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.
Oxygen gas measurement technologies
Gas analysis is one of the tools in the broad field of industrial instrumentation. Today, gas analysers are present in all fields. 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 used gas analysers in industry and research. But the technologies employed 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 most widely used 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 :
There are in fact two distinct detection principles, and therefore two equally different types of paramagnetic oxygenanalysers .
But each exploits, as the name suggests, the "paramagnetic" property, or "magnetic susceptibility", of the oxygen molecule.IEC 61207-3 formalizes the performance of paramagnetic oxygen analysers .
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 a magnetic field, the dumbbell tends to rotate, and the mirror in the middle of the axis returns a light signal different from that at rest. This signal difference, once processed, is proportional to the partial pressure of oxygen in the cell, and therefore, once theanalyser has been calibrated, to the oxygen concentration in % by volume.
The advantages of paramagnetic oxygen analyser with optical detection:
This method has a very good reputation.
Paramagnetic oxygen analyser with optical detection are often required by local regulations or trades.
The cell is compact.
It's a high-performance metrological tool, subject to very little interference from other gaseous compounds.
Disadvantages of paramagnetic oxygen analyser with optical detection:
It is, however, 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 microflowmeter.
The advantages of paramagnetic oxygen analyser with micro mass flow meter :
The paramagnetic oxygen analyser with mass microflowmeter is renowned for its robustness, as there is no contact between the sensing element and the gas to be measured, and no moving parts.
It's also fast.
It suffers little from cross-interference.
It benefits from a broad dynamic scale.
Disadvantages of the paramagnetic oxygen analyser with mass microflowmeter :
Paramagnetic oxygen analyser with a micro mass flow meter, on the other hand, requires the permanent use of a reference gas (often nitrogen).
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 advantages of electrochemical oxygen analyser :
Robust.
Compact design.
Inexpensive.
Low interference due to acid electrolyte.
Disadvantages of electrochemical oxygen analyser :
The electrochemical oxygen cell has the disadvantage of degrading with use. It should therefore be replaced periodically.
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)
The generated electromotive force (E) satisfies the Nernst equation.
The advantages of zirconia oxygen analyser :
Its robustness has been proven for decades.
The zirconia oxygen analyser is precise and stable, with calibration intervals of up to 6 months or even a year.
The sensing element is not a consumable 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 zirconia oxygen analyser :
The downside of heated technology: the analyser must be able to evacuate its calories. The extractive version is less easily installed in a multi-gas analyser , and is often the subject of a small external box, to be mounted separately within the system.
Due to the technology itself, based on high-temperature ionization, it is not possible to use a zirconia oxygen analyser to measure oxygen concentration in a fuel gas.
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.
The advantages of laser oxygen analyser :
Laser oxygen analyser offer unrivalled performance, particularly in terms of calibration stability, scale dynamics and cross-interference.
The in-situ version has the advantage of having no components in contact with the gas to be measured. It is therefore possible to analyze gases that are, for example, humid and/or very dusty.
Last but not least, the absence of moving parts makes it extremely robust.
The disadvantages of laser analyser :
The main disadvantage of laser oxygen analyser remains its price. Although this technology has become much more widely available over the past 25 years, it remains an expensive alternative to more traditional techniques.
In the in-situ version, it must also be permanently purged with an oxygen-free gas. In most cases, this will be nitrogen. Except at high temperatures, when the absorption line analyzed 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 oxygenanalyser will therefore systematically be the subject of a multi-criteria study.
