Fiber Optic Hydrogen Sensor (FOHS)

The ability to detect hydrogen gas leaks economically and with inherent safety is an important technology that could facilitate commercial acceptance of hydrogen fuel in various applications. In particular, hydrogen fueled passenger vehicles will require leak detectors to signal the action of various safety devices. Such detectors will be required in various locations within a vehicle, wherever a leak could pose a safety hazard. It is therefore important that the detectors be very economical. For purposes of early detection a fast response time (<–1 second) is also desired. An optical fiber coated with a thin film of a chemochromic (color change induced by a chemical reaction) material offers the possibility of meeting these objectives.

Chemochromic materials such as tungsten oxide and certain lanthanide hydrides can react reversibly with hydrogen in air while showing significant changes in their optical properties. Thin films of these materials applied to the end of an optical fiber have been used as sensors to detect low concentrations of hydrogen in air. The coatings include a thin layer of gold in which a surface plasmon is generated, a thin film of the chemochromic material and a catalytic layer of palladiumthat facilitates the reaction with hydrogen. The gold thickness is chosen to produce a guided surface plasmon wave between the gold and the chemochromic material.

A dichroic beam splitter separates the reflected spectrum into a portion near the resonance and a portion away from the resonance and directs the portions to two separate photodiodes. The electronic ratio of these two signals cancels most of the fiber transmission noise and provides a stable hydrogen signal.

A fiber optic sensor based on the palladium catalyzed reaction of amorphous tungsten oxide and hydrogen was first proposed by Ito (1984). This simple sensor design was found to be adequate in terms of sensitivity but too slow in response time for the intended use. A different design using a surface plasmon resonance (SPR) configuration was therefore investigated. The SPR shifts in response to subtle changes in the refractive index of the coating. This shift can be monitored to give a faster response.

Technology for Hydrogen Sensors

Hydrogen may be emerging as the fuel of choice for an energy carrier. It can be stored, handled, reacted or combusted to deliver large quantities of energy to an end use safely, conveniently, and efficiently with very little environmental impact. However, it is a combustible gas, and the public has been sensitized to dangers associated with its use.

Safe practices and codes for handling hydrogen will require convenient and reliable methods of detecting hydrogen leaks in spaces where combustible or explosive concentrations may be reached. The U. S. Department of Energy has undertaken many of the long-range tasks associated with bringing a new energy carrier into widespread use and has initiated study of new sensor technology that will meet the requirements imposed by new technology.

Expanded use of hydrogen in the public domain brings new requirements for safety monitoring, which have not been considered until recently. For instance, the use of hydrogen for a transportation fuel will necessitate the outfitting of each vehicle and each fueling area with multiple sensors to detect low concentrations of hydrogen and to initiate a set of hierarchical actions such as setting off alarms, activating fans, etc. prior to the onset of the explosive limit.

The sensors must be rugged, reliable, and inexpensive enough to incorporate several into each vehicle. Additionally, the sensors need to be lightweight and have minimal energy requirements themselves. In order to meet such challenges, solid-state hydrogen sensors was designed. The technologies are based upon either chemochromic or resistance changes in the properties of thin films in the presence of hydrogen.

The Fiber Optic (chemochromic) sensor requires no electrical power at the sensing point and is ideal for high electromagnetic environments. Furthermore, a modification of the fiber optic sensor has shown promise as an analytical tool for measurement of diffusible hydrogen in welded steel. The thick film (resistive) sensor is versatile and can operate from a small battery. Data from combinations of multiple sensors can be fed into a central processing unit via fiber optics or telemetry to provide hydrogen situational awareness for small and large areas.