A coriolis meter is usually one or more pipes with longitudinally or axially displaced section(s) that are excited to vibrate at resonant frequency. Coriolis meters are used with liquids and gases. When the fluid within the displaced section is at rest, both the upstream and downstream portion of the displaced section will vibrate in phase with each other. The frequency of this vibration is determined by the overall density of the pipe (including its contents). This allows the meter to measure the flowing density of the gas in real time. Once the fluid begins to flow, however, the Coriolis force comes into play. This effect implies a relationship between the phase difference in the vibration of the upstream and downstream sections and the mass flow rate of the fluid contained by the pipe.