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Basic Concepts

Of

Measuring system and their

performances

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Block diagram of a generalized instrumentation system

Dr. Bahauddin Karagozuglu 3

The Bourdon Gauge

Dr. Bahauddin Karagozuglu 4

Block diagram of the pressure gauge based

on Bourdon tube Physical variable to be measured Pressure

Detector – transducer stage Bourdon tube Pressure to mechanical displacement

Intermediate stage Gearing arrangement that amplifies the displacement signal

Pressure (input)

Pressure (calibration)

Displacement ( transduced signal)

Amplified displacement signal

Calibration signal from a source with known pressure values

Final stage Pointer and dial arrangement

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Characteristics of measurement systemsThe system characteristics are to be known, to choose an instrument that most suited to a particular measurement application.The performance characteristics may be broadly divided into two groups, namely ‘static’ and ‘dynamic’ characteristics. Static characteristicsthe performance criteria for the measurement of quantities that remain constant, or vary only quite slowly. Dynamic characteristicsthe relationship between the system input and output when the measured quantity (measurand) is varying rapidly.

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Static Performance of InstrumentThe static characteristics of instruments are related with steady state response.The relationship between the output and the input when the input does not change, or the input is changing at a slow rate.Range & SpanLinearity & SensitivityEnvironmental effectsHysteresisResolutionRepeatabilityDeath space

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Generalized Static Characteristics

• Accuracy• Precision • Resolution• Tolerance• Static sensitivity• Span / Range

• Sensitivity drift• Linearity• Repeatability• Dead space /

Threshold• Zero drift

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Accuracy Data points with

low accuracy

high accuracy

valuetrue

valuemeasuredvaluetrueaccuracy

Accuracy: closeness with which an instrument reading approaches the true or accepted value of the variable (quantity) being measured. It is considered to be an indicator of the total error in the measurement without looking into the sources of errors.

Accuracy is often expressed in percentage

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Precision Data points with

low precision

high precision

1. A measure of the reproducibility of the measurements; i.e., given a fixed value of a variable, precision is a measure of the degree to which successive measurements differ from one another.

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Resolution

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

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Tolerance• Maximum deviation allowed from the conventional true

value. • It is not possible to built a perfect system or make an

exact measurement. All devices deviate from their ideal (design) characteristics and all measurements include uncertainties (doubts).

• Hence, all devices include tolerances in their specifications. If the instrument is used for high-precision applications, the design tolerances must be small.

• However, if a low degree of accuracy is acceptable, it is not economical to use expensive sensors and precise sensing components

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Static sensitivity

Sensorsignal

Measurand

Sensorsignal

Measurand

A low-sensitivity sensor has low gain

A high sensitivity sensor has high gain

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Zero and sensitivity drifts

Range & Span

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Defined as the range of reading between minimum value and maximum value for the measurement of an instrument.

Has a positive value e.g..:The range of span of an instrument which has a reading range of –100°C to 100 °C is 200 °C.

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Linearity

Output

Input

Output

Input

A linear system fits the equation y = mx + b.

A nonlinear system does not fit a straight line

Repeatability

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Repeatability:a measure of how well the output returns to a given value when the same precise input is applied several times.

Or the ability of an instrument to reproduce a certain set of reading within a given accuracy.

Death Space : Threshold : Dead space is defined as the range of different input values over which there is no change in output value.

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