Every flow meter covered so far measures volume — gallons per minute, cubic meters per hour. But chemical reactions, billing, and combustion all care about mass. Volume changes with temperature and pressure; mass doesn't. To get mass flow from a volumetric meter, you need a separate density measurement and a computer. Coriolis meters skip the middleman: they measure mass flow directly, by exploiting the same physics that deflects hurricanes.

The principle. Bend a tube into a U-shape and vibrate it at its natural frequency (typically 80–1000 Hz) using a magnetic drive coil at the bend. With no flow, both legs of the U vibrate in perfect sync. Now push fluid through it. The fluid entering the inlet leg has to be accelerated sideways to follow the vibrating tube; on the outlet leg, it has to be decelerated. Newton's third law: the fluid pushes back on the tube. The inlet leg gets pushed one way, the outlet leg the other. The U twists, and the twist is proportional to mass flow rate.

Two pickoff sensors (one near each end of the U) detect when each leg crosses zero. With no flow, both crossings happen simultaneously. With flow, there's a tiny time delay Δt between them — typically microseconds. The relationship is beautifully direct:

• ṁ = K · Δt, where K is a meter calibration constant set by tube geometry and stiffness.

That's it. No density needed, no temperature compensation, no Reynolds number worries, no straight pipe runs upstream. Accuracy is typically ±0.1% of reading — an order of magnitude better than most volumetric meters.

Bonus measurement: density. The natural frequency of the vibrating tube depends on the total mass it carries (tube + fluid). Heavier fluid → lower frequency. So the same instrument measures density to ±0.0005 g/cc essentially for free. Divide mass flow by density and you also get volumetric flow. One meter, three readings.

Real-world example. Custody transfer of crude oil at a pipeline terminal. Two parties are exchanging millions of dollars of product per day. A 6-inch Coriolis meter measures the mass directly — buyer and seller both trust it because temperature swings and entrained gas don't bias the reading the way they would a turbine meter. Same instrument is used to dose ingredients in pharmaceutical batching, where ±0.1% on a 500 kg batch matters.

Limitations. Expensive ($5k–$50k+). Pressure drop is significant — the fluid has to flow around a bent tube. Entrained gas bubbles wreck accuracy (the tube can't tell mass from inertia of slugs). And large sizes get heavy fast: a 10-inch meter can weigh 800+ kg.