements are usually done in order to measure respiration of a community or an organism, but it has also been used as a method to measure primary production of algae. The traditional way of measuring oxygen concentration in a water sample has been to use wet chemistry techniques e.g. the Winkler titration method. There are however commercially available oxygen sensors that will measure the oxygen concentration in liquids with great accuracy. There are two types of oxygen sensors available: electrodes (electrochemical sensors) and optodes (optical sensors).

Electrodes


The Clark-type electrode is the most used oxygen sensor for measuring oxygen dissolved in a liquid. The basic principle is that there is a cathode and an anode submersed in an electrolyte. Oxygen enters the sensor through a permeable membrane by diffusion, and is reduced at the cathode, creating a measurable electrical current.

There is a linear relationship between the oxygen concentration and the electrical current. With a two-point calibration (0% and 100% air saturation), it is possible to measure oxygen in the sample.

One drawback to this approach is that oxygen is consumed during the measurement with a rate equal to the diffusion in the sensor. This means that the sensor must be stirred in order to get the correct measurement and avoid stagnant water. With an increasing sensor size, the oxygen consumption increases and so does the stirring sensitivity. In large sensors there tend to also be a drift in the signal over time due to consumption of the electrolyte. However, Clark-type sensors can be made very small with a tip size of 10 µm. 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.

Optodes
An oxygen optode is a sensor based on optical measurement of the oxygen concentration. A chemical film is glued to the tip of an optical cable and the fluorescence properties of this film depend on the oxygen concentration. Fluorescence is at a maximum when there is no oxygen present. When an O2 molecule comes along it collides with the film and this quenches the photoluminescence. 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.

The signal (fluorescence) to oxygen ratio is not linear, and an optode is most sensitive at low oxygen concentration. That is, the sensitivity decreases as oxygen concentration increases following the Stern-Volmer relationship. The optode sensors can, however, work in the whole region 0% to 100% oxygen saturation in water, and the calibration is done the same way as with the Clark type sensor. No oxygen is consumed and hence the sensor is insensitive to stirring, but the signal will stabilize more quickly if the sensor is stirred after being put in the sample.