Finding the Web document differential pressure transducer you want can be easy or seem impossibly difficult. This is in part due to the sheer size of the WWW, currently estimated to contain 3 billion documents. It is also because the WWW is not indexed in any standard vocabulary. Unlike a library's catalogs, in which can use standardized Library of Congress subject headings to find books in most large, general libraries in the U.S., in Web searching you are always guessing what words will be in the pages you want to find or guessing what subject terms were chosen by someone to organize a web page or site covering some topic.
