The automotive, chemical, petrochemical or metallurgical industries, scientific and medical research, energy production and environmental protection... Here are some of the major areas involving a wide range of technologies, especially in terms of instrumentation and gas analysis.
These include traditional industries, from food processing to cement and refining. We are increasingly involved in new energy sectors, such as anaerobic digestion or fuel cells, among other examples.
Recent activities often require innovative, more efficient measurement technologies. But our traditional industries, at the same time, are rapidly evolving to keep pace. At the crossroads of these paths, the instrumentation and gas analysis technologies available on the market are just as varied.
Gas analysis is one of the tools in the broad field of industrial instrumentation. Today, gas analysers can be found in every field. They play an active role in raising manufacturing quality levels and optimizing the efficiency of industrial processes. The stakes are generally both economic and ecological.
Oxygen analysers are by far the most widely established gas analysers in industry and research. But the technologies used and implementation methods vary widely from one application to another.
These technologies have come a long way in the last two decades. Here we review the oxygen measurement principles in industry and research today. We will point out the advantages and disadvantages of each oxygen analysis technology.
In reality, there are two distinct detection principles, and therefore two equally different types of paramagnetic oxygen analyser.
Each exploits, as the name suggests, the "paramagnetic" property, or "magnetic susceptibility", of the oxygen molecule.
Dumbbell-type paramagnetic oxygen analysers use optical detection. The small balls of the dumbbell are filled with nitrogen (N2). When oxygen enters the cell, through which a magnetic field is passed, the dumbbell tends to rotate, and the mirror located in the middle of the axis reflects a different light signal from the one at rest. This signal difference, once processed, is proportional to the partial pressure of oxygen in the cell, and therefore, once the analyser has been calibrated, to the oxygen concentration in % by volume.
When the gas sample is placed in a magnetic field, oxygen molecules are attracted. The resulting pressure is detected by a mass-micro-flow meter.
The O2 electrochemical cell is a kind of fuel cell. The oxygen from the sample to be analysed is diffused through the diaphragm. An electrical current flows between the 2 electrodes through an electrolyte. It also passes through a resistance and a thermistor at the terminals of which a voltage is measured. This voltage is proportional to the oxygen concentration.
The zirconia oxygen analyser uses the peculiarity of zirconium oxide (ZrO2) which, when heated to high temperature, behaves like a solid electrolyte in relation to oxygen ions. If the platinum electrodes attached to each side of the solid electrolyte are exposed to a different partial pressure of oxygen on each side, an electrochemical reaction occurs and an electromotive force is collected on the electrodes.
From a microscopic point of view, this electrochemical reaction occurs at the triple interface between solid electrolyte, electrode and oxygen.
High partial pressure side: O2 + 4e- >> 2O2- (ionization)
Low partial pressure side: 2O2- >> O2 + 4e- (molecularization)
The electromotive force generated (E) responds to the Nernst equation.
The laser gas analyser is more precisely known as Tunable Diode Laser (TDL) analyser. It is an optical instrument that uses an infrared laser beam starting from a transmitter and pointing at a receiver. The measurement technique relies on the absorption of light by the gas molecules present between the emitter and receiver.
Most gases absorb light at specific wavelengths, and absorption is a direct function of gas concentration.
The laser wavelength is analysed on a given absorption line, specific to the desired molecule, thus avoiding almost any interference from other molecules present. The measured gas concentration is therefore proportional to the amplitude of the absorption line.
