The Application of Critical Nozzles in Series for the Determination of Small Flow Rates and the Generation of Gas Mixtures |
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| Rainer Kramer, Bodo Mickan, Roland Schmidt |
- Abstract:
- Because of the low uncertainty and very good long term stability the application of critical nozzles became very important especially for test rigs. The majority of nozzles are used in the so-called “suck mode” using atmospheric air as test medium. In order to use nozzles for the generation of different flow rates as well as to generate gas flows with different kinds of gases the application with increased input pressure became more important. An other field of extended application is the usage for small flow rates. Nozzles which have a shape in accordance to ISO 9300 are available with throat diameters down to 80 µm. Below this value the shapes have to be simplified. Even for simplified shapes the mechanical manufacturing of nozzles allows at the time being throat diameters of d = 15 µm.
In order to extend the flow rates to values smaller than the flow rates possible with a single nozzle the use of two nozzles in series is a possible approach. The paper shows that the properties of nozzles, especially the very high reproducibility, is the main precaution for this solution.
The paper will describe an arrangement of two micro nozzles in series. The throat diameters are d1 = 15 µm and d2 = 25 µm, respectively. The generated flow rates are compared with LFE measurements. The main source of uncertainty is the reproducibility of the pressure transducers. A nozzle bridge and its potential to decrease this source of uncertainty will be proposed and discussed.
Gas mixtures for the calibration of sensors are usually manufactured by gravimetric methods. The so called dynamic generation is based on the mixture of two ore more flows of gas components with known flow rates. The arrangement of two nozzles in series is an interesting approach for two-component mixtures. The flow rate of the main component is given by the flow rate of the upstream nozzle. The second component is inputted at the connection point of the two nozzles. The amount of this additional flow can be determined by the pressure change at the inlet of the second nozzle. The application of two nozzles in series for generating nitrogen-methane mixtures will be described using a third nozzle for calibrating the mixing unit. In order to get experience with this kind of application the generated mixtures were compared with measurement results of a commercially available gas analyser. The results show the feasibility of this kind of application. - Keywords:
- critical nozzles, flow rate determination, gas mixture, micro nozzle
- Download:
- IMEKO-TC9-2010-050.pdf
- DOI:
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