On a more practical note, take a look at the Figaro 2610-C/D LP gas sensor= s. These work a little differently to my previous explanation in that there= is a heating element that is not used for sensing, and a thick film sensin= g element made from tin dioxide. The -D version has a membrane filter of so= me kind to apparently increase selectivity by filtering out interfering com= bustible vapours. Power consumption is 280mW, which is manageable for inter= mittent operation on battery power. https://static.rapidonline.com/pdf/50-9095.pdf David --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .