Ultrasonic transit time flowmeters measure the difference of the transit time of ultrasonic pulses propagating in and against the direction of flow. This time difference is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated. Using the two transit times formula_18 and formula_19 and the distance between receiving and transmitting transducers formula_15 and the inclination angle formula_21 one can write the equations: