According to legislation or quality systems management, calibration of a Gas analyzer is required in many applications. This is the case for Air Pollution Monitoring or Continuous Emissions Monitoring devices installed in cabinets; measuring in continuous, manual, and remote locations. Measurements at ppb/ppm levels are performed and the analytical device needs to be checked regularly to avoid any drifts. Rather than a single-point calibration or validation, the goal is to perform a linearity validation throughout the entire measurement range.
To accomplish this task, gas mixers can be used to generate a range of different mixture concentrations in a very accurate and reproducible way; preferentially with automatic routines. Two groups of mixture generation for calibration purposes are available and described by ISO norms.
Gas calibration methods according to the ISO
The first group is called gravimetric methods. According to ISO 6142, a mixture is generated by weighing the cylinders before putting the final mixture into a new cylinder. This method comes to mind as measuring the mass is a well-established method and a quick calculation eliminates uncertainty in the final result. However, field deployment of this method has exposed some limitations; • only one unique concentration is available, • some compounds, such as formaldehyde, cannot be stored in cylinders, • the critical stability of compounds at low concentration (SO2 in ppb range), • high costs, when several mixtures are required A second group of methods is described by the ISO 6145 and consist in several parts, under the general title “Gas analysis — Preparation of calibration gas mixtures using dynamic volumetric methods”. It includes; volumetric pumps, continuous syringe injection methods, capillary calibration devices, critical orifices, thermal mass-flow controllers, diffusion methods, saturation methods, permeation methods, and electrochemical generation. The main benefits of dynamic methods include a better compatibility with industry requirements; mixture generation is achieved, only when it is required and several concentrations (ranges) can be generated. We will describe here the principle and details of Part 6; sonic nozzles technology.
A sonic nozzle works according to the principle of critical flow (also referred to as “choked”); an effect generated with compressible gas conditions associated with the Venturi effect. When a flowing gas, at given conditions, passes through a restriction (such as the throat of a convergent-divergent nozzle) into a lower-pressure environment, the fluid velocity increases. At initially subsonic upstream conditions, the conservation of mass principle requires the fluid velocity to increase as it flows through the smaller cross-sectional area of the restriction. When the ratio inlet pressure/outlet pressure becomes higher than 2, then the supersonic speed is reached into the restriction and the mass flow does not increase with a further decrease in the downstream pressure. The critical flow is reached. The critical flow of gases is used in many engineering applications because the mass flow rate is independent of the downstream pressure; depending only on the temperature and pressure on the upstream side of the restriction. For instance in de Laval nozzles used in rocket engines, to avoid loss of efficiency when exit pressure is lower than ambient (atmospheric); diving rebreathers, where precise constant mass flow gas addition is required at any depth and temperature conditions. Finally, it is also used in gas pipeline flow measurements and is covered by the ISO standard 9300. Sonic nozzles should not be mixed with capillary devices where the supersonic speed is not reached and then no critical flow conditions are generated.
The critical flow is determined by the parameters of the equation below. The inlet pressure should be carefully managed to generate a precise flow.
Configuration of a sonic nozzle calibrator
A basic sonic nozzle gas calibrator has two main lines; one for each gas to be mixed. A high precision pressure regulator maintains a constant inlet pressure, 3 bar at each gas inlet, and with repeatability better than ± 1 mbar. As one sonic nozzle can deliver only one flow, a combination of nozzles is created for each line to generate different concentrations. When 2 nozzles can generate 4 mixtures, up to 1024 concentrations steps can be reached by using 10 nozzles in different combinations (1024 = 2*10). A dilution range from 1/1 up to 1/1000 can be generated. Fig. 3 shows the dilution point “26.6%” with a 4 sonic nozzles device (16 concentrations). The mechanical setup is configured to have all nozzles at the same temperature and to generate a homogeneous gas mixture.
Mechanical setup and performance of a sonic nozzle
Sonic nozzles are used either as stand-alone devices into gas circuits or, integrated in mixers/diluters. They are manufactured in stainless steel, gold, or silconert for corrosion gas compatibility and can work at room temperature with up to 7 bars of inlet pressure. The nozzle is encapsulated into a metallic body for easy integration into the regulating device
Fig 4: Sonic Nozzle gas calibrator in a 19" enclosure and portable version
Side-by-side comparison of sonic nozzle and mass flow controller (MFC) technologies
As the MFC is a well-known technology for gas diluters and calibrators, it is of interest to compare both methods. Each method has advantages and disadvantages. While Mass Flow Controllers are more flexible by allowing a mix of several gases at the same time, the sonic nozzle generates only binary mixtures but at a better accuracy and in the long term.
How a high accuracy is reached with sonic nozzles?
High accuracy means low uncertainty. This can be generated in two ways: either by reducing the sources of uncertainty and/or by reducing the uncertainty of remaining sources. The sonic nozzle combines both ways, assuming that the gas source purity is constant: Simplicity – No electronic signal measurement or flow regulation is needed as flow conditions are blocked by gas physics conditions of the critical flow. The simple setup of a sonic nozzle calibrator is key in reducing potential sources of uncertainty. Only orifice diameter and pressure regulation remain.
Long-term stability – A high-precision pressure regulator is a full metal device capable of maintaining the inlet pressure within variations of less than 2 mbar. The nozzle is made of nickel or gold with unaffected dimensions or surface properties over time and even for corrosive gas applications. Both mechanical devices show excellent stability over the years and no aging effect has been found. Their contribution to the device uncertainty is almost negligible.
Benefits
The sonic nozzle technology provides several benefits:
Metrological superior performances - The uncertainty and repeatability are better than 0.5% and across the entire dilution range of the device. Unlike Mass Flow Controllers, working below 5 % of the dilution range is possible within the specifications
Flow rate is constant - not affected by downstream flow or pressure disturbances Extended dilution capability – By combining several nozzles up to 1024 mixtures ratios can be generated. The dilution ratio range goes from 1/1 up to 1/1000
Lower running cost – Even if acquisition costs are higher, the long-term stability of components reduces the frequency of validation and calibration of a calibrator
Proven performances – Finally, metrological performances of installed devices have been identified in accredited laboratories
Traceability of methods for gas analysis
As mentioned initially, gravimetric methods mentioned in ISO 6142 have the shortest link to primary standards (the mass), but field deployment is not very convenient. The critical orifices principle, (mentioned in the ISO 6145 Part 6) although less direct maintains a full traceability with gas standards and methods as shown in Fig. 6.
Conclusion
A gas calibrator with sonic nozzle technology involves single mechanical devices without any electronic measurement or regulation. The flow stability and gas mixture accuracy are mainly generated by gas physics and the critical flow effect generated with a sonic nozzle. Uncertainty sources are therefore limited and the accuracy reached is excellent over several months of use. This provides a huge advantage for calibrators having to work without frequent care and validations in remote locations such as cabinets installed for Air Pollution Monitoring. This has also a positive impact on reducing the cost of ownership as fewer calibrations are required. Over many years installed calibrators with sonic nozzles have shown superior reliability and metrological performances and therefore may be considered an ideal transfer standard for gas calibration purposes.
Contact an LNI Gas Calibration Solutions Specialist
LNI Swissgas (North America)
50 Inwood Rd., Suite 104
Mailbox #3
Rocky Hill, CT 06067
Tel: +1-860-216-4967
M: +1-713-677-1980
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