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 advantages and disadvantages. We also described in the previous article what the advantages and disadvantages of each technology were.
The oxygen analyser is the most widely used gas analyser in industry and research. As a result, the range of applications is therefore extremely diverse. This is because an oxygen analyser is used whenever the measurement of oxygen concentration is important to ensure the quality, safety or effectiveness of a product or process.
For example, oxygen analysers are used to monitor breathing air in the cockpit of an aircraft, to regulate the combustion of waste incinerator, to measure the amount of oxygen in vacuum food packaging, or to prevent the risk of explosion by measuring oxygen content in oil storage tanks.
These are all applications that require various rules and methods of implantation of 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 purpose of this article is to list and describe the criteria to be evaluated to make this choice.
Criteria N°1 : The level of oxygen concentration concerned
Criteria N°2 : The overall composition of the gas mixture to be analysed
Criteria N°3 : Environmental conditions and implantation constraints
Criteria N°4 : Utilities available on site
Criteria N°5: Allocated budgets
The choice of oxygen analyser in terms of technology will depend in particular on the level of oxygen concentration within the gas mixture to be analyzed, and on the required measurement performance.
For low oxygen levels (below 1%, or at ppm level, for "parts per million"), gas chromatography analysis will often be required, but some electrochemical oxygen analysers, zirconia oxygen analysers, and some Laser oxygen analysers are also capable of it.
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 is important to choose the right technology to ensure accurate and reliable measurements depending on the level of oxygen concentration involved.
Each technology also has its specificities in terms of metrological performance.
And although the measurement accuracy is relatively close, it is notable that the Laser oxygen analyzer stands out for its very fine resolution and wider dynamic range than for competing technologies. We will also see later in this article the greater calibration stability of this technology, and the benefits it provides 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 oxygen analysers certified QAL1 by TÜV for these applications are based on these technologies.
When choosing an oxygen analyser, in addition to the concentration level of oxygen itself, it is important to consider the overall composition of the analyzed gas mixture.
Paramagnetic gas analysers and Laser gas analysers are known to be the most "independent of the gas matrix". In other words, Laser and paramagnetic technologies are the least sensitive to cross-interference. In a very large 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.
Conversely, the use of a zirconia oxygen analyser should be avoided when the analyzed mixture contains large quantities of sulfur compounds, but also if it is flammable. Indeed, in the first case, the zirconia sensor would be prematurely degraded, and in the second, the measurement would be totally inhibited.
It will also be interesting to choose a Laser oxygen analyser in case of corrosive gas mixture, provided that the Laser gas analyser is of the in situ cross-stack type. In this case, there is no contact between the corrosive gas mixture to be analyzed and the components of the analyser since a purge with air or nitrogen protects the latter.
Finally, in addition to the corrosivity of the gas mixture to be analyzed, it can also be highly loaded with solid particles. The more traditional gas analysers such as paramagnetic or electrochemical oxygen analysers should also be avoided as they are generally designed to receive gases which are considered clean. Conversely, here too, the specificities of the so-called cross-stack Laser gas analyser will allow measurement in a very dusty gas matrix.
When choosing an oxygen analyser for a given application, the most delicate criterion to understand is certainly that of the environment and installation constraints.
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Either of these configurations will be favored based on the criteria of size and accessibility, environmental conditions, required performance, maintainability, budgets, and the life cycle of the solution.
In general, an in situ oxygen analyser will be preferred when space is limited near the measuring point. But it will then be necessary to pay attention to the ambient conditions at this place. We are talking here about constraints in terms of vibrations, temperatures, explosive zones, or the presence of a strong magnetic field.
Most oxygen analysis technologies come in situ and extractive versions.
However, some of these technologies are more suitable for either of these configurations.
The dumbbell-type optical sensing paramagnetic analyser is more suitable for an extractive configuration, especially because of its dumbbells that require special care in terms of measurement environment. For example, vibrations in the industrial process will be avoided by deporting the analyser from the measuring point, making it extractive in fact.
Both the zirconia oxygen analyser and the Laser oxygen analyser are very conventionally used in both configurations.
The benefits of the Laser oxygen analyser are much better used when it is mounted in situ and through. Indeed, as we have seen above, the permanent purging of optics allows a direct analysis, without maintenance, and with a very short response time. However, attention should also be paid to the possible presence of low-frequency vibrations, which may disturb the alignment of the optics.
Finally, the electrochemical oxygen analyser is almost exclusively used in protected environments in extractive mode.
Depending on the technology used, an oxygen analyser may require utilities such as a power supply or reference gas.
It is therefore important when choosing the analysis technology to check the needs of the gas analyser on the one hand, and the utilities that can be made available on the other hand.
Except in the case of a portable instrument equipped with a battery, an oxygen analyser always requires a power supply.
It will most of the time be able to connect directly to the local AC network (115-230 VAC) but may require a converter in the case of a device supplied with DC voltage (24VDC in general).
It is therefore necessary, in the majority of projects, regardless of the technology used, to route a suitable power supply to the oxygen analyser installation place.
On the other hand, from one technology to another, the utility gases required are different.
All oxygen analysers will need calibration gas in order to calibrate their zero and span, at frequencies depending on the technology.
Paramagnetic and electrochemical gas analysers will need to be recalibrated daily, weekly, or even monthly, depending on the measurement drift you want to allow, and therefore the measurement accuracy you want to keep. Standard gas cylinders will then have to be installed permanently, whether manual calibration is used or automatically with a dedicated gas injection system by solenoid valves.
For both technologies, the zero calibration gas will have to be an oxygen-free gas: mostly pure nitrogen. However, a mixture containing a nitrogen base and a few ppm of another component used for the calibration of a second analyser may also be possible.
For example, if the installation is equipped with an infrared analyser measuring CO (carbon monoxide) between 0 and 1.000 ppm, and a paramagnetic oxygen analyser measuring between 0 and 21%, the same cylinder containing 900 ppm CO in nitrogen will be possible to use both for zero oxygen calibration and for CO span calibration.
Regardless of the technology used for the oxygen analyser, the span calibration gas shall be an oxygen content close to the full scale of the analyser. If the measurement range is 0-21%, a cylinder containing 20% oxygen in nitrogen, for example, can be used.
For reasons of both economics and ease of operation, ambient air will also often be used as a span calibration gas. Indeed, at low altitude, the air we breathe contains a stable amount of about 21% oxygen. But attention should be paid to variations in altitude on the one hand, and air humidity on the other, so as not to risk distorting measurements via erroneous calibration operations in case of variable oxygen levels for these reasons.
It should also be noted that in the case of zirconia oxygen analysers, 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.
The zirconia oxygen analyser and the Laser oxygen analyser also often benefit from greater calibration stability. Calibration periods can be up to 6 months, or even one year in the case of Laser technology. In this case, it is not absolutely necessary to maintain large capacity standard cylinders permanently in the vicinity of the analyser. A smaller bottle, or even portable, can be used, punctually.
In addition to calibration gases, which, as their name suggests, are used to calibrate analysers, some oxygen analysers also require the application of a reference gas. This is particularly the case for the paramagnetic oxygen analyser with micro mass flow meter. Indeed, the latter needs to operate a permanent injection of a small flow of nitrogen or air, according to the selected measurement ranges.
As for the Laser oxygen analyser, as we have seen above, it will need a permanent purge gas to guarantee perfect metrology and cleanliness of the optics of the transmitter and receiver, if it is in situ version ("cross-stack"). Depending on the temperatures of the process gas, this purge gas may be air or nitrogen.
It is very important to anticipate these utility needs, firstly because they are essential to the operation of the oxygen analysers concerned. If the uses are not put in place before the commissioning of the instrument, the latter can not be exploited. On the other hand, the implementation of these utilities often represents a significant cost in the primary phase of the project, but also an operating cost to be taken into account when choosing the type of oxygen analyser.
For any project, the budgeting phase is decisive. Technical requirements guide project teams and procurement departments. But the opposite is also true, as the budget allocated to the project or module concerned will also have an impact on the latitude that the engineer will have to design the desired solution.
The costs to be considered are purchase costs and operating costs.
The purchase costs are themselves composed of the costs related to the acquisition of the oxygen analyser, and the costs related to the installation work, commissioning, and handling of the new equipment.
The utilities necessary for the operation of the oxygen analyser also represent a purchase cost since their installation will require not only purchases of equipment, but also installation services, ranging from simple logistics to potentially heavy civil engineering work, boilermaking with often work at height requiring the erection of scaffolding.
The cost of purchasing an oxygen analyser varies according to the technology chosen. Electrochemical oxygen and zirconia analysers are generally the least expensive. Then there are paramagnetic oxygen analysers, whose technology is slightly more expensive. Finally, Laser oxygen analysers require higher budgets for purchase.
However, medium- or long-term project planning will often show a rebalancing of budgets when considering not only purchase costs, but also oxygen analyser operating costs.
For example, an electrochemical oxygen analyser that is financially more attractive at the time of purchase, will have a large operating budget because it will require the regular replacement of its measuring cell. More regular and potentially significant maintenance will also be required to ensure that it is maintained in flawless operational conditions from a metrological point of view. In the case of an extractive type oxygen analyser, sampling elements such as filters, pumps or dryers will have to be maintained or even replaced periodically. We are talking about preventive maintenance, but also corrective maintenance.
The same level of analyser maintenance will be required for optical sensing paramagnetic oxygen analysers (dumbbell type). Even if the cell is considered permanent, it is relatively fragile and will eventually have to be replaced, at a relatively high cost.
The more robust micro-mass flow meter paramagnetic oxygen analyser 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 will need to be maintained in the same way.
Zirconia oxygen analysers mounted in situ (analyser installed directly on the industrial process, the pipe, the chimney, the furnace ...) require very little maintenance. In addition, and provided they are carefully selected and installed, they are robust and their service life is important.
When zirconia oxygen analysis technology is used in an extractive analyser, this same robustness remains an asset in terms of reduced maintenance operations, but there is still a need for maintenance of the sampling system.
The Laser oxygen analyser, if it is of extractive type, will have the same constraints, and therefore maintenance costs as an extractive gas analyser. But the stability of its calibration being more important, it will require fewer calibration operations and logically a lower consumption of calibration 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 fast and the cost of the power supply negligible, compared to an extractive analysis installation, often very consuming electricity. On the other hand, the operating budget will have to include the cost of the permanent consumption of purge gas, whether nitrogen or clean compressed air.
Once the technical need has been defined, it is therefore important to evaluate not only the cost of purchasing and installing the oxygen analyser, but also the operating costs that will be related to the proper functioning of the equipment throughout the project, or even at the end of the product life cycle. The latter components will be increasingly high in future years due to the evolution of the costs of labour, energy, and raw material.
