The hydrogen sensors are important items supporting the hydrogen safety at every phase of hydrogen production, transportation, storage, filling and consumption. A catalytic combustion type gas sensor is mainly used for the gas alarm system of hydrogen stations, and it is recognized as a reliable detection method. However, the further improvement of the sensor is still necessary about power consumption, response time and durability to support safety of the future hydrogen energy society. So, we had developed the catalytic combustion type hydrogen sensor in the shape of the miniature beads. It is using the optimized Pd-Pt/Al₂O₃ catalyst and the Pt micro-heater coil. Both warm-up time and response time of this sensor achieved less than 1 second by downsizing the element to 200μm diameter. Furthermore, we improved the resistance of sensor poisoning to silicone vapor and confirmed long term stability within ±10% of output error up to 8 years. Therefore, we assume that our sensor technology contributes to hydrogen safety.

Hydrogen gas concentration sensing using ultrasonic was developed and demonstrated. It uses the difference in sound velocity between hydrogen and other mixed gases such as air. Hydrogen concentration can be calculated from the sound velocity alteration measurement after hydrogen is mixed with air or other known gases. The sensor using ultrasonic is very fast in response time and low cost with a smaller size equipment. The response time of measuring gas concentration was less than 0.1 sec. It was demonstrated that it is possible to measure absolute hydrogen concentration accurately from 100 ppm to 100% with one sensor head. The ultrasonic sensor head probes distance can be as small as 20 mm. Concentration of hydrogen gas flowing inside a pipe can be also measured by using ultrasound from exterior of the pipe without extracting the gas from the pipe. Drilling a hole in the pipe is not necessary for the gas concentration measurement. Various gas flow rate was tried and it was estimated that it is possible to measure the hydrogen gas concentration in the flow rate of more than 50 m/sec.

Hydrogen-sensing properties of diode-type gas sensors utilizing noble-metal (M) sensing electrodes and a thin titania film fabricated by anodization of a titanium plate (M/Ti0₂) have been reviewed in this paper. The nonlinear current-voltage characteristics of all the M/Ti02 sensors at 250°C in air and nitrogen (base gases) originated from the Schottky contact formed at the interface between M and Ti0₂, and the magnitude of current drastically increased with the addition of H₂ into the base gases, because of the reduction in the Schottky barrier height. The Pd/Ti0₂ sensor showed relatively large H₂ response than the Pt/Ti0₂ sensor, especially in air, and an optimal mixing of Pt into the Pd electrode enhanced the H₂ response of the Pd/Ti0₂ sensor. The H₂ response of these sensors were largely dependent on oxygen concentration in the gaseous atmospheres, but the slight surface modification of the M electrode (esp. M: Pt) with Au was drastically effective in reducing the effects of oxygen concentration on the H₂ response, especially under wet atmosphere. In addition, the Au-modified Pt/Ti0₂ sensor showed excellent H₂ selectivity against hydrocarbon gases in air as well as in nitrogen.

KOA have been developing novel catalytic combustion gas sensors using quartz resonators to detect hydrogen gas leakage. This sensor has a platinum catalyst on electrodes of a quartz resonator; it detects a temperature change due to catalytic reaction heat of hydrogen gas as a shift of its resonance frequency. In this paper, the structure of the sensor was explained using schematic diagram and photograph. In addition, by referring to the experimental data, some advantages of the sensor such as high response speed, high repeatability and linearity of sensitivity were discussed. Finally, as an example of sensor application, we introduced the prototype of stationary hydrogen gas leakage detector and its experimental results.

Based on know-how on compressed natural gas dispensers, we have developed element devices with consigned development of New Energy and Industrial Technology Development Organization, and at the same time we have fed back the verification test results such as durability to the products. In this paper, we describe the constitution of the hydrogen station and the hydrogen dispenser and requirement for the hydrogen dispenser, especially requirements on sensing.

This dissolved hydrogen sensor utilizes the principle of concentration cell. The dissolved hydrogen sensor is composed of sample electrode and reference electrode made of a hydrogen permeable metal film, with electrolyte solution having hydrogen metal conductivity. Based on the electromotive force generated between the standard pole and the specimen pole, the hydrogen partial pressure at the standard pole, and the temperature of the specimen, it is possible to calculate the hydrogen partial pressure at the specimen pole by using the following Nernst equation.

In the field of space development, especially space transportation, a large amount of hydrogen has been treated and consumed. Leakage detection with hydrogen sensors is indispensable in order to deal with hydrogen safely as in other systems on the ground. On considering requirements for hydrogen sensors used in space development, the concept of hydrogen leakage detection system should be firstly considered. In this paper, the hydrogen leakage detection system for a reusable ballistic-flight rocket was considered so that the requirements of the sensors used in the leakage detection system were investigated. One of characteristics that are needed for the reusable rocket is working under low pressure and without oxygen. Development study of the hydrogen sensor that can work under low pressure utilizing phenomenon of hydrogen adsorption on solid surface is introduced.