It was Napoleon who asked de Coriolis to explain why his cannon balls never went straight. History doesn't tell us whether he ever gave Napoleon a good explanation - that phenomenon is caused by an entirely different effect - but de Coriolis' research led to his discovery of the Coriolis effect. A century or so later, Coriolis technology spawned a whole new family of accurate meters.

Applied to instrument design, Coriolis flowmeters can measure mass flow directly, as well as volume and density. In process plants, mass is a vital measurement,"the basis of all chemical reactions, mass and energy balances, and almost all process flow operations.

Before the availability of Coriolis meters, mass was measured by weighing liquids on a scale. This can be appropriate for batch process operations, but not for continuous processes. There, methods such as orifice plates and magnetic flow tubes measured only volumetric flow and required temperature and pressure corrections and calculations using additional instruments to convert readings to mass.

A volumetric flowmeter is affected by changes in the temperature, as well as by pressure, density, viscosity, and the flow profile of the process fluid. By comparison, a Coriolis flowmeter provides a direct mass measurement that is unaffected by changing process fluid characteristics.

When there's no flow, fluid on both sides of the U-shaped tube is subject to the same force, so there is no twist (Figure 1). When fluid is flowing, it accelerates (changing direction) on the inlet side and decelerates on the outlet side. As a result, reactive force on the inlet side is opposed to the motion of the tube, and reactive force on the outlet side operates in the same direction as the motion of the tube. The tube is twisted ever so slightly by these opposing forces. The amount of twist is proportional to the mass flow rate.

To exploit the Coriolis phenomenon, a magnet is attached to one tube and a pickoff coil is attached to the other tube at both the inlet and the outlet of both tubes (Figure 2). Because of vibration, the coil moves into the magnetic field and generates a sine wave proportional to that motion. Because the coils and magnets are referenced to each other (one element moves with each tube), the sine wave represents the relative velocity and position of the tubes. When the waves cross zero, the tubes are momentarily at rest before changing direction.

Under no-flow conditions, the two sine waves are in phase, or on top of each other. Under flow conditions as the tubes twist, these sine waves shift apart. The instrument measures the twisting, which is directly proportional to the mass flow.

At the inlet side, the tube is rotating in one direction, say upward, but the fluid is restrained by the wall and the substrate and now it is vibrating up, creating a force downward. When the fluid passes the centerline lengthwise and is rotating in the opposite direction, the tube wall restricts the rotation, which creates a force in the opposite direction.

Figure 3 shows an exaggeration of the deflected shape. Here again, there are two forces proportional to the mass flow rate. The Coriolis effect is generated, as it is in the double curved tube, through the rotation of the tube about fixed ends. The two sine waves on the inlet and the outlet are shown as Point A and Point B in Figure 3. The inlet side lags the outlet side, which is identical for either straight or curved tubes.