flowmeters and data acquisition

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November 21, 20081

FLOWMETERS AND DATA

ACQUISITION

Ravi kumar, Ph.D.Associare ProfessorDepartment of Mechanical & Industrial EnggIndian Institute of Technology, Roorkee


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Magnetic Flow meter

A magnetic flow meter is a volumetric flow meterwhich does not have any moving parts and isideal for wastewater applications or any dirty

liquid which is conductive or water based. Magnetic flowmeters will generally not work with

hydrocarbons, distilled water and many non-

aqueous solutions). Magnetic flowmeters arealso ideal for applications where low pressuredrop and low maintenance are required.


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Principle of Operation

The operation of a magnetic flowmeter is based

upon Faraday's Law, which states that the voltageinduced across any conductor as it moves at rightangles through a magnetic field, is proportional tothe velocity of that conductor.

Faraday's Formula:E is proportional to V x B x D where:

E = The voltage generatedV = The velocity of the conductorB = The magnetic field strengthD = The length of the conductor


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Coriolis Mass Flow Rate Meter

The fluid to be measured runs through a U-shaped tubethat is caused to vibrate in a direction perpendicular tothe fluid flow.

As the flowing fluid is made to run through the tube andinteract with the vibration, causing it to twist. Because ofthe twist action the two limbs of the tube do not pass

through the mean position simultaneously. The greaterthe angle of the twist, the greater the flow.


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coriolis mass flow rate meter

Mathematically it can be shown that

Therefore, the mass flow rate is directlyproportional to the time lag.

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Hall effect flow meter| The flowmeter includes a housing including a

first port, a second port, a chamber in fluid

communication with the first port and thesecond port, and a fluid passageway extendingfrom the first port to the second port.

| An impeller having a non-magnetized metalpole piece is located within the chamber of thehousing and is rotatable about an axis inresponse to fluid flow through the fluidpassageway.

| A Hall Effect sensor and a magnet are located

outside of the fluid passageway. The magneticfield generated by the magnet enables the HallEffect sensor to sense rotation of the impellerand thereby the rate of fluid flow through thefluid passageway.


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Turbine Flow Meter| The turbine flow meter (better

described as an axial turbine)

translates the mechanical action ofthe turbine rotating in the liquid flowaround an axis into a user-readablerate of flow (gpm, lpm, etc.). Theturbine tends to have all the flow

traveling around it.| The turbine wheel is set in the part of

a fluid stream. The flowing fluidimpinges on the turbine blades,imparting a force to the blade surfaceand setting the rotor in motion. whena steady rotation speed has beenreached, the speed is proportional tofluid velocity.


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Vortex flow meter

A bluff body is placed in the path of thefluid. As the fluid passes this body,

disturbances in the flow called vortices arecreated. The vortices trail behind thecylinder, alternatively from each side ofthe bluff body.

The frequency at which these vorticesalternate sides is essentially proportionalto the flow rate of the fluid.

Downstream there is a sensor for measuring the frequency ofthe vortex shedding.

This sensor is often a piezoelectric crystal, which produces asmall, but measurable, voltage pulse every time a vortex is

created.


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vortex flow meter

Since the frequency of such a voltage pulse is alsoproportional to the fluid velocity, a volumetric flowrate is calculated using the cross sectional area ofthe flow meter. The frequency is measured and theflow rate is calculated by the flowmeter electronics.

With f= SV/L where,

f = the frequency of the vorticesL = the characteristic length of the bluff bodyV = the velocity of the flow over the bluff bodyS = Strouhal number,

which is essentially a constant fora given body shape within itsoperating limits


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Thermal mass flow meter| Thermal mass flow meters generally use

combinations of heated elements and temperature

sensors to measure the difference between static andflowing heat transfer to a fluid and infer its flow with aknowledge of the fluid's specific heat and density.

| Technological progress allows today to manufacture

thermal mass flow meters on a microscopic scale asMEMS sensor, these flow devices can be used tomeasure flow rates in the range of nano litres ormicro litres per minute.


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Pressure Transducers

pressure transducer is a transducer thatconverts pressure into an analog electricalsignal.

1. strain-gage base transducer

2. Piezoelectric transducer


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Pressure Transducers

| Millivolt Output Pressure Transducers(30mV)

| Voltage Output Pressure Transducers

0-5 VDC; 0-10 VDC; -5 +5 VDC| 4-20 mA Output Pressure Transducers

known as pressure transmitters (L>300 m)


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Pressure Sensors


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Details of Pressure Transducers


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Data Acquisition


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Signal Conditioning

z

Isolationz Amplification

z Filtering

z Switchingz Sample & Hold

z Excitation


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Electrical Isolation

Electrical isolation breaks the galvanic path between the

input and output signal. That is, there is no physicalwiring between the input and output. The input is normallytranferred to the output by converting it to an optical ormagnetic signal then it is reconstructed on the output. By

breaking the galvanic path between input and output,unwanted signals on the input line are prevented frompassing through to the output. Isolation is required when

a measurement must be made on a surface with avoltage potential far above ground. Isolation is also usedto prevent ground loops.


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Amplification

When a signal is amplified, the overallmagnitude of the signal is increased.Converting a 0-10mV signal to a 0 -10V signalis an example of amplification


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Signal Conditioning Requirements for Thermocouples

Amplification for High-Resolution ADC

Cold-Junction CompensationThermocouples requiresome form of temperature reference to compensate forunwanted parasitic thermocouples.

FilteringA thermocouple can act much like an antenna,making it very susceptible to noise from nearby 50/60 Hzpower sources. Therefore, apply a 2 Hz or 4 Hz lowpass filterto your thermocouple signal to remove power line noise.

LinearizationThe output voltage of a thermocouple is notlinear with temperature. Therefore, the system must performlinearization either through hardware or software.


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Excitation

Many sensors require some form ofexcitation for them to operate. Strain gagesand RTDs are two common examples.


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Virtual Instrumentation

VI is a paradigm shift from a hardware centricapproach of traditional instrumentation methodsto a software centric approach where the users

have the flexibility to develop his ownmeasurement instruments.


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NYQUIST THEOREM: Sampling Rate Versus Bandwidth

The Nyquist theorem states that a signal must be sampled

at least twice as fast as the frequency of the signal to

accurately reconstruct the waveform; otherwise, the high-frequency content will alias at a frequency inside the

spectrum of interest (passband). An alias is a false lower

frequency component that appears in sampled data

acquired at too low a sampling rate.


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Lab View

Front PanelControls = Inputs

Indicators = Outputs

Block Diagram program for front panel

Components wired

together


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THANK YOU..

flowmeters and data acquisition

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