static characteristics of measuring instruments

7/23/2019 Static Characteristics of Measuring Instruments

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Static Characteristics of

Measuring Instruments

Anurag Arya

2K11/EE/016


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Static Characteristics of a

Measuring Instrument include

Accuracy

Precision

Repeatability

Reproducibility

Range/Span

Sensitivity

Hysteresis

Drift

Dead Band

Readability


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Accuracy

Accuracy is the degree of conformity of the measured value

with the accepted standard or ideal value, which we can take

as the true physical variable.


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PrecisionPrecision is the degree of exactness for which an instrument is

designed or intended to perform.

It is composed of two characteristics;

1. Conformity

2. Number of significant figures


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RepeatabilityThe closeness of agreement among a number of consecutive

measurements of the same variable (value) under the same

operating conditions, approaching in the same direction


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Reproducibility

The closeness of agreement among a number of

consecutive measurements of the same variable (value)

under the same operating conditions over a period of

time, approaching from both directions.

Reproducibility includes hysteresis, dead band, drift

and repeatability


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Range/Span

Range represents the minimum and maximum values which

can be determined by an instrument or equipment.

Difference between upper and lower range is known as Span.

Span can be the same for two different range instruments.


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Sensitivity

This is the relationship between a change in the

output reading for a given change of the input. (This

relationship may be linear or non-linear.)

Sensitivity is often known as scale factor or

instrument magnification and an instrument with a

large sensitivity (scale factor) will indicate a large

movement of the indicator for a small input change.


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Load Cell

Force, F

Output, Vo

K

Input, F(kN) Output, Vo (V)

Sensitivity, K= 5 V/kN

Output, Vo (V)

Input, Fi(kN)

Slope = 5 V/kN


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Hysteresis

This is the algebraic difference between the average

errors at corresponding points of measurement

when approached from opposite directions, i.e.

increasing as opposed to decreasing values of the

input.

Hysteresis is caused by

energy storage/ dissipation

in the system.

Actual/

Input Value

Measured

Value Ideal


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Drift

Zero driftis variation in the output of an instrument

which is not caused by any change in the input; it is

commonly caused by internal temperature changes

and component instability.

Sensitivity driftdefines the amount by which

instruments sensitivity varies as ambient conditions

change.


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input

Output

zero

drift

input

Output

sensitivity drift

Outputsensitivity drift

zero

drift

Input


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Dead Band

This is the range of different input values over which

there is no change in output value. This is produced

by friction, backlash or hysteresis in the instrument.


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Readability

This is defined as the ease with which readings may

be taken with an instrument. Readability difficultiesmay often occur due to parallax errors when an

observer is noting the position of a pointer on a

calibrated scale.


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Noise

A spurious current or voltage extraneous to the current or

voltage of interest n an electrical or electronic circuit is

called NOISE.

It is a signal that does not convey any information.


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Sources of Noise

Generated Noise : The noise present due to the effects

of internal resistances, capacitances and transistors etc.

This includes Johnson noise.

Conducted Noise : The noise present due to thepresence of extra harmonics that may result in spikes

and ripples.

Radiated Noise : Electric or magnetic fields in theenvironment may tend to distort the original signal. This

is radiated noise.


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Signal to Noise Ratio

The ratio of desired signal to the unwanted noise is

termed as Signal to Noise Ratio.

S/N = SIGNAL POWER

NOISE POWER

= SIGNAL IN VOLTS2

NOISE IN VOLTS2


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NOISE FACTOR

It is defined as :

F= S/N at input

S/N at output

Noise factor measurements are important because they

are a measure of noise added to a signal by a device in a

measuring system.

If noise factor is expressed in decibels it is known asnoise figure

nf = 10logF


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References

A course in Electrical and Electronic Measurements and

Instrumentation by A.K. Sawheny

A report on Industrial Instrumentation by Dr. Ing Naveed

Ramzan

A lecture on Instrumentation and Product Testing


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Thank You!

static characteristics of measuring instruments

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