Analysis of gases dissolved in liquids is a common technological challenge in monitoring the gas content, for example of oil and water, in industrial and environmental applications. An important example is the condition monitoring of large power transformers, which is mainly based on transformer oil dissolved gas analysis.
The document “Oil-filled electrical equipment—Sampling of gases and analysis of free and dissolved gases—Guidance, International Electrical Commission, International Standard, Edition 4.0 (2011), ISBN 978-2-88912-768-9” deals, for example, with techniques for sampling gases from gas collecting relays from power transformers.
Currently available devices for on-line measurement of gases or gas mixtures dissolved in oil contained by electric power transformers and their load tap changers typically use the so-called headspace sampling method or sample the dissolved gases or gas mixtures through semipermeable membranes. The headspace sampling method is inherently rather inaccurate way to measure dissolved gas contents, because of variation in the solubility coefficients for example between different species of oil. Same applies to methods utilizing semipermeable membranes which are also plagued by contamination problems. The high-end on-line devices use vacuum extraction sampling, but incorporate complicated technology, for example vacuum pumps, and are consequently expensive.
The Finnish patent application 20135951 further describes a method and system for gas concentration measurement of gas or gas mixtures dissolved in liquids. A gas or gas mixture dissolved in a liquid sample is extracted from the liquid sample using an extraction system and conducted into a measurement chamber. Then a measurement signal is generated by means of a radiant source and the measurement signal is directed to a measurement object in a measurement chamber containing the gas or gas mixture to be measured. The measurement signal is filtered using at least two wavelengths, whereupon the filtering is preferably implemented by means of an electrically tunable, short-resonator Fabry-Perot interferometer. Then the filtered measurement signals are detected my means of a detector.
Additionally, document EP 1950560 A1 discloses a method which involves filling a separation vessel with transformer oil, such that a gas chamber remains above the transformer oil in the separation vessel. A measuring chamber is filled with gas from the gas space above the transformer oil. A concentration of a gas component is measured in the measuring chamber with the help of sensor devices, particularly arranged in the measuring chamber. The sensor devices measure the concentration of one or multiple gas components selected from dihydrogen, carbon monoxide, ethylene and acetylene. The document also teaches a device for quantitative analysis of gases in a transformer oil.
Document U.S. Pat. No. 6,443,001 B1 teaches an extraction, analysis and measuring method and system intended for constituents carried along by a well fluid during drilling operations. The volume of drilling fluid contained in a vessel is subjected to stirring in order to favour extraction through degasing and evaporation. The stirring operation may take place by means of a rotating agitator. The measuring means comprises an expensive vacuum pump in order to create an underpressure that can reach several millibars.
Document JP 560253840 A discloses a method to prevent the penetration of dissolved gas into an oil immersed electric machinery tank. A bellows is used in order to provide a space in a gas extraction chamber. Air is flowed into the space and a gaseous mixture is guided to a detection chamber.
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