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Plastic Fiber Detects Methanol

Photonics Spectra
Sep 2001
Richard Gaughan

Methanol is a promising energy source for fuel cells. But while the primary product of a methanol fuel cell is warm water, a leaky cell can pose a hazard because the alcohol is toxic and explosive. A safe, sensitive method for detecting methanol is needed, therefore, and a plastic optical fiber sensor is a potential solution.

Researchers have developed a plastic optical fiber sensor that detects methanol. The refractive index of the cladding changes in the presence of alcohol, and the fiber switches from leaky to guided operation. Courtesy of Yamanashi University.

Professor Masayuki Morisawa and his colleagues at Yamanashi University, with the help of Chuan Xin Liang of TRI Chemical Laboratory Inc. in Uenohara, Japan, have constructed a proof-of-principle plastic fiber sensor from Arton, a transparent plastic produced by JSR Inc. of Tokyo that has a refractive index of 1.51. They replaced a section of the normal cladding with Novolac polymer resin coating. Novolac's standard index of refraction is 1.6, but it falls to 1.45 when exposed to alcohol. The fiber typically is leaky, but it displays guided transmission in the presence of alcohol.

To verify their theoretical predictions, the researchers coupled a green LED source to the fiber and then placed the 5-cm-long sensor section in a chamber where they could control the saturated methanol vapor pressure. They monitored the intensity of the light that was transmitted by the sensor with either a photomultiplier or a photodiode.

As expected, the intensity increased by approximately an order of magnitude with the addition of methanol vapor.

Improvements and applications

Morisawa is evaluating thinner claddings and finer control of the cladding's refractive index to improve the response time of the sensor. The researchers also are refining the composition of the polymer film coating and have evaluated a Novolac-iron-polyvinylidenfluoride mixture. They are working with an industrial partner to build a more compact sensor and are developing an intelligent optical sensing system for residential applications.

Compared with existing electronic alcohol detection methods, the plastic fiber sensor offers safety, immunity from electromagnetic interference and low cost. Besides serving as a gas leakage sensor for fuel cells, it could function as an atmosphere monitor in a brewery or as an optical system for identifying drunken drivers.

A report about the sensor was published in the July issue of Measurement Science and Technology.

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