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Coulometric trace humidity measurement in industrial gases

  • Coulometric sensors are applied for trace humidity measurements in various technical gases. The use of this sensor type is demanded in some standards by the European Pharmacopoeia for medical gases. Coulometric sensors allow the measurement of water vapour in gases such as e.g. air, Cl2, H2, N2, N2O, CH4 between the concentration ranging from 0.1 to 2,500 µmol∙mol-1 which corresponds to frost point temperature -90 °C to -10 °C, respectively. The sensing principle is based on Faraday’s law of electrolysis whereby water is decomposed to hydrogen and oxygen. The sensor signal is the measured electrical current which is proportional to the mass of water that is absorbed on the hygroscopic phosphorous pentoxide layer. The signal is dependent on the gas flow at a given voltage, gas pressure and temperature. The sensors need to be calibrated to measure the accurate signal for humidity in air. However, the signal is dependent on the type of gas matrix. This dependency has not beenCoulometric sensors are applied for trace humidity measurements in various technical gases. The use of this sensor type is demanded in some standards by the European Pharmacopoeia for medical gases. Coulometric sensors allow the measurement of water vapour in gases such as e.g. air, Cl2, H2, N2, N2O, CH4 between the concentration ranging from 0.1 to 2,500 µmol∙mol-1 which corresponds to frost point temperature -90 °C to -10 °C, respectively. The sensing principle is based on Faraday’s law of electrolysis whereby water is decomposed to hydrogen and oxygen. The sensor signal is the measured electrical current which is proportional to the mass of water that is absorbed on the hygroscopic phosphorous pentoxide layer. The signal is dependent on the gas flow at a given voltage, gas pressure and temperature. The sensors need to be calibrated to measure the accurate signal for humidity in air. However, the signal is dependent on the type of gas matrix. This dependency has not been quantified so far. Therefore, the impact of reactive gases such as hydrogen and nitrous oxide on the calibration curve was investigated. Furthermore, a possible rationale in relation to the interaction of water with the gas matrix and its impact on the electrode reactions is suggested. The experimental setup consists of a gas supply, dryer, humidifier, test chamber and reference hygrometer. The test gas is generated by mixing the dry and the wet gas flow. First the carrier gas is dried by an activated carbon filter and then split into two flows. One flow is dried again with a molecular sieve. The other flow is humidified by passing it through a bubbler filled with pure water. After this, the rate of both flows is controlled by mass flow controllers and then mixed with the test gas. A calibrated precision chilled mirror dew-point hygrometer is used as a reference instrument. The coulometric sensors were exposed to different humidified gases and the sensor signal was recorded till a constant value was obtained. Calibration curves were calculated for the frost point temperature in the range of about -70 °C to -10 °C according to the equation, tf = A + B∙ln(I), (I is the electrolysis current and A, B are constants) followed with a linear regression fit. Comparison of the results of air and nitrogen showed no significant differences. In contrast, there are remarkable differences for humidified hydrogen and nitrous oxide, respectively. The difference might be due to increased recombination of hydrogen with the produced oxygen to form new water molecules in humidified hydrogen. In conclusion, coulometric trace humidity sensor is a robust hygrometer for various technical applications. However it needs to be calibrated for the specific gas matrix.zeige mehrzeige weniger

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Metadaten
Autor*innen:Marc Detjens
Koautor*innen:Carlo Tiebe, Ulrich Banach, J. Majewski, Thomas Hübert
Dokumenttyp:Posterpräsentation
Veröffentlichungsform:Präsentation
Sprache:Englisch
Jahr der Erstveröffentlichung:2016
DDC-Klassifikation:Naturwissenschaften und Mathematik / Chemie / Analytische Chemie
Freie Schlagwörter:Coulometric sensor; Industrial gases; Trace humidity
Veranstaltung:Tempmeko 2016
Veranstaltungsort:Zakopane, Poland
Beginndatum der Veranstaltung:27.06.2016
Enddatum der Veranstaltung:01.07.2016
Verfügbarkeit des Dokuments:Datei im Netzwerk der BAM verfügbar ("Closed Access")
Datum der Freischaltung:07.07.2016
Referierte Publikation:Nein
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