Mass flow meter

A Coriolis mass flowmeter ( CMD) is a flow meter that measures the mass flow of the flowing liquids or gases. The measurement method is based on the Coriolis principle.

• 4.1 Areas of application
• 4.2 Product Quality

Construction

Today's generation of devices of Coriolis mass flowmeters used the Geradrohrgeometrie in the single-tube or two-tube version.

Function

The pipe bend is rotated by actuators in vibration. The rotational axis of the base of the arch, that is, the direction of inflow and outflow. The legs swing and thus perform a partial circle. In this case, the web speed of a point on the arm is all the greater the greater the distance from the rotational axis.

Without flow

The two legs of the pipe bend, left and right, rocking the same without temporal difference. Seen from the front side of the assembly of moving one behind the other congruently. The deflection of the pipe by the vibration is not detected by the eye. The amplitude is too small, however, they can be felt by hand.

The frequency of the output signal is in this case, where k is the torsional spring constant ( stiffness compared to the acting Auslenkmoment ) of the tube and J the moment of inertia (which is proportional to the mass of the substance in the tube ) is. Thus, by the density of the object is determined.

With flow

If the pipe passes through, must match the entry of the fluid, eg be brought to an ever increasing line speed in the left leg of the arch, the mass. According to the law of inertia, therefore, the liquid in the left leg delayed = lagging. Cause is the Coriolis force, or which is caused by the Coriolis acceleration. The medium in the apex of the sheet has reached the maximum web speed.

The medium flows back to the axis of rotation in the right leg, constantly achieved places a lower web speed. The inertial force of the Coriolis acceleration now presses before the liquid in the legs in the vibration direction. The liquid in the right leg stands proud.

If you look again the moving pipe loop from the front page, so the two legs do not move in a row. The time difference depends on the vibration frequency, the mass of the medium and the flow velocity and the (temperature- specific ) elasticity modulus of the tube. Thus, this method is able to measure the mass flow directly, rather than indirectly to determine him about other characteristics (volume, density, viscosity).

Simplified representation

The influence of the Coriolis force is visible when a garden hose is held between the hands and slightly stretched.

Signal Conversion

A the inlet and outlet are mounted on the vibrating system sensors which generate sinusoidal signals. Without flow, both signals are in phase. For mass flow results from the different in the inlet and outlet Coriolis force, a phase shift of the two signals. This phase shift is proportional to the mass flow.

Application

Depending on the medium, whether gases or liquids, tubes with different tube inner diameters are used. The meter is only suitable for multi-phase flow, since the non-uniform density distribution in the measuring tube can adversely affect the oscillating system. Are the phases, however, homogeneously mixed, two phases can be on the density measurement and knowledge of the fluid properties proportionally calculate (eg dissolved sugar in water, can be converted into Brix and output directly ). The pressure drop varies depending on the type, pipe inner diameter, and properties of the medium. The sensors have no columns and can be used excellently therefore also in the food or pharmaceutical industry. The resonance frequency of the oscillating system is in addition to the mechanical properties of the flow tube (or tubes ) of the density of the medium to be measured depends. Thermal expansions in the measuring tube (s ) and the carrier structure also change the frequency. Therefore, in general, the temperature is measured on the measuring tube and the support structure and is used for temperature compensation. Thus, the density can be precisely recorded m³ with an accuracy of ± 2 kg / and also issued with knowledge of the material data equal temperature compensated.

Devices are available for high temperatures and pressures available. An application in hazardous areas is also possible. Versions for use in legal metrology are also available.

Other areas of application include:

• Natural gas fuel pumps
• Pipeline accounting measurement
• Dosing
• Truck and rail loading test bench.

The instruments should not be confused with similarly structured density meters after the bending vibrator principle.

Areas of application

Since this technology is independent of properties of the medium such as: is conductivity, flow profile, density, viscosity, etc., almost all substances can be measured such as oils and fuels, cleaning agents and solvents, greases, silicone oils, alcohol, methane, fruit solutions, thickness, colors, vinegar, ketchup, mayonnaise, beer, milk, sugar solutions, gases, liquid gases.

Product quality

Many users just estimate the density measurement and set them a target for quality control. Due to the simultaneous detection of the density and the temperature of the fluid, a simultaneous quality assessment of the medium is possible. Differs from the density of the medium from the nominal value, this indicates the quality problems in the process close. Also air inclusions can be detected directly from the density signal. Conventional Density Meters often cost many times with similar accuracy.

Benefits

In summary, a Coriolis mass flow meter has the following advantages:

• Universal measuring system for flow, density and temperature, regardless of - Conductivity
• - Entry and exit routes
• - Flow profile

• - Mass Flow
• - Density
• - Temperature

Disadvantages

• Usage limits for multiphase media or excessive gas content.
• Deposits can lead to errors, particularly in the density measurement
• Limited selection of materials for wetted parts, corrosion must be checked particularly
• The medium to be measured must be homogeneous.