Vol.4 No.2 2011

Research paper : Thermoelectric hydrogen gas sensor (W. Shin et al.)−105−Synthesiology - English edition Vol.4 No.2 (2011) expensive, costs around 3 million yen, and it combines a semiconductor type gas sensor and a gas chromatograph. The thermoelectric sensor can detect ppm level hydrogen concentration, and can be a simple alternative solution, which does not need gas chromatograph, reducing the size, cost, and analysis time significantly. Currently, a prototype has been commercialized as an AIST venture, and is expected to be applied to various fields as well as to hydrogen-energy facilities such as hydrogen stations.5 International standard making use of research output The state of the market of hydrogen sensor strongly correlated to the spread of the hydrogen energy use and the social receptivity of hydrogen are also important issues for commercialization. However, the situation of the current market is inactive and conservative. The sensors of the old technology are already adopted in the present hydrogen stations operated in Japan, and the new technology cannot be accepted easily. Even though there is new demand for a wide range of hydrogen detection, the thermoelectric hydrogen sensor could just end up as a development study. To promote the application of our new sensor technology, we also carried out a study for the standardization of the gas sensor, making a new proposal for hydrogen sensors. The standardization work has been proceeded in parallel with the technology development of the thermoelectric sensor, propelled by the policy of the Ministry (METI).We prepared a proposal based on the performance of our newly developed "thermoelectric hydrogen sensor with a wide range of hydrogen concentrations with hydrogen selectivity", and submitted it as a new proposal (NWIP) to the committee of ISO/TC197 (Hydrogen technologies) in 2005. This proposal was accepted and WG13 (Hydrogen Detectors, host country is Japan) started to discuss a new international standard of hydrogen detectors. This international standard was issued in June 2010. This was one example of proposed international standards development efforts utilizing the technology, and the best detection technology that satisfied many of the contents of the proposed standard was the thermoelectric hydrogen sensor developed by us. However, the draft did not pass as it was, and the final draft was edited and changed significantly from the 2005 draft. This International Standard set the performance requirements for hydrogen detectors as follows:・ measuring range, concentration calibration and alarm set points,・ stability (short and long term),・ time of response and recovery, selectivity, poisoning,・ temperature, pressure, humidity (standard test conditions),・ operation above the measuring range, power supply variation and interruptions.The most discussed issue was the measuring range of the detector as shown in Fig. 6. In the related IEC standard on the inflammable gas detectors, only the upper limit of gas concentration is specified and this is declared to be the range. However, the tolerance required in the calibration is defined as 5 % of the upper limit or 10 % of the indication, and the error becomes seriously large in the low gas concentration range. We have explained the importance of low-concentration detection for the multi-level safety operation, and proposed the standard of detectors covering a wide range of hydrogen concentration. This claim of our Japanese delegate was accepted and a committee draft containing the measuring range from 500 ppm to 2 % as shown in Fig. 6 was drawn. However, several countries did not agree to this, because their technology works well at high gas concentration. In the end, as the international standard is not intended to exclude any specific technologies that meet the performance requirements, the measuring range of the detector including the tolerance of the detection was decided to be declared by each sensor manufacturer.6 SummaryWe have invented a novel thermoelectric gas sensor, integrating the elemental technologies of catalytic combustion of hydrogen and thermoelectric conversion from thermal gradient to voltage, for gas-leak detection systems in hydrogen stations. By integrating various elemental technologies, completely new performances such as robustness, hydrogen-selectivenes, wide-range hydrogen detection from 0.5 ppm to 5 % in air, fast and linear response are realized, which are impossible by the current sensor technology. Furthermore, taking advantage of this new technology, we have proposed and published a new ISO Fig. 6 Measuring range and calibrationIt was extremely difficult to write down the measuring range of the hydrogen detectors to satisfy all the members in the ISO working group for hydrogen detectors. This figure shows the meeting material suggested and used in the final stage of the committee meeting, showing a range from 500 ppm to 2 %, which was the result of the previous meeting. This was finally rejected and the measuring range of the detector was decided to be declared by each sensor manufacturer. 4 %2 %1 %1000 ppm500 ppm100 ppm±125 ppmor ±50 %of test gasconcentrationDeclaration bymanufacturerLower explosion limit(LEL)3 points within the measuring rangeDeclaration by manufacturer(Specification of the lowest and highest detection concentration)Declaration bymanufacturerBelow ±25 % of test gas concentration±125 ppm or ±50 %of test gasconcentrationHydrogen concentrationMeasuring rangeStandard test gas for calibrationTolerance


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