introduction to measurement system

D1MC Semester 1 / Energy Conservation Management ( ECM06) / May 2007 /

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Instrumentation and Measurement Techniques

•Introduction •Instrumentation System•Errors•Transducers

•Other Measurements:flow, level, pressure, temperature, miscellaneous


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Objectives

•Introduction to general elements of measurement system

•Introduction to general functions of instruments


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Specific Objective• Define the basic concept of instrumentation :

– Measurement– Calibration– Primary and Secondary Standards

• Describe the measuring system by block diagrams representing:– Transducer– Signal Conditioner– Display unit /recorder

• Explain various classification of the above objective • Explain instrumentation system performance with reference to :

– Static performance– Dynamic performance


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Measurement SystemA measurement system converts the unknown quantity under measurement to a numerical unit using instrument.

Number + measured unit6.8 kg/(ms)2

Measurement can be defined as a comparison with a standard therefore calibration and routine maintenance are prerequisites of accurate measurement. It is multidiscipline subject which embraces physics, thermodynamics, mechanics , fluids chemistry and electrotechnology.


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Instrument• 1) Instrumentation is the application of

instruments for monitoring, sensing and measurement .Some of its applications are

• a) product testing• b) monitoring- health, safety, costing • c) control system • d) R&D


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Instrument• 2) The advantage of electronic instrument over

mechanical instruments,• a) rapid response • b) flexibility of electrical signal –amplitude,

distance


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CalibrationCalibration can be performed in two ways :

• a) varying one input quantity with allother parameters kept constant and observing the output b) marking or graduating • an output scale as the primary quantity is varied through

its full range .

Two type of standards are used primary standard and secondary standard (working standard)


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•Measurement - Comparison between an unknown quantity and a predefined standard

Terms

•Measurand- the unknown quantity to be measured

•Instrument - physical device uses to determine measurand numerically.


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Transducer• a) Transducer- an element which converts physical

quantity being measured.• b) Signal conditioner – modify the tranduced signal

in a form that can be recorded.• c) Recorder /display – to record or display the

measured quantity.• Example: Bourdon gauge - Bourdon tube

(transducer), Links and gear(Signal conditioner), Graduated scale and pointer (Display )


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Transducer

•A device to which change or converts physical quantity in a more easily measurable quantity

Transducer

(Output)Actuator

(Input)Sensor


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Transducer Basic Requirement• Ruggedness• Linearity• Repeatability• High Signal to Noise Ratio• High stability and reliability


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SensorA device which senses and detects the physical quantity of measurand and converts to electrical form.Example of sensors:

Mechanical : Bourdon tube pressure meter.

Electrical : Potentiometer

Optical : Photon counter

Chemical : Thermocouples

*All sensors are transducers but not all transducers are sensors


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ActuatorA device that senses and detects the electrical quantity and converts to physical form.

Example of actuator:• Valve in heat exchanger system

• Motor speed control where the motor is driving the conveyor belt

• Magnetic relays that turn on/off of the fans

• Compressor in a control air conditioning


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Variable Conversion ElementA device that converts analogue signals to digital form or vice versa.

Example of converters:

ADC -- Analogue to digital converter

DAC -- Digital to analogue converter


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Variable manipulation element

A device that manipulate the signal presented to it while preserving the original information.

Example:

Amplifier : Signal gain

Buffer : Unity conversion

Attenuator : Signal reduce


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Signal conditioning

• Operation performed on the signal to bring it to the desired form.

• Include variable conversion & variable manipulation element.

Example:

Filter, amplifier, comparator circuit, differential circuit, ADC, multiplexer and etc.


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TelemetryTransmission of data from remote sources to serve specific purposes.

Example:

Remote control, coaxial cable, wireless transmission, optical fiber cable and etc...


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Data output element

To convey the measured quantity for further action.

Display, recording and control.

Display : Monitor(CRT), LCD

Recording : Magnetic tapes, printer

Control : Data managing system


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Essential DevicesThese following devices must always present for all instruments:

Power supply : To supply power(current) to instruments

Pulse generator : To generate clock to digital system

Protection device : To supervise current flow.

Shielding system : To protect instrument from interfere by external disturbance.


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Function of Instruments & Measurement Systems

Indicating instruments :Meter display, digital display

Example: speedometer in car, pressure gauge

Recording function : Data Keeping

Example: Printer, magnetic disc

Controlling function :Temperature, position, speed, liquid level, flow control.


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Performance Indication An ideal measuring system is one where the output

signal has a linear relationship with the measurand. Error is the difference between the indicated value and the true value. Measuring and control system performance can be examined in two ways ,

• Static performance –when steady or constant input signals are applied

• Dynamic performance – when changing input signals are applied


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Static Performance Indication • Static sensitivity is defined as the ratio of the

change in output to the corresponding change in input under static or steady state conditions, . For a system having static sensitivities of K1 ,K2,K3 ……, the overall system sensitivity is given by


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Dynamic Performance Indication• Dynamic performance of both measuring and control

system is specified by response to certain standard test inputs

• Step input- abrupt change from one steady value to another ,will give the transient response

• Ramp input –which varies linearly with time, will give the ramp response

• Sine wave input – will give the frequency response


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Error Calculation• Accuracy can be stated in terms of errors

introduced

• Percentage error = Indicated value – True value . X . 100%Maximum scale value

%100xvalueTrue

valueTruevalueIndicated −

Precision is used to specify the closeness of output result when a measuring device is subjected to the same input on a number of occasions


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EXAMPLEA measuring system consist of a transducer , an

amplifier and a recorder, with an individual sensitivities as follows :

• Transducer sensitivity 0.2 mV /0 C• Amplifier gain 2.0 V/mV• Recorder sensitivity 5.0 mm /V• Determine the overall system sensitivity• K= K1 x K2x K3• = 0.2 mV/0C x 2.0V/mVx 5.0 mm/V• =2.0 mm/0 C


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EXAMPLEA 0 to 10 bar pressure gauge was found to have an error of ±

0.15 bar when calibrated by the manufacturer .Calculate (a) the percentage error of the gauge and (b) the possible error asa percentage of the indicated value when reading of 2.0 bars was obtained in a test .

Percentage error = 0.15/10. x 100 = ± 1.5%Possible error = ± 0.15%∴ error at 2.0 bars = 0.15/10. x 100 =± 7.5%

• The gauge is therefore more unreliable at the lower end of its range, and alternative gauge with a more suitable range should be used .


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EXAMPLE• For a general measuring system where the errors in

the transducer, signal conditioner , and recorder are ±2 % , ± 3%,and ±4 % respectively, calculate the maximum possible system error and the probable or root- sum –square error.

• Maximum possible error = ± (2+3+4)% =±9%• Root –sum-square error = ± √(22+32+42)%• = √ 29% =± 5.4%• Thus the error is possibly as large as ± 9% but

probably not larger than ± 5.4%.


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Transducer• Resistance transducers• Resistance strain gauge • Resistance temperature transducer• Photo-emisive cell• Capacitive transducer• Inductive transducer• Linear variable differential transformer• Piezos-electric transducer• Electromagnetic transducers• thermoelectric transducer• Photoelectric cell

introduction to measurement system

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