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Basic concepts of Measurements

Instrument

A device for determining the value or magnitude of a quantity or variable

Accurac *

Closeness with which an instrument reading approaches the true value of thevariable being measured.

Precision*

Measure of the reproducibility of the results

Measure of the degree to which successive measurements differ from one

another ens v y

Ratio of output signal or response of the instrument to a change of input ormeasured variable.

The smallest change in measured value to which the instrument will respond

Errors

Deviation from the true value of the measured variable

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Accuracy

Precision Degree of agreement within a group of measurements or instruments

Composed of two characteristics

Conformity

Significant Figures

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Accuracy Vs Precision

Example 1.

Consider an ammeter which posses high degree of precision by virtue

, ,

with mirror arrangement to remove parallax. LC=(1/100) of Ampere. But the zero adjustment is wrong

, , ,

and the readings are consistent. However, the readings are not accurate

since they do not confirm to the truth.

Example 2.

Consider a known voltage of 100V. Five reading are taken: 104V,

, , .precision is 1%.

The instrument can be calibrated so that it could be used properly.

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Accuracy Vs Precision

By calibration, accuracy can be improved upon but not theprecision of the instrument.

Althou h the readin s are close to ether the have a small scatter andthus have a high degree ofprecision, but the results are far fromaccurate.

When it is stated that a set of reading shows precision, itmeans that the results agree among themselves.

A reement is no uarantee ofaccurac as there ma be some

disturbing effect that cause all the measured values to be in error.

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Significant Figures (SF)

Convey the actual information regarding the magnitude and the

.

The more SF, the greater is the precision of the measurement

The result is only as accurate as the least accurate measurement

The number of SF in multiplication may increase rapidly

Examples of SF

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Direct method

Unknown quantity (Measurand) is directly compared against a standard

,

Indirect method

Measuring Instruments

Absolute Instruments (AI)

Give the magnitude of the quantity in terms of physical constant of the

instrument

Secondary Instruments (SI)

Quantity can be measured by observing the output indicated by the instrument

Calibrated by comparison with an absolute instrument

Working with AI for routine work is time consuming since every time ameasurement is made, it takes a lot of time to compute the magnitude

Therefore, SI is most commonly used.

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Types of Instruments

Depending upon the way the instruments present the results of measurements,they are classified into two broad categories:

Deflection Type (DT)

Value of the measurand will be inferred from deflection or mechanicaldisplacement at the point of balance

Opposing effect = cause producing the deflection

Example: PMMC

Null Type (NT)

A zero indication leads to the determination of magnitude of measurand

Attempt to maintain the deflection at zero by suitable application of an

affect opposing that generated by the measurand Example: DC Potentiometer

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Comparison between Deflection and Null Type

Accuracy is higher for NT

Because the o osin effect is calibrated with the hel of standards whichhave high degree of accuracy

Whereas for DT, calibration depends upon the instrument constant whichare normally not known to a high degree of accuracy

Measurand is balanced out for NT

Because the detector have to cover a small range around the balance point

Whereas for DT, it has to be large in size, more rugged and less sensitive

D namic measurement

NT is not suitable because it require many manipulation before balance

DT is suitable because it can follow the rapid variations of the measurand

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Characteristic of Instrument & Measurement System

Static (measurand are either constant or vary slowly with time)

Static Characteristic

Accuracy

Sensitivity

Drift

Static error

Dead zone

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True Value Defined as the average of an infinite number of measured value when

the average deviation due to the various factors tend to zero.

Variation between the largest and the smallest reading (or largest one)

Scale Span

Difference between the largest and the smallest reading

Frequency Range requenc es over w c measuremen s can e per orme w a

specified degree of accuracy

Static Error

Difference between the measured and true vale

Static Correction

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Drift

Perfect reproducibility means no drift

i.e., with a given input, the values do not vary with time

Zero Drift

If the whole calibration gradually shift due to the slippage, permanent set,

or due to the undue warming up of the electronic circuit.

Can be prevented by zero setting

Span Drift or Sensitive Drift

Zonal Drift

Occurs onl over a ortion of s an

Drift is an undesirable quantity for the industries.

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Static Sensitivity Ratio of the magnitude of response or output signal to the magnitude of

the measurand.

When a calibration curve is linear sensitivit can be defined as the

slope of curve Constant over the entire range

, .

Dead Time

Time re uired b a measurement s stem to be in to res ond to a

change in the measurand.

Time before the instrument begins to respond after the measurand has been

changed

Dead Zone

Largest change of input for which there is no output of the instrument

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Resolution

The smallest increment in the input which can be detected with

So, the resolution defines the smallest measurable input change While, the threshold defines the smallest measured input.

Loading Effect

The ori inal si nal should not be distorted b introduction of an

element in the measurement system

However, practically it is not possible.

phase shift, etc.