Sonic nozzles for gas calibrators, when simplicity provides accuracy!

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Courtesy of LNI Swissgas SA

Gas analyser’s calibration is a task required in many applications according to either legislation or quality systems management. This is the case for Air Pollution Monitoring or Continuous Emissions Monitoring devices installed in cabinets, for continuous analysis in remote locations. Measurements in traces range are performed and analytical devices specifications need to be validated and corrected over the time. Rather than a single point calibration, the goal is to perform a linearity validation throughout the entire measurement range. 

In order to accomplish this task, a gas device should generate a range of different concentrations, by mixing a dilution gas like nitrogen and a main component in very accurate and reproducible ways. Two groups of gas mixtures generation for calibration purpose are described by ISO norms and explained below.

Gas calibration methods according the ISO

The first group is called gravimetric methods. According the ISO 6142 procedure, individual components are weighed in before mixed into a new cylinder. The weighing process is one of the most accurate physical measuring processes known, as weight is a primary standard and a short connection to this standard reduces uncertainties of the final resulting gas mixture. The closer we can be from a primary standard the better it is. However the field deployment of this method has seen some limitations:

  • only one unique concentration is available, 
  • some compounds, such as formaldehyde cannot be stored in cylinders,
  • critical stability of compounds at low concentration (SO2 in ppb range),
  • high costs, when several gas concentrations 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 method, capillary calibration devices, critical orifices, thermal mass-flow controllers, diffusion method, saturation method, permeation method, electrochemical generation. The main benefits of dynamic methods include a better compatibility with industry requirements, the mixture generation is done only when it is required and several concentrations (ranges) can be generated. We will describe here the principle and specificities of the Part 6: sonic nozzles technology.

How does a sonic nozzle work?

A sonic nozzle works according the principle of critical flow (also referred to as “choked”), an effect generated with compressible gases and flow 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.

When the pressure ratio in-/outlet becomes higher than 2, the supersonic speed is reached into the restriction and the mass flow does not increase anymore with further decrease in the downstream pressure environment, keeping the upstream pressure fixed, the critical flow is reached.

The critical flow effect is used in several 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. Examples are found in de Laval nozzles used for 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 covered by the ISO standard 9300.

Sonic nozzles should not be confused with capillary devices where the supersonic speed is not reached and then no critical flow conditions are present.

Design 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 a repeatability better than ± 1 mbar.

As one sonic nozzle can deliver only one flow, a combination of nozzles is created for each line in order to generate different concentrations. When 2 nozzles can generate 4 mixtures, up to 1024 concentrations steps can be reached by using 16 nozzles in different combinations (1024 = 216).

A dilution range from 1/1 up to 1/1000 can be generated.

The 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 an 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 nickel or gold, for corrosion gases compatibility, and can work up to 80°C and 10 bar maximal working pressure or 25 bar acceptable overpressure.
The nozzle is encapsulated into a metallic body for easy integration into the calibration device.

Fig. 4 A sonic nozzle calibrator in a 19” enclosure

Side by side comparison 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 of dilution.

Sonic nozzles for gas calibrators, when simplicity provides accuracy!
Sonic nozzles for gas calibrators, when simplicity provides accuracy!

There is no real winner and each technology has pro and cons. When the MFC technique is more flexible by allowing a mix of several gases at the same time, the sonic nozzle technology generates only binary mixtures but at better accuracy and for long term operations.

How a high accuracy is reached with sonic nozzles?
High accuracy means low uncertainty. This can be generated by two ways: either by reducing the sources of uncertainty into a calibrator or to reduce the uncertainty of each source. The sonic nozzle combines both ways, assuming that the gas sources purity is constant: few mechanical components with high precision performances components.

Simplicity – Because a calibration device with sonic nozzles has a simple design with fewer parts, this is the key reason to reduce potential sources of uncertainty. No electronic signal measurement or regulation are needed as flow condition are blocked by gas physics conditions of the critical flow. Only orifice diameter and pressure regulation remain as potential sources of uncertainty.

Long-term stability – The upstream pressure is regulated by a high mechanical precision regulator  which maintains the inlet pressure within variations of less than 2 mbar. Nozzles are made of nickel or gold with unaffected dimensions or surface properties over the time, even for corrosive gas. Both mechanical devices show excellent stability across several years and no aging effect has been found. Their contribution to the total uncertainty of the calibrator is almost negligible.

Benefits

The sonic nozzle technology provides several benefits:

Superior metrological performances - Uncertainty and repeatability are better than 0.5%, for the dilution range of the device. Unlike Mass Flow Controllers, working below 5 % of the whole dilution range is possible within specifications

Constant flow rate - Not affected by downstream flow or pressure disturbance

Extended dilution capability – By combining several nozzles up to 1024 mixtures ratios can be generated. The a dilution ratio range goes from 1/1 up to 1/1000

Lower running cost – Even if acquisition costs are higher, long term stability of components reduce the frequency of calibration

Proven performances – As recommended, metrological performances should be certified in ISO 17025 laboratories. Reports show the excellent performance of calibrators with sonic nozzles.

Traceability of methods for gas analysis

As mentioned initially, gravimetric methods mentioned in the ISO 6142 have the shortest link to primary standards (weigh), but field deployment is not very convenient. The critical orifices principle, (mentioned in the ISO 6145 Part 6) although less direct maintains the traceability with gas standards and methods as shown in Fig. 7.

Fig. 7 Flow chart of norms related to gas analysis
and their links to primary standards

For gas analysis, five norms are involved:
ISO 6141:2000 - Requirements for certificates for calibration gases and gas mixtures
ISO 6142:2001 - Preparation of calibration gas mixtures -- Gravimetric method
ISO 6143:2001 - Comparison methods for determining and checking the composition of calibration gas mixtures
ISO 6144:2003 - Preparation of calibration gas mixtures -- Static volumetric method
ISO 6145:2009 - Preparation of calibration gas mixtures using dynamic volumetric methods

Conclusion
A gas calibrator designed with the sonic nozzle technology involves simple mechanical design without any electronic measurement or regulation. Flow and generated gas mixtures are driven exclusively by the physics of the critical flow. Uncertainty sources are limited, providing a huge benefit for calibrators which have to work without frequent care and validation tasks on remote locations.  
Over many years, installed calibrators with sonic nozzles have shown superior reliability and metrological performances, therefore they may be considered as an ideal transfer standard for gas calibration purpose.

Sonic nozzles for gas calibrators, when simplicity provides accuracy!

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