The measurement of oxygen requires a dedicated analyser. At low temperatures the paramagnetic method may be used and for high temperatures a zirconia detector is preferred.
For the measurement of oxygen concentrations at high temperatures the most common sensor is a zirconium oxide element, known as a zirconia cell or electrochemical sensor. The sensor has two platinum electrodes which are separated by a porous ceramic layer. One of the electrodes is placed in the gas to be analysed and the other reference electrode remains in ambient air. It develops a voltage difference across two electrodes when there is a difference in oxygen concentration between them.
The correct mixing of fuel and air in automotive internal combustion engines is critical to achieve fuel efficiency and minimise automotive emissions. Operation at close to the 1:1 stoichiometric oxygen to fuel ratio is the target for steady state operation of the engine. An excess of fuel is used during cold starts and is referred to as rich, when exhaust emissions will be high in CO and hydrocarbons. An excess of air is referred to as lean and can result in jerky performance of the car and engine, which may result in mechanical damage if allowed to continue over an extended period of time.
Control of the engine fuel-to-air ratio in the automotive industry is made by an oxygen sensor known as a lambda sond. It is located in the hot engine exhaust gases. The first lambda sond designed to measure the oxygen content of exhaust gases in a passenger car was produced by Bosch in 1976. Within 30 years after the first commercialisation, Bosch had produced almost 500 million lambda sensors. Production of engine sensors requires the use of high precision and low blend tolerance specialty gases calibration mixtures.
In modern cars it is possible to switch between similar fuels. For example, in Australia many cars are fitted with a dual fuel option for petrol or LPG. Also, at most pumps it is possible to select regular unleaded petrol or the E10 fuel grade, which contains 10% ethanol. So considering the list below, it is clear the range of air to fuel ratios changes as the fuel composition changes. This is one reason that the engine control unit and the lambda sond have an important job to do. There is no real way to predict the correct fuel-to-air injection ratio and a feedback process control loop is therefore required.
Air-to-fuel ratio (by mass) for 1:1 stoichiometric combustion:
In recent years the zirconia oxygen sensor has become an industry standard for online measurement of flue gases. Paramagnetic measurement of oxygen is an alternative online technique which is not suitable for the high temperatures in the combustion exhaust gases because they are sensitive to both temperature and moisture. The option to cool the gas in a side stream would theoretically exist, but since flue gases often contain between 10 and 20% of water the sample composition would be dramatically altered as the water condenses.
The stoichometry in a combustion furnace is always run slightly lean, meaning that there will be an excess of oxygen. For clean burning fuels such as natural gas, the excess of oxygen may be as low as 10%. For fuel oil, the oxygen excess might be 15 to 20% and for coal, closer to 20 or 30%. The target oxygen concentration in the exhaust gases is therefore between 2 and 6% oxygen. A typical flue gas process control measurement sensor will be designed to measure up to 20% oxygen and will typically be calibrated with a gas mixture of 8% oxygen in a balance of nitrogen.
The paramagnetic principle of operation to measure oxygen concentration relies on the paramagnetic susceptibility of the oxygen in the sample gas. The paramagnetism of oxygen is significantly higher than other common gases. This operation principle is one of the most accurate and reliable procedures to determine the oxygen concentration in a gas mixture from 0 to 100%. A paramagnetic measurement cell can be constructed with a small stagnant volume meaning that a fast response time is possible. The technique also has a low drift, absolute linearity and the negligible cross sensitivity to other sample gas components. With proper sample conditioning and good control of pressure, the cell should never need replacing.
Inside the measurement cell there is a nitrogen-filled dumbbell mounted in a magnetic field. The presence of oxygen in the measurement cell causes a rotation of the dumbbell. This rotational torque is detected and provides an electrical current which is linearly proportional to the oxygen concentration.
Paramagnetic oxygen analysers have found many applications in petrochemical processing and power generation.
Calibration of the paramagnetic oxygen analyser man be achieved using ambient air. However, for maximum accuracy, a span gas at 75 to 100% of the target measurement value is recommended. The analyser also requires a zero setting, for which Nitrogen 5.0 grade is ideal.