PdfMESB 33hi3 Chapter 2x1(1)

8/11/2019 PdfMESB 33hi3 Chapter 2x1(1)

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INSTRUMENT PERFORMANCE CHARACTERISTIC

1. Static the desired input to theinstrument is constant or varying slowly withrespect to time

2. Dynamic the desired input is notconstant but varies rapidly with the time


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ERRORS AND UNCERTAINTIES INPERFORMANCE PARAMETERS

Type of errors and uncertainties associatedwith measurement system

Systematic or cumulative errorsInstrument error

Environmental errors

Loading errors


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Accidential or random error

Inconsistencies associated with accuratemeasurement of small quantitiesPresence of certain system defects

Effect of unrestrained and randomly varying

parameterMiscellaneous Type of Gross Errors

Personal or human errors

Error due to faulty components/adjustmentImproper application of the instrument.


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TYPES OF UNCERTAINTIES

External estimate of uncertainty, U EThe resolution of the instrument which is thesmallest confidently measurable input

Internal estimate uncertainty, U IIs inherent in the data itself


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PROPAGATION OF UNCERTAINTIES INCOMPOUND QUANTITIES

To compute the overall uncertainty due to thecombined effect of the uncertainties ofdifferent variables.


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STATIC PERFORMANCE PARAMETER

Static performance parameters of theinstruments are:-

1. Accuracy- the closeness of the instrumentoutput to the true value of the measuredquantity.

2. Precision- the ability of the instrument toproduce a certain set of reading within a givenaccuracy.


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ACCURACY VS PRECISION


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RESOLUTION (DISCRIMINATION)

The smallest increment in the measured valuethat can be detected with certainty by theinstrument.

THRESHOLD

The minimum value of input below which nooutput can be detected.


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STATIC SENSITIVITY

The ratio of the magnitude of response(outputsignal) to the magnitude of the quantity beingmeasured (input signal).

Static sensitivity, K=

= 0


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Example,

The sensitivity of a typical linear spring, whoseextension is directly proportional to the appliedforce can be defined as, 450N/mm


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LINEARITY

The output is a linear function of the input

Never completely achieved

Linearity specification:-i. Independent of the input

ii. Proportional of input

iii. Combined independent and proportional tothe input


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RANGE AND SPAN

The range of the instrument is specified by thelower and upper limits in which it is designed tooperate for measuring.


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HYSTERESIS

The magnitude of error caused in the output for given value of

input, when this value approached from opposite direction, i.efrom ascending order and then descending order.

Arithmetic mean , =( 0 ) 1+( 0 ) 2

2


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DEAD BAND

The largest change of the measurand to whichthe instrument does not response.

BACKLASH

The maximum distance or angle through whichany part of the mechanical system may bemoved in one direction without causing motionof the next part.


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DRIFT

The variation of output for a given input causeddue to change in the sensitivity of theinstrument due to certain interfering inputs liketemperature change, component instabilities,etc.

PdfMESB 33hi3 Chapter 2x1(1)

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