39604873 flow transmitters

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Mini Project Report

On

Flow Transmitter using Differential Pressure Sensor

Submitted by

Mohammed Sarwar Shaikh

Arif Shamsher Khan

Mohammed Nasiri

Under the Guidance of

Prof.N.B.Joshi.

INSTRUMENTATION DEPARTMENT

Smt. Indira Gandhi College of Engineering

Koparkhairane ,Navi Mumbai.

University of Mumbai

2009-10

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Smt. Indira Gandhi College of Engineering

INSTRUMENTATION DEPARTMENT

CERTIFICATE

This is to certify that

Mohammed Sarwar Shaikh

Arif Shamsher Khan

Mohammed Nasiri

Have submitted this mini project report entitled

FLOW TRANSMITTER USING DIFFERENTIAL PRESSURE SENSOR

as part of their work in partial fulfillment of requirement for V semester of BACHELOR

OF ENGINEERING in INSTRUMENTATION Engineering (University of Mumbai)

course during the academic year 2009-10.

GUIDE H.O.D

(Prof. N. B. JOSHI) (Prof. S. D. Gaikwad)

EXAMINER PRINCIPAL

(Dr. S. K. Narayankhedkar)

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Flow Transmitters

Flow transmitters are circuitry which are used to

amplify and condition the signal coming from theparticular sensor and transmit it to suitable use (such

as display, transmission or further signal processing).

Flow transmitters can take signals from various type of

sensor depending upon their circuitry, or environment

or process place in which they are going to be installed.

A Flow transmitter must be able to perform at least

three basic operations on the incoming signal from the

sensor i.e. amplifying, signal conditioning, converting

(favorably to the current parameter if used for long

distance transmission).

There are various type of flow transmitters they are,

Differential Pressure flow transmitters, Ultrasonic flow

transmitters, Mass flow transmitters, Wheel flow

transmitters.

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Differential Pressure Flow Transmitters

In a differential pressure drop device the flow is calculated bymeasuring the pressure drop over an obstructions inserted in the flow.The differential pressure flowmeter is based on the BernoullisEquation, where the pressure drop and the further measured signal is afunction of the square flow speed.

The DP transmitter operation is dependent on the pressure differenceacross an orifice, venturi, or flow tube. This differential pressure isused to position a mechanical device such as a bellows.The bellows acts against spring pressure to reposition the core of adifferential transformer. Thetransformers output voltage on each of two secondary windings

varies with a change in flow.

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Differential Pressure Flow Detection Block Diagram

A loss of differential pressure integrity of the secondary element, theDP transmitter, will introduce an error into the indicated flow. Thisloss of integrity implies an impaired or degraded pressure boundary

between the high-pressure and low-pressure sides of the transmitter. Aloss of differential pressure boundary is caused by anything thatresults in the high- and low-pressure sides of the DP transmitter beingallowed to equalize pressure.As previously discussed, flow rate is proportional to the square root ofthe differential pressure.The extractor is used to electronically calculate the square root of thedifferential pressure and

provide an output proportional to system flow. The constants aredetermined by selection of theappropriate electronic components.The extractor output is amplified and sent to an indicator. Theindicator provides either a local or a remote indication of system flow.

Recovery of Pressure Drop in Orifices, Nozzles and VenturiMeters

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After the pressure difference has been generated in the differentialpressure flow meter, the fluid pass through the pressure recovery exitsection, where the differential pressure generated at the constrictedarea is partly recovered.

As we can see, the pressure drop in orifice plates are significanthigher than in the venturi tubes.

Differential Pressure Flow Transmitters using

Piezoresistive Differential Pressure sensor

The resistance change in a monocrystallinesemiconductor (a piezoelectric effect) is substantially

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higher than that in standard strain gauges, whoseresistance changes with geometrical changes in thestructure. Because most control systems operate withelectrical signals, pressure or force must be convertedto current or voltage before further processing oranalysis. Capacitive and resistive signal transducers arecommonly used for this purpose.

In resistive sensors, pressure changes the resistance bymechanically deforming the sensor, enabling theresistors in a bridge circuit, for example, to detectpressure as a proportional differential voltage acrossthe bridge. Conventional resistive pressuremeasurement devices include film resistors, straingauges, metal alloys, and polycrystallinesemiconductors.

Specific advantages are:

High sensitivity, > 10mV/V

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Good linearity at constant temperature

Ability to track pressure changes without signal hysteresis, up tothe destructive limit

Disadvantages are:

Strong nonlinear dependence of the full-scale signal ontemperature (up to 1%/kelvin)

Large initial offset (up to 100% of full scale or more) Strong drift of offset with temperature

Within limits, these disadvantages can be compensated with electroniccircuitry.

The signal generated from piezoresistive differential pressure sensorare then given as input preferably to an instrumentation amplifierwhich are potent for amplifying and signal conditioning .

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Block Diagram description.

1. PIEZORESISTIVE SENSOR :

This is a Low pressure sensor uses the piezo resistive method toconvert the air pressure into electrical signal. This electricalsignal is given to the amplification block.

2. AMPLIFIER :

The electrical signal from the piezo resistive sensor is given tothe amplifier where the noise & disturbances are suppressedwhereas the amplitude level is raised by the suitable gain factor

proper shaping if the signal is alos provided by this block we getthe pure equivalent electrical signal which high amplitude. Theoutput is given to the V to I Converter.

3. VOLTAGE TO CURRENT CONVERTER :

V to I converter takes the output from the Amplifier in electricalform (voltage ) and is converted into the electrical current in the

range of 4-20mA.

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Instrumentation Amplifier

A brief description of an instrumentation amplifier is given belowalong with its most commonly used circuit configuration.

An instrumentation (orinstrumentational) amplifier is a type ofdifferential amplifierthat has been outfitted with input buffers, which

eliminate the need for input impedance matching and thus make theamplifier particularly suitable for use in measurement and testequipment. Additional characteristics include very low DC offset, lowdrift, low noise, very high open-loop gain, very high common-moderejection ratio, and very high input impedances. Instrumentationamplifiers are used where great accuracy and stability of the circuitboth short- and long-term are required.

Although the instrumentation amplifier is usually shownschematically identical to a standard op-amp, the electronicinstrumentation amp is almost always internally composed of 3 op-amps. These are arranged so that there is one op-amp to buffer eachinput (+,), and one to produce the desired output with adequateimpedance matching for the function.[1][2]

The most commonly used instrumentation amplifier circuit is shownin the figure. The gain of the circuit is

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http://en.wikipedia.org/wiki/Differential_amplifierhttp://en.wikipedia.org/wiki/Electronic_test_equipmenthttp://en.wikipedia.org/wiki/Electronic_test_equipmenthttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Drifthttp://en.wikipedia.org/wiki/Noise_(physics)http://en.wikipedia.org/wiki/Open-loop_gainhttp://en.wikipedia.org/wiki/Common-mode_rejection_ratiohttp://en.wikipedia.org/wiki/Common-mode_rejection_ratiohttp://en.wikipedia.org/wiki/Input_impedancehttp://en.wikipedia.org/wiki/Accuracyhttp://en.wikipedia.org/wiki/BIBO_stabilityhttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Differential_amplifierhttp://en.wikipedia.org/wiki/Electronic_test_equipmenthttp://en.wikipedia.org/wiki/Electronic_test_equipmenthttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Drifthttp://en.wikipedia.org/wiki/Noise_(physics)http://en.wikipedia.org/wiki/Open-loop_gainhttp://en.wikipedia.org/wiki/Common-mode_rejection_ratiohttp://en.wikipedia.org/wiki/Common-mode_rejection_ratiohttp://en.wikipedia.org/wiki/Input_impedancehttp://en.wikipedia.org/wiki/Accuracyhttp://en.wikipedia.org/wiki/BIBO_stabilityhttp://en.wikipedia.org/wiki/Electrical_network


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The rightmost amplifier, along with the resistors

labelled R2 and R3 is just the standard differentialamplifier circuit, with gain = R3 / R2 and differentialinput resistance = 2R2. The two amplifiers on the leftare the buffers. With Rgain removed (open circuited),they are simple unity gain buffers; the circuit will workin that state, with gain simply equal to R3 / R2 and highinput impedance because of the buffers. The buffergain could be increased by putting resistors betweenthe buffer inverting inputs and ground to shunt awaysome of the negative feedback; however, the singleresistor Rgain between the two inverting inputs is a muchmore elegant method: it increases the differential-modegain of the buffer pair while leaving the common-modegain equal to 1. This increases the common-moderejection ratio (CMRR) of the circuit and also enables

the buffers to handle much larger common-modesignals without clipping than would be the case if theywere separate and had the same gain. Another benefitof the method is that it boosts the gain using a singleresistor rather than a pair, thus avoiding a resistor-matching problem (although the two R1s need to be

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matched), and very conveniently allowing the gain ofthe circuit to be changed by changing the value of asingle resistor. A set of switch-selectable resistors oreven a

potentiometer can be used for Rgain, providing easychanges to the gain of the circuit, without thecomplexity of having to switch matched pairs ofresistors.

The ideal common-mode gain of an instrumentationamplifier is zero. In the circuit shown, common-modegain is caused by mismatches in the values of theequally-numbered resistors and by the mis-match incommon mode gains of the two input op-amps.Obtaining very closely matched resistors is a significantdifficulty in fabricating these circuits, as is optimizingthe common mode performance of the input op-amps.

The output of the instrumentation amplifier is supplied

to the Voltage-Current converter, which converts thevoltage output of the instrumentation amplifier intocurrent attribute, which are suitable for long distancetransmission and various other application.

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Voltage To Current Converter

For a variety of reasons, in low-voltage electronics, voltage is a morefrequently used data carrier. Thus electronic devices tend to be labeled

with voltage inputs and outputs. However some devices are labeled interms of current-input and -output (for example, abipolar transistor).In such cases, a component is needed to convert (change) the electricattributes into a relay of information.

A voltage-to-current converter changes the electric attribute carryinginformation from voltage to current. It acts as a linear circuit withtransfer ratio k = IOUT/VIN [mA/V] having dimension of conductivity.

That is why the active version of the circuit is referred also as atransconductance amplifier.

Typical applications of voltage-to-current converter are measuringvoltages by using instruments having current inputs, creating voltage-controlled current sources, building various passive and activevoltage-to-voltage converters, etc.

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Voltage-to-current converters feeding to grounded loads often findtheir way into industrial measurements and control applications. Theconventional textbook circuit needs both positive and negative-supplyrails.

This circuit uses one half of the quad operational amplifier LM324.The first amplifier is configured as a subtractor, while the secondamplifier is configured as a current converter.

The output of the first amplifier at A equals e1 minus ein. Here, e1 isderived from the positive power supply by potentiometer P1. Thevoltage at B equals V minus IL RS.

Op amp inputs at A and B are the same, so:

e1 ein = V IL RS

IL = ein/RS + (V e1)/RS

The first term is proportional to the input voltage, with the secondterm a constant. RS is chosen so that the first term gives 16 mA forfull-scale input voltage, and the potentiometer is adjusted such that thesecond term supplies a constant 4 mA. In effect, the output is 4 to 20mA, corresponding to zero to full input voltage. Thus, this circuitworks without using a negative power-supply rail. For the circuitshown in Figure 2, the current varies from 4 to 20 mA with an inputof 0 to 1 V.

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Using such circuits we can effectively transmit the signal comingfrom the piezoresistive differential pressure sensor, and effectivelyamplify and conditioning the signal, as well as converting the voltage

attribute to the current attribute.ADVANTAGES :

1. Measurement is accurate and precise as compare to othermethods.

2. Easy to Install.

3. Can be used for corrosive and viscous liquids.

DISADVANTAGES :

1. Design is complex.

2. Sensor is expensive.

APPLICATION :

1. It is most suitable for measurement of corrosive and viscous

fluid.

2. It is used for open tank applications.

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Future Scope :

Such circuitry and technique have tremendous use and future scope(i.e. development and implementation) in various fields, speciallymostly in process industry where constant reading and analyzing of

the fluid flow is an important aspect in production cycle.

Flow transmitters have found their way in to various fields, fromprocess industry, power generation, water supply, oil and gas industry,etc. Flow transmitter have penetrated into every industry, thus have awide use in present as well as future developing industry and units,and becoming a non avoidable part of Industrial Cycle.

References:- Differential Piezoresistive Pressure Sensor.

Firtat, B. Moldovan, C. Iosub, R. Necula, D. Nisulescu

A Linear voltage-to-current converter.(eBook).

Chen, R.Y. Tsung-Shuen Hung.

Op-Amp and its applications(eBook).

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Walter G. Jung

A.K. Sawhney

Instrumentation design by Ranghan, Mani & Sharma.

A handbook of process control by Bela G. Liptak.

Web Source:-

www.wikipedia.org.

www.artikel-software.com/handbook of instrumentation andcontrols/

www.maxim-ic.com
http://www.wikipedia.org/http://www.artikel-software.com/handbook%20of%20instrumentation%20and%20controls/http://www.artikel-software.com/handbook%20of%20instrumentation%20and%20controls/http://www.maxim-ic.com/http://www.wikipedia.org/http://www.artikel-software.com/handbook%20of%20instrumentation%20and%20controls/http://www.artikel-software.com/handbook%20of%20instrumentation%20and%20controls/http://www.maxim-ic.com/

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