In many areas of the industry is more interesting to measure mass flow to volumetric flow. In some processes in the food industry, for example, products such as pastes, pulps or yogurt are often packaged by weight, not volume. For this reason, the labels on the packaging of these products to inform consumers of the product weight rather than volume. The reason is that the volume of most liquids can vary greatly influenced by the physical conditions of pressure, temperature and density.

By contrast, the mass of fluid is not affected by these influences - so that the mass flow measurement has some advantages that the volumetric flow simply can not offer. This is a particularly important aspect in the count of flows for the packaging and billing.

The usual way to determine the mass of a body is weighed. But from the standpoint of engineering major difficulties arise when trying to direct a mass despite continuously flowing through a piping system. However, in recent decades there have been a measurement principle which allows a direct measurement and continuous mass flow in pipes, namely the mass flow measurement by Coriolis. In some applications it is more reasonable to apply this principle to determine the mass from indirect methods of measuring volume flow and density (volume x density = mass).

Measuring principle

The first description of this principle is commonly attributed to French mathematician and physicist whose name is known: Gustave Gaspard Coriolis (1792-1843). The effect only occurs in rotating systems, eg in roundabouts or rotating surface of our own planet, but not to be confused with centrifugal force. Although the use of the term "Coriolis force" is widespread, the description of this force is often difficult, and much explanation. This force appears when a system is superimposed straight line movement and rotary motion.

Figure below shows a practical example:

A person still on a rotating circular platform halfway between center and edge simply knock your weight slightly inward to counter the centrifugal force (left). However, if the person moves from the center to the edge of the turntable, as it moves seen an increase in speed and the Coriolis force occurs in reaction to the forces of inertia. The Coriolis force tends to deflect the person of the shortest path on the turntable (ie, the straight line on the radius of the circular platform). The higher the speed of rotation of the platform, the greater the weight Give it more individual and their speed of movement towards the edge of the circular platform (the "mass flow"), the greater the effect of inertia and the greater the effect will be perceived Coriolis force.

In mathematical terms, the value of the Coriolis force (Fc) is directly proportional to the moving mass (m), the angular velocity (co) and radial velocity (vr) in the rotating system:

Ill.: Causes and effects of the Coriolis force in a rotating circular platform.

Coriolis forces are always presented in a linear movements are superimposed rotational movements (right). In the absence of linear motion (left, a person at rest), only perceived centrifugal forces.

In a flow meter Coriolis mass flow, each individual particle mass is subjected to the same influence that person's body on the turntable we see in the illustration above (see Figure below). The rotational movement that causes the Coriolis force in the above description is replaced in the flow by an oscillating movement of the measuring tube in its resonant frequency.

- A zero flow when the fluid is at rest, there is no linear movement (a). Therefore, there are no Coriolis forces.

- However, when the mass of fluid flows, the movement induced oscillation (equivalent to a rotation) of the measuring tube is superimposed on the linear motion of fluid in circulation, the effects of the Coriolis force "twist" tubes measurement (b, c), and sensors (A, B) at the entrance and exit recorded a time difference in this movement, ie a phase difference. The higher the mass flow, the greater the phase difference (see Figure A).

Ill.: The Coriolis measuring principle (for a detailed explanation, see Figure below).

a = zero flow: state of oscillation of the measuring tube at zero flow

b - Traffic flow -> state of oscillation of the measuring tubes in the time interval 1

c = Traffic flow -> oscitación state of the measuring tubes in the time interval 2

Figure A: Coriolis forces and geometry of the oscillation in the measuring tubes.

When the fluid flows, mass particles move along the measuring tube and are subject to a superimposed lateral acceleration due to Coriolis forces (Fc). At the entrance of the tube, the particle mass (m) experience a shift away from the center of rotation (Z1), and back again to the center (Z2) as they approach the outlet end. Coriolis forces act in opposite directions at the entrance and exit and wing tube measurement starts a. "Buckle". This change in geometry on the oscillation induced in the measuring tube is recorded on the sensors (A, B) at each end of the tube as a phase difference. This phase difference (Δω) is directly proportional to the mass of the fluid and velocity (v) the same, so also the flow máslco.

An important aspect when applying Coriolis flowmeters is the possible presence of external influences such as vibration of the pipe. The vibrations in piping systems often have vibration frequencies between 50 and 150 Hz on the other hand, the resonance frequencies typical Coriolis flowmeters E + H are between 600 and 1,000 Hz, these flowmeters are, therefore immune to vibrations induced in the system of this nature. In addition, for the same reason, these measuring devices require no special mounting vibration inhibitor.

The field of values ​​of the standard nominal sizes available ranging from DN 1 to 300 (1 / 24 to 12 "). However, in practice we can find from dispensing very small amounts in pharmaceutical applications to applications for loading and unloading of ships merchant. The choice of models is correspondingly wide.

Measurement of densities:

The measuring tubes are in constant oscillation frequency of resonance. If the fluid density changes, and therefore the mass of the oscillating system (more fluid measuring tube), the frequency of oscillation is adjusted accordingly. The resonant frequency is thus a function of fluid density and can be used as an additional output signal.

Temperature measurement:

The temperature of the measuring tubes is determined to calculate the compensation factor, which takes into account the effects of temperature. This signal corresponds to the process temperature and is also available as output signal.