About Coriolis Mass Flow Meters
A Coriolis mass flow meter, also known as an inertial flow meter is a device that measures mass flow rate of a fluid traveling through a tube. The mass flow rate is the mass of the fluid traveling past a fixed point per unit time.
The Coriolis mass flow meter does not measure the volume per unit time (e.g., cubic meters per second) passing through the device; it measures the mass per unit time (e.g., kilograms per second) flowing through the device. Volumetric flow rate is the mass flow rate divided by the fluid density. If the density is constant, then the relationship is simple. If the fluid has varying density, then the relationship is not simple. The density of the fluid may change with temperature, pressure, or composition, for example. The fluid may also be a combination of phases such as a fluid with entrained bubbles.
Density and Volume Measurements
The mass flow of a u-shaped coriolis flow meter is given as:
where Ku is the temperature dependent stiffness of the tube, K a shape-dependent factor, d the width, τ the time lag, ω the vibration frequency and Iu the inertia of the tube. As the inertia of the tube depend on its contents, knowledge of the fluid density is needed for the calculation of an accurate mass flow rate.
If the density changes too often for manual calibration to be sufficient, the coriolis flow meter can be adapted to measure the density as well. The natural vibration frequency of the flow tubes depend on the combined mass of the tube and the fluid contained in it. By setting the tube in motion and measuring the natural frequency, the mass of the fluid contained in the tube can be deduced. Dividing the mass on the known volume of the tube gives us the density of the fluid.
An instantaneous density measurement allows the calculation of flow in volume per time by dividing mass flow with density.