The use of ultrasound in flow measurement began in the last century when, in 1928, Oskar Rütten patented this method and apparatus for measuring the volume of flowing liquids, gas, and steam. The first ultrasonic flowmeters were introduced in the 1950’s, and in the 1960’s different companies developed the first ultrasonic flowmeters using the differential transit time method. Around this same time the first clamp-on ultrasonic flowmeters were also launched onto the market. The first multi- path ultrasonic flowmeters for measuring gases were finally introduced at the start of the 1980’s. However the main breakthrough in ultrasonic flow measurement has come in the last 10 to 20 years, which has seen the breakthrough of digital signal processing. With this highly precise method, exact recording and analysis of transit times has become increasingly accurate, which of course has made the measuring device extremely interesting to industry.