By measuring mass rather than volume, many problems associated with fluid expansion can be avoided. Most mass flow meter designs only satisfy specialized applications. Three classes of mass meter techniques can be found.
'Hybrid' meters use two meters, one measuring velocity and the other kinetic energy. Many possibilities exist, but most are specialized curiosities.
Two principles of 'Inferential' meters can be found, the most popular being the combination of a volume meter and a densitometer. This technique is popular and uses proven technology, but it suffers from the uncertainties associated with two measurements.
A critical flow (sonic) nozzle accelerates gas to the speed of sound within the nozzle. At this point, mass flow is proportional to the gas properties and upstream pressure. This provides very accurate measurement (and control) of mass flow of gas, albeit with a high pressure drop.
'True' mass meters utilize four principles. The first makes use of the momentum of the fluid by imparting swirl to the fluid, then the force generated by removing it is measured. A driven rotor followed by a fixed measuring rotor is one design example. Other momentum measuring techniques are also used.
The second principle uses Differential Pressure, usually produced by forcing unbalanced flow through Orifices or Venturis. Problems involving the provision of a steady unbalanced flow however allow very few practical flow meters.
Thermal meters measure the energy removed from a heated element. Heat loss and temperature rise techniques, although dependent on the heat capacity of fluids, provide good metering results, especially for low flows of gas.
The fourth true mass flow meter principle depends on the Coriolis force. This force is produced by a moving body subjected to an angular acceleration. In this case, the moving body is the flowing fluid while the angular acceleration is provided by a vibrating element oscillating within the fluid. The resulting Coriolis force reacts on the vibrating element in the same direction of the flow. This causes the vibrating element to bend slightly. This bending is detected by the resultant phase differences. Normally a tube is anchored at each end and vibrated in the center. Sensors between the anchors detect the phase difference, which is proportional to mass flow. Twin tubes increase noise rejection, and bending the tube into loops or a 'U' shape improves sensitivity. A computer provides control, makes corrections and calculates mass flow and density. Future designs may replace the vibrating tube with 'tuning fork arrangements.'
The Coriolis meter design is the first practical, accurate, true mass flow meter with a variety of uses for many different fluids.