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Mass flow meterFrom Wikipedia, the free encyclopedia

A mass flow meter, also known as an inertial flow meter is adevice that measures mass flow rate of a fluid travelingthrough a tube. The mass flow rate is the mass of the fluid

traveling past a fixed point per unit time.

The mass flow meter does not measure the volume per unittime (e.g., cubic meters per second) passing through thedevice; it measures the mass per unit time (e.g., kilograms persecond) flowing through the device. Volumetric flow rate isthe mass flow rate divided by the fluid density. If the densityis constant, then the relationship is simple. If the fluid hasvarying density, then the relationship is not simple. Thedensity of the fluid may change with temperature, pressure, orcomposition, for example. The fluid may also be a

combination of phases such as a fluid with entrained bubbles.

Operating principle of a coriolis flow meter

A mass flow meter

Contents

1 Operating principle of a coriolis flow meter 2 Density and volume measurements 3 Calibration 4 See also 5 External links

There are two basic configurations of coriolis flow meter: the curved tube flow meterand the straighttube flow meter. This article discusses the curved tube design.

Theanimationsontherightdo

notrepresentanactuallyexistingcoriolisflowmeterdesign.Thepurpose

oftheanimationsis

A rotating mass flow meter to illustrate the

operating principle of the vibrating massflow meter.

double sized version

When fluid is flowing through the tubes, the

tubes twist slightly.double sized version

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toillustrate the operating principle, and to show the connection with rotation.

Fluid is being pumped through the mass flow meter. When there is mass flow, the tube twists slightly.The arm through which fluid flows away from the axis of rotation must exert a force on the fluid, toincrease its angular momentum, so it bends backwards. The arm through which fluid is pushed backto the axis of rotation must exert a force on the fluid to decrease the fluid's angular momentum again,

hence that arm will bend forward.

In other words, the inlet arm is lagging behind the overall rotation, and the outlet arm leads the overallrotation.

The animation on the right represents how curved tubemass flow meters are designed. When the fluid isflowing, it is led through two parallel tubes. An actuator(not shown) induces a vibration of the tubes. The twoparallel tubes are counter-vibrating, to make the

measuring device less sensitive to outside vibrations.The actual frequency of the vibration depends on the sizeof the mass flow meter, and ranges from 80 to 1000vibrations per second. The amplitude of the vibration istoo small to be seen, but it can be felt by touch.

When no fluid is flowing, the vibration of the two tubesis symmetrical, as shown in the animations.

The animation on the right represents what happensduring mass flow. When there is mass flow, there issome twisting of the tubes. The arm through which fluidflows away from the axis of rotation must exert a forceon the fluid to increase its angular momentum, so it islagging behind the overall vibration. The arm through

which fluid is pushed back towards the axis of rotationmust exert a force on the fluid to decrease the fluid'sangular momentum again, hence that arm leads theoverall vibration.

The inlet arm and the outlet arm vibrate with the samefrequency as the overall vibration, but when there ismass flow the two vibrations are out of sync, the inletarm is behind, the outlet arm is ahead. The twovibrations are shifted in phase with respect to each other,and the degree of phase-shift is a measure for the amountof mass that is flowing through the tubes.

Density and volume measurements

The principle design of a curved tube mass

flow meter. The vibration pattern during no-

flow.


Vibration pattern of the tubes when there is

mass flow.



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The mass flow of a u-shaped coriolis flow meter is given as:

where Ku

is the temperature dependent stiffness of the tube, Ka shape-dependent factor, dthe width, the time lag, the vibration frequency andI

uthe inertia of the tube. As the inertia of the tube

depend on its contents, knowledge of the fluid density is needed for the calculation of an accuratemass flow rate.

If the density changes too often for manual calibration to be sufficient, the coriolis flow meter can beadapted to measure the density as well. The natural vibration frequency of the flow tubes depend onthe combined mass of the tube and the fluid contained in it. By setting the tube in motion andmeasuring the natural frequency, the mass of the fluid contained in the tube can be deduced. Dividingthe mass on the known volume of the tube gives us the density of the fluid.

Such an instantaneous density measurement in turn allow us to calculate the flow in volume per time,by dividing mass flow with density.

Calibration

Both mass flow and density measurements depend on the vibration of the tube. This depends on therigidity of the tube which in turn depend on its temperature. Calculations must therefore take thetemperature of the fluid into account.

See also

Coriolis effect Flow measurement Gaspard-Gustave Coriolis

External links

Lecture slides on flow measurement, University of Minnesota

Retrieved from "http://en.wikipedia.org/wiki/Mass_flow_meter"Categories: Classical mechanics | Flow meters | Mass

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