Static and Dynamic characteristics of Measuring Instrument

The performance of an instrument is described by means of a quantitative qualities termed as characteristics. They are characterized into two types static and Dynamic.

1. Static and Dynamic
Characteristics of Measuring
Instruments
Mrs.K.V.Archana
Assistant Professor
Department of Electronics and Communication
Engineering
Avinashilingam Institute For Home Science and Higher
Education For Women, Coimbatore

2. Characteristics ofInstruments
Theperformanceofaninstrumentisdescribedbymeansofa
quantitativequalitiestermedascharacteristics.Thesearebrokendown
into:
1.Static Characteristics: These characteristics pertain to a system
where the quantities to be measures are constant or vary slowly with
time
2.Dynamic Characteristics: Performance criteria based on dynamic
relations (involving rapidly varying quantities)
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3. StaticCharacteristics
Accuracy: It is the closeness with which an instrument reading approaches
the true value of the quantity measured.
Precision : The degree to which repeated measurements show the same
results.
Low Accuracy
Low Precision
Low Accuracy
High Precision
High Accuracy
Low Precision
High Accuracy
High Precision
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4. Accuracy andPrecision
Accuracy may be specified in terms of inaccuracy or limit of errors and can be
expressed in the following ways:
1.Point Accuracy
2.Accuracy as “Percentage of Scale Range”
3.Accuracy as “Percentage of True value”
IndicationofPrecision
SignificantFigures:Itisanindicationofprecisionofmeasurement.Itconveytheactual
informationregardingthemagnitudeandthemeasurementprecisionofaqty.Themorethe
significantfigures,thegreatertheprecision.
e.g.302A=3S.F
302.10V=5S.F0.00030=5S.F
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5. StaticCharacteristics
Resolution or Discrimination: The smallest detectable incremental change of the input
parameter that can be detected in the output signal. Eg; Scale, Multi range meters.
Sensitivity: For an instrument or sensor with input x and output y. Sensitivity = dy/dx
output
Static sensitivity = Infinitesimal change in output/infinitesimal
change ininput
output
input
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6. Continued
Repeatability:Closenessofoutputreadingwhenthesameinputisappliedrepeatedly
overashortperiodoftimewiththesamemeasurementconditionssameinstrumentand
observer,samelocationandsameconditionsofusemaintainedthroughout.
 Reproducibility:Closenessofoutputreadingsforthesameinputwhenthereare
changesinmethodofmeasurement,observer,location,conditionsofuse,andtimeof
measurement.
Span&Range:
•Range : High measurement possible
•Span : Difference between max. and min measurement possible
•E.g. Thermocouple (700 0C to 1200 0C) Ammeter (0 to 10 A)
Dead zone : The largest of a measured variable for which the instrument
•does not respond Cause: friction in mechanical measurement system
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7. Continued
•Dead Time :The time before the instrument begins to respond after the
measured quantity has been changed. E.g: Camera, Data acquisition
card,Ammeter
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8. Continued
Drift : It is an undesired gradual departure of the instrument o/p over a period of time that
is unrelated to changes in i/p , operating conditions or load.
•The drift may be caused by the following factors:
1)Mechanical vibrations
2)Temp. changes
3)Wear and Tear etc.
Classification:
1)Zero drift : If the whole of instrument calibration/ characteristics gradually shifts one by same amount. It
may be due to presence set or slippage and can be corrected by shifting pointer position.
Zero
output
Characteristics with zerodrift
Normalcharacteristics
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9. Continued
2) Span or sensitivity Drift : If the calibration from zero upwardschanges proportionally
output
Span Drift
Normal Characteristics
3) Zonal Drift : When the drift occurs only over a portion of span of aninstrument.
output
Zonal Drift
Normal Characteristics
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10. Continued
Linearity:Ifthecalibrationfromzeroupwardschangesproportionally.If
input-outputrelationshipisastraightlinepassingthroughorigin
•Nonlinearitycauselotofproblemduringsignalconditioningeventhoughitis
moreaccurateinsomecasese.g.LVDT(linear),Thermistor(Non-linear)
Error: Error is the degree to which a measurement conforms to the expected
or true value .Errors are due to measuring instruments (causing the change in
the value of the parameter being measured) or due to persons carrying out the
measurements (human errors).Errors may be expressed as absolute or
percentage.
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11. Error Types
Types of Errors
Grosserrors
Humanerrors
Systematicerrors
Instrumenterrors
Environmentalerrors
Observationalerrors
Randomerrors
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12. Dynamic Characteristics
1)Speed of Response: It is defined as the rapidity with which a
measurement system responds to changes in the measurement
quantity.
2)MeasurementLag:Itreferstoretardationordelayintheresponse
ofmeasurementsystemtochangesinmeasuredquantity.Thelagis
causedbyconditionssuchascapacitance,inertiaorresistance.
•Measuringlagareoftwotypes:
a)Retardationtypelag
b)Timedelaytypelag
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13. Continued
3)Fidelity:Itisdefinedasthedegreetowhichameasurementsystem
indicateschangesinthemeasuredquantitywithoutanydynamic
error.
4)Dynamicerrorormeasurementerror:Itisthedifferencebetween
truevalueofthequantitychangeswithtimeandthevalueindicated
bythemeasurementsystemifnostaticerrorisassumed.
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14. THANK YOU
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