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.
Oxygen gas measurement technologies
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.
The most widely used oxygen measurement technologies are :
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.
Paramagnetic oxygen analyser with optical detection
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.
The advantages ofanalyser paramagnetic oxygen with optical detection:
This method has a very good reputation.
The optical sensing paramagnetic oxygen analyser is often required by local or business regulations.
The cell is compact.
It is a powerful metrological tool, and very little subject to interference from other gaseous compounds.
Disadvantages of analyser paramagnetic oxygen with optical detection:
However, it is relatively fragile due to the meticulousness of the dumbbell assembly.
Expensive to purchase and replace in the event of accidental damage.
Paramagnetic oxygen analyser with micro mass flow meter
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.
Advantages of theanalyser paramagnetic oxygen micro mass meter:
The paramagnetic oxygen analyser with micro mass flow meter is renowned for its robustness, as there is no contact between the detection sensor and the gas to be measured, and because it has no moving parts.
It is also fast.
It suffers little from cross-interference.
It benefits from a wide dynamic scale.
Disadvantages of theanalyser paramagnetic oxygen micro mass meter:
Theanalyser paramagnetic oxygen micro mass meter, on the other hand, requires the permanent use of a reference gas (often nitrogen).
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 advantages of electrochemical oxygen analysers :
Robust.
Compact.
Inexpensive.
Little subject to interferents thanks to the acid electrolyte used.
Disadvantages of electrochemical oxygen analysers :
The electrochemical oxygen cell has the disadvantage of degrading with use. It should therefore be replaced periodically.
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)
The electromotive force generated (E) responds to the Nernst equation.
Advantages of the zirconia oxygen analyser :
Its robustness has been proven for decades.
The zirconia oxygen analyser is precise and stable. Its calibration intervals are very spaces (up to 6 months or even a year).
The sensitive element is not a consumable part and has a lifespan of up to 5 or even 10 years.
The cost of the solution is relatively low.
The sensitive element is regulated at high temperature (800°C), so it can be used to analyze both cold and hot, humid gases.
Disadvantages of the zirconia oxygen analyser :
The flip side of a heated technology : the analyser must be able to evacuate its calories. In the extractive version, it will be less easily mounted in a multi-gas analyser, and will often be integrated into a small external housing, to be mounted separately within the system.
Because of the technology itself, based on high-temperature ionization, it is not possible to use a zirconia oxygen analyser to measure oxygen concentration in a combustible gas.
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.
The advantages of laser oxygen analysers :
The Laser oxygen analyser offers unmatched performance, including calibration stability, scale dynamics and cross-interference.
With the in-situ version, it has the advantage of having no element in contact with the gas to be measured. It is therefore possible to analyse gases that are, for example, wet and/or very dusty.
Finally, it has a certain robustness ue to the absence of moving parts.
Disadvantages of the laser analyser :
The main disadvantage of the Laser oxygen analyser remains its price. Although this technology has become much more democratized over the past 25 years, it remains an expensive alternative to more traditional techniques.
In the in-situ version, it must also be purged continuously with an oxygen-free gas. In most cases, this will be nitrogen. Except at high temperatures, when the absorption line analysed is specific above a certain temperature.
Let's study your needs together and choose the ideal oxygen analyser.
We have examined the four technologies for measuring oxygen gas, and presented the advantages and disadvantages of each. Of course, depending on the intended use, the budget available and the skills of the teams involved, one or other will be more appropriate. The choice of the ideal oxygen analyser will therefore systematically be the subject of a multi-criteria study.
