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In a previous article entitled "How an oxygen analyser works", we detailed and explained the principles of the technologies most commonly used in gas analysis to determine the oxygen concentration of a gas mixture.
We reviewed the following oxygen measurement technologies:
Depending on the measurement method used, each oxygen analyser has its advantages and disadvantages. In the previous article, we also described the advantages and disadvantages of each technology.
Oxygen analyser is the most widely used gas analyser in industry and research. The range of applications is therefore extremely diverse. In fact, an oxygen analyser is used whenever measurement of oxygen concentration is important to guarantee the quality, safety or efficiency of a product or process.
Oxygen analysers are used, for example, to control breathing air in the cockpit of an aircraft, to regulate combustion in a waste incinerator, to measure the amount of oxygen in vacuum-packed foodstuffs, or to prevent the risk of explosion by measuring the oxygen content in oil storage tanks.
All these applications call for different rules and methods of installation for the measuring instruments.
Once you are aware of the different oxygen analysis technologies available on the market, the next step is to choose the right oxygen analyser for your application. The aim of this article is to list and describe the criteria to be evaluated when making this choice.
Criteria N°1 : Oxygen concentration level and performance
Criteria N°2 : Overall composition of the gas mixture to be analyzed
Criteria N°3 : Ambient conditions and site constraints
Criteria N°4 : Utilities available on site
Criteria N°5: Allocated budgets
The choice of oxygen analyser in terms of on-board technology will depend in particular on the level of oxygen concentration in the gas mixture to be analyzed, and the measurement performance required.
For very low oxygen levels (below 1%, or at ppm level, for "parts per million"), analysis by gas chromatography will often be required, but some electrochemical oxygen analysers , zirconia oxygen analysers , and some Laser oxygen analysers are also capable of this.
For higher oxygen levels (from 1 to 21%, or even higher), the most widely used oxygen analyser is the paramagnetic oxygen analyser . Zirconia oxygen analyser and electrochemical oxygen analyser are also widely used to measure oxygen levels between 0% and 25%.
It's important to choose the right technology to ensure accurate and reliable measurements according to the oxygen concentration level concerned.
Each technology has its own specific features in terms of metrological performance.
And although measurement accuracies are relatively close, it is notable that laser oxygen analyser stands out for its very fine resolution and wider dynamic range than competing technologies. Later in this article, we'll also look at the greater calibration stability of this technology, and the benefits this brings to the user.
However, the oxygen analysers most commonly used to measure oxygen in flue gases for regulatory purposes in air emissions control, for example, remain those employing zirconia and paramagnetic technologies. For this reason, the vast majority of TÜV QAL1-certified oxygenanalysers for these applications are based on these technologies.
When choosing an oxygen analyser , in addition to the concentration level of the oxygen itself, it is important to consider the overall composition of the gas mixture being analyzed.
Paramagnetic gas analysers and laser gas analysers are renowned for being the most "gas matrix independent". In other words, laser and paramagnetic technologies are the least sensitive to cross-interference. In the vast majority of applications, the measurements of a paramagnetic gas analyser and a laser gas analyser will not be affected by the presence of any other gaseous compound in the mixture.
On the other hand, the use of a zirconia oxygen analyser should be avoided when the mixture being analyzed contains high levels of sulfur compounds, or if it is flammable. In the former case, the zirconia sensor would be prematurely degraded, and in the latter, measurement would be totally inhibited.
It's also worth choosing a laser oxygen analyser for corrosive gas mixtures, provided that the laser gas analyser is of the in-situ flow-through type. In this case, there is no contact between the corrosive gas mixture to be analyzed and the analyser's components, which are protected by air or nitrogen purging.
Finally, in addition to the corrosiveness of the gas mixture to be analyzed, it may also be heavily laden with solid particles. The most traditional gas analysers , such as paramagnetic or electrochemical oxygen analysers , should also be avoided, as they are generally designed to receive clean gases. On the other hand, here too, the specific features of through-flow laser gas analyser will enable measurement in a very dusty gas matrix.
When choosing an oxygen analyser for a given application, the most delicate criterion is certainly the environment and installation constraints.
The first step is to decide whether to install an extractive oxygen analyser or an in situ oxygen analyser . We have previously produced a carousel presenting the elements to be considered when choosing between an in situ and an extractive gas analyser : See the carousel.
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One or other of these configurations will be favored according to criteria of size and accessibility, ambient conditions, required performance, maintainability, budgets and solution life cycle.
Generally speaking, in-situ oxygen analyser is preferable when little space is available near the measurement point. However, attention must be paid to the ambient conditions at the point of measurement. We're talking here about constraints in terms of vibration, temperature, explosive zones, or the presence of a strong magnetic field.
Most oxygen analysis technologies come in in situ and extractive versions.
However, some of these technologies are better suited to one or other of these configurations.
The barbell-type paramagnetic analyser with optical detection is better suited to an extractive configuration, notably because its barbells require particular care in terms of the measurement environment. For example, vibrations in the industrial process can be avoided by moving theanalyser away from the measurement point.
The zirconia oxygen analyser and the laser oxygen analyser are both classically used in both configurations.
However, the benefits of laser oxygen analyser are much better exploited when it is mounted in situ and traversing. In fact, as we saw above, permanent purging of the optics enables direct, maintenance-free analysis, with a very short response time. However, attention must also be paid to the possible presence of low-frequency vibrations, which could disrupt the alignment of the optics.
Finally, electrochemical oxygen analyser is used almost exclusively in protected environments in extractive mode.
Depending on the technology employed, an oxygen analyser may require utilities such as a power supply or reference gas.
When choosing an analysis technology, it is therefore important to carefully consider both the gas analyser requirements and the utilities that can be made available.
Except in the case of a battery-powered portable instrument, an oxygen analyser always requires a power supply.
In most cases, it can be plugged directly into the local AC mains (115-230 VAC), but will require a converter if it is to be supplied with DC voltage (usually 24VDC).
In the majority of projects, therefore, and whatever the technology employed, it is necessary to route a suitable power supply to the point of installation of the oxygen analyser .
On the other hand, different technologies require different utility gases.
All oxygen analysers will need standard gases to calibrate their zero and span, at more or less frequent intervals depending on the technology.
Paramagnetic and electrochemical gas analysers will need to be recalibrated on a daily, weekly or even monthly basis, depending on the measurement drift you wish to allow, and therefore the accuracy of measurement you wish to maintain. Cylinders of standard gas will then have to be installed "permanently", whether you're working with manual or automatic calibration, with a dedicated gas injection system using solenoid valves.
For both these technologies, the zero calibration gas must be an oxygen-free gas: in most cases, pure nitrogen. However, a mixture containing a nitrogen base and a few ppm of another component used to calibrate a second analyser is also possible.
For example, if the plant is equipped with an infrared analyser measuring CO (carbon monoxide) between 0 and 1000 ppm, and a paramagnetic oxygen analyser measuring between 0 and 21%, the same bottle containing 900 ppm CO in nitrogen can be used for both oxygen zero and CO span calibration.
Whatever the technology used for oxygen analyser , the scale calibration gas should have an oxygen content close to the full scale of theanalyser. If the measurement scale is 0-21%, a cylinder containing 20% oxygen in nitrogen, for example, can be used.
For reasons of both economy and ease of operation, ambient air is often used as the calibration gas. At low altitudes, the air we breathe contains a stable oxygen content of around 21%. However, attention must be paid to variations in altitude on the one hand, and in air humidity on the other, so as not to risk distorting measurements through erroneous calibration operations when oxygen levels vary for these reasons.
Please note that in the case of the zirconia oxygen analyser , and only in this case, the zero calibration gas must not be oxygen-free, but must contain a small amount of oxygen. If the zirconia oxygen analyser measures on a scale from 0 to 21%, the zero calibration gas should contain, for example, 1 to 2% oxygen.
Zirconia oxygen analyser and laser oxygen analyser often benefit from greater calibration stability. Calibration periods can be as long as 6 months, or even a year in the case of Laser technology. In this case, it is not necessarily necessary to keep large-capacity standard bottles permanently close to theanalyser. Smaller, portable bottles can be used from time to time.
In addition to standard gases, which are used to calibrate analysers, some oxygen analysers also require the application of a reference gas. This is particularly true of the paramagnetic mass micro flow meter analyser . In fact, to operate, it needs a permanent injection of a small flow of nitrogen or air, depending on the measurement scale selected.
As for laser oxygen analyser , as we have already seen, they require a permanent purge gas to guarantee perfect metrology and clean transmitter and receiver optics, if they are used in a cross-stack in-situ version. Depending on process gas temperatures, this purge gas may be air or nitrogen.
It is very important to anticipate these utility requirements, firstly because they are essential to the operation of the oxygen analysers concerned. If the utilities are not installed before the instrument is commissioned, it will not be possible to operate it. On the other hand, the installation of these utilities often represents a not inconsiderable cost in the primary phase of the project, but also an operating cost to be taken into account when choosing the type of oxygenanalyser .
For any project, the budgeting phase is crucial. Technical requirements guide project teams and purchasing departments. But the reverse is also true, as the budget allocated to the project or module in question will also have an impact on the latitude available to the engineer to design the required solution.
The costs to be taken into account are purchase costs and operating costs.
Purchase costs include the cost of purchasing the oxygen analyser , as well as the cost of installation, commissioning and commissioning of the new equipment.
The utilities required to operate the oxygen analyser also represent a purchase cost. Their installation will require not only the supply of equipment, but also the provision of installation services. These range from simple logistics to potentially heavy civil engineering and boiler-making work, often involving scaffolding at height.
The cost of purchasing an oxygen analyser varies according to the technology chosen. Electrochemical and zirconia oxygen analysers are generally the least expensive. Next come paramagnetic oxygen analysers , whose technology is slightly more expensive. Finally, laser oxygen analysers require a higher purchase price.
However, medium- to long-term project planning will often show a rebalancing of budgets when taking into account not only the purchase costs, but also the operating costs of oxygen analyser .
For example, an electrochemical oxygen analyser that is financially more attractive at the time of purchase will have a substantial operating budget, as it will require regular replacement of its measuring cell. More regular and potentially more extensive maintenance will also be required to ensure that the analyzer remains in irreproachable operating conditions from a metrological point of view. In the case of extractive oxygen analyser , sampling elements such as filters, pumps and dryers will have to be maintained or even replaced on a regular basis. This is not just preventive maintenance, but also corrective maintenance.
The same level of analysers maintenance will be required for paramagnetic oxygen analysers with optical detection (dumbbell type). Even if the cell is considered permanent, it is relatively fragile and will eventually need to be replaced, at a relatively high cost.
The more robust paramagnetic oxygen analyser with mass microflowmeter does not require cell replacement, but the application of a reference gas, the operating cost of which must be taken into account in the overall calculation. Always used in an extractive analysis system, the sampling elements must be maintained in the same way.
In-situ zirconia oxygen analysers analyser installed directly on the industrial process, pipe, chimney, furnace, etc.) require very little maintenance. What's more, provided they are carefully selected and installed, they are often robust and usually have a long service life.
When zirconia oxygen analysis technology is used in extractive analyser , this same robustness remains an asset in terms of reduced maintenance operations, but the sampling system still needs to be serviced.
The laser oxygen analyser , if it's of the extractive type, will have the same constraints, and therefore the same maintenance costs, as any extractive gas analyser . However, since its calibration is more stable, it will require fewer calibration operations and, logically, less consumption of standard gas.
If the laser oxygen analyser is mounted in situ on either side of a pipe, furnace or chimney, maintenance operations will be rare and rapid, and the cost of the power supply negligible, compared with an extractive analysis system, which often consumes a lot of electricity. On the other hand, the operating budget will have to take into account the cost of permanent consumption of purge gas, whether nitrogen or dried and de-oiled compressed air.
Once the technical requirements have been defined, it is important to evaluate not only the cost of purchasing and installing the oxygen analyser , but also the operating costs. These will be linked to the proper operation of the equipment throughout the project, and even at the end of the product's life cycle. These latter components will be increasingly high in the years to come, due to changes in the cost of labor, energy and raw materials.
In this article, we have reviewed and detailed the 5 main criteria to be taken into account when choosing an oxygen analyser : the oxygen concentration sought and the metrological performance required, the gas matrix to be analyzed, the measurement environment, the utilities required and the relative costs.
Although this list is not exhaustive, it does provide a basis for a multi-criteria study, enabling project teams or operations managers to choose the solution best suited to their needs.
| Criteria | Description | Recommended technology |
| Concentration and Performance | Technology adapted to different levels (ppm or % oxygen) and precision requirements. | Laser for ppm, Paramagnetic/Zirconia for %. |
| Gas mixture composition | Impact of interfering gases on measurement. | Laser/Paramagnetic for matrix independence |
| Environmental conditions and constraints | Harsh environment (temperature, vibrations) and space. | Zirconia for robustness, Laser in situ for vibrations |
| Available utilities | Power supply and calibration gas required. | Zirconia/Paramagnetic for extractive configuration |
| Budget | Initial cost and long-term operating expenses. | Electrochemical for low budget, Laser for low maintenance |
Vincent MECKLER
Technical manager - Gas analysis expert
