Certain kinds of cellulose, such as a hydroxyethylcellulose, swell in a humid atmosphere and show a decrease in refractive index. Using this effect, a simple optical humidity sensor with fast response and high sensitivity, which is based on the plastic optical fibre structure change from leaky to guided, was easily fabricated and could be used for real-time humidity monitoring.

A simple optical fiber sensing system of alcohol concentration in liquors has been studied. In this sensor head, a mixture polymer of novolac resin and polyvinylidenefluoride (PVDF) with a ratio of 9 : 1 was coated as a sensitive cladding layer on the plastic fiber core made of polystyrene-(PS-)coated polycarbonate (PC). Using this sensor head and a green LED light source, it was confirmed that alcohol concentration in several kinds of liquors from beer to whisky can easily be measured with a fast response time less than 1 minute.

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