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.