Continuous dynamic-gravimetric preparation of calibration gas mixtures for air pollution measurements |
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Authors: | Dorothea Knopf |
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Institution: | (1) Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany e-mail: Dorothea.Knopf@ptb.de Tel.: +49-531-592 3325 Fax: +49-531-592 3015, DE |
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Abstract: | The preparation of calibration gas mixtures for air pollution measurements by the dynamic-gravimetric method was investigated
using sulphur dioxide in nitrogen as a model. The target mole fraction was 200×10–9 mol/mol, with the option of also getting smaller mole fractions. Thermal mass flow meters calibrated with reference mass
flows were used to measure the dilution gas flow (nitrogen). The relative standard uncertainty of the dilution gas flows between
10 mg/s (approx. 500 ml/min) and 40 mg/s (approx. 2000 ml/min) was 0.15%. The mass flow of the target component measured as
the permeation rate was determined via the quasi-continuous observation of the loss in the permeation tube mass during the
measuring time. A magnetic coupling system and an adapted microbalance were used for this purpose. The results presented show
permeation rates measured over the lifetime of a tubular permeation source. The measurement cycles took between 3 days and
7 h at least. The relative standard uncertainty of the mixture composition did not exceed 2%. First comparisons with gas mixtures
prepared by the static-gravimetric method show compatibility. The applicability of the system is not restricted to the SO2/N2 mixture. It can also be used for preparing other gas mixtures in this field of application.
Received: 26 April 2000 / Accepted: 12 September 2000 |
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Keywords: | Gas mixtures Mass flow Permeation tube MFM Uncertainty determination |
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