The automotive, chemical, petrochemical and metallurgical industries, scientific and medical research, energy production and environmental protection... these are just some of the major sectors that call on a wide range of technologies, particularly in terms ofgas instrumentation andanalysis.
These include traditional industries, from food processing to cement and refining. We are increasingly involved in new energy sectors, such as methanization and fuel cells, to name but a few.
New businesses often require innovative, more efficient measurement technologies. But our traditional industries, meanwhile, are rapidly evolving to keep pace. At the crossroads, the gas instrumentation and analysis technologies available on the market are just as varied.
Gas analysis is one of the tools used in the broad field of industrial instrumentation. Gas- analysers s are now found in every industry. They play an active role in improving manufacturing quality and optimizing the efficiency of industrial processes. The challenges involved are generally both economic and environmental.
Oxygen analysers s are, by far, the most widely used gas analysers s in industry and research. However, the technologies employed and installation 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 most widely used in industry and research today. We will point out the advantages and disadvantages of each oxygen analysis technology.
In fact, there are two distinct detection principles—and therefore two equally distinct types of paramagnetic oxygenanalysers .
But each one, as the name implies, takes advantage of the “paramagnetic” property—or “magnetic susceptibility”—of the oxygen molecule.IEC 61207-3 formalizes the specification of performance for paramagnetic oxygen analysers .
Paramagnetic “dumbbell”-type oxygen analysers s use optical detection. The small spheres of the dumbbell are filled with nitrogen (N₂). When oxygen enters the cell, which is subjected to a magnetic field, the dumbbell tends to rotate, and the mirror located at the center of the axis reflects a light signal different from that at rest. This difference in the signal, once processed, is proportional to the partial pressure of oxygen in the cell and, therefore—once theanalyser s calibrated—to the oxygen concentration in volume percent.
When the gas sample is placed in a magnetic field, oxygen molecules are attracted. The resulting pressure is detected by a mass microflowmeter.
The electrochemical O2 cell is a kind of fuel cell. Oxygen from the sample to be analyzed is diffused through the diaphragm. A current flows between the 2 electrodes through an electrolyte. It also passes through resistors and thermistors, across which a voltage is measured. This voltage is proportional to the oxygen concentration.
The zirconia oxygen analyser uses the special feature of zirconium oxide (ZrO2) which, when heated to high temperatures, behaves like a solid electrolyte with respect 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 takes place and an electromotive force is collected on the electrodes.
From a microscopic point of view, this electrochemical reaction takes place 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 generated electromotive force (E) satisfies the Nernst equation.
Laser analyser are more precisely known as Tunable Diode Laser (TDL) analyser . It's an optical instrument that uses an infrared laser beam directed from a transmitter to a receiver. The measurement technique relies on the absorption of light by the gas molecules present between the transmitter and receiver.
Most gases absorb light at specific wavelengths, and absorption is a direct function of gas concentration.
The laser's wavelength is analyzed on a given absorption line, specific to the molecule in question, thus virtually avoiding interference from other molecules present. The gas concentration measured is therefore proportional to the amplitude of the absorption line.
Vincent MECKLER
Technical manager - Gas analysis expert
