Static characteristic of Static characteristic of measuring instrumentsmeasuring instruments

Scale (Graduation)Scale (Graduation)

Prepared by dr. Mohamed Ahmed Prepared by dr. Mohamed Ahmed AwadAwad

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Scale value (resolution)Scale value (resolution)

This specification is the smallest detectable incremental change of input parameter that

can be detected in the output signal

0 5قيمة

التدريج = 5

0 1 2 3 4 5قيمة

التدريج = 1

0 5قيمة

التدريج = 1

This specification is the smallest detectable This specification is the smallest detectable incremental change of input parameter that can incremental change of input parameter that can be detected in the output signal. be detected in the output signal.

It also may be defined as the value of one It also may be defined as the value of one division on the scale.division on the scale.

Resolution can be expressed either as a Resolution can be expressed either as a proportion of the reading (or the full-scale proportion of the reading (or the full-scale reading) or in absolute terms.reading) or in absolute terms.

010

20

30

4050

60

70

80

90

10987654321

1 mm

0

0.1mm

10

11

12

13

N=100

Z=10

Sm

0 10

20

30

405060

70

80

90

Scale DivisionScale Division

طول التدريج = مم40

40

طول التدريج =

مم10

10

10

10

10

It is the distant between two adjacent scale marks in mms.

Measuring Capacity (Scale range)Measuring Capacity (Scale range)

5

10

25

30

20

15

35

40

مم10

50

طاقة التدريج =-

50

The range is the maximum and minimum values of applied parameter that can be measured.

If the maximum reading of the measuring system If the maximum reading of the measuring system is Xis Xmaxmax and its minimum reading is X and its minimum reading is Xmin, min, thenthen

   Range = XRange = Xmin min to X to Xmaxmax

   If XIf Xminmin = 0 then Range = X = 0 then Range = Xmaxmax

  

  

SpanSpan

If the maximum reading of the measuring If the maximum reading of the measuring system is Xsystem is Xmaxmax and its minimum reading is X and its minimum reading is Xmin, min,

thenthen    Span =XSpan =Xmaxmax - X - Xminmin

The range of the sensor is the maximum and The range of the sensor is the maximum and minimum values of applied parameter that can minimum values of applied parameter that can be measured. be measured.

For example, a given pressure sensor may For example, a given pressure sensor may have a range of -400 to +400 mm Hg. have a range of -400 to +400 mm Hg.

Alternatively, the positive and negative ranges Alternatively, the positive and negative ranges often are unequal. often are unequal.

Measuring RangeMeasuring RangeY

X

Measuring Range

Measuring Capacity

Equipment capacityEquipment capacity

ReadabilityReadability

ReadabilityReadability

It is the ease with which readings may be It is the ease with which readings may be observed. This term is frequently used for observed. This term is frequently used for analogue type instruments where the human analogue type instruments where the human observer noting the position of the pointer on observer noting the position of the pointer on the scale. It is obvious that as the scale the scale. It is obvious that as the scale division increases the readability improved. division increases the readability improved.

SensitivitySensitivity

minimum input of physical parameter that will create a detectable output change.

The sensitivity of the sensor is defined The sensitivity of the sensor is defined as the slope of the output as the slope of the output

characteristic curve characteristic curve

It is the ratio of the change in output signal to It is the ratio of the change in output signal to the change in the input signalthe change in the input signal

In case the output is in a form of a graduated In case the output is in a form of a graduated scale, and a pointer, thenscale, and a pointer, then

Deflection FactorDeflection Factor

It is the inverse of the sensitivityIt is the inverse of the sensitivity

In some cases , the sensitivity is defined as the In some cases , the sensitivity is defined as the input parameter change required to produce a input parameter change required to produce a standardized output change. In others, it is standardized output change. In others, it is defined as an output voltage change for a defined as an output voltage change for a given change in input parameter. given change in input parameter.

Sensitivity ErrorSensitivity ErrorThe sensitivity error is a departure from the The sensitivity error is a departure from the ideal slope of the characteristic curve. ideal slope of the characteristic curve.

Sensitivity errorSensitivity error

AccuracyAccuracy

It is the closeness with which an instrument It is the closeness with which an instrument reading approaches the true value of the reading approaches the true value of the quantity being measured. Thus, accuracy of a quantity being measured. Thus, accuracy of a measurement means conformity to truth.measurement means conformity to truth.

AccuracyAccuracy

The accuracy is the The accuracy is the maximummaximum differencedifference that that will exist between the will exist between the actual valueactual value (which (which must be measured and the must be measured and the indicated valueindicated value at at the output.the output.

The accuracy can be expressed either as a The accuracy can be expressed either as a percentage of full scale or in absolute terms.percentage of full scale or in absolute terms.

Accuracy as “Percent of Measured Accuracy as “Percent of Measured ValueValue““

This to specify the accuracy in terms of the This to specify the accuracy in terms of the measured value, measured value,

i.e. accurate to within ±percent of the i.e. accurate to within ±percent of the measured value.measured value.

This statement means that as the readings get This statement means that as the readings get smaller so do the error.smaller so do the error.

Accuracy as “Percent of Scale Accuracy as “Percent of Scale RangeRange““

Accuracy of measuring instruments is often Accuracy of measuring instruments is often guaranteed to a certain percent of its full scale guaranteed to a certain percent of its full scale deflection (f.s.d.). deflection (f.s.d.).

Specification of accuracy in this manner can be Specification of accuracy in this manner can be highly misleading. highly misleading.

As the readings get smaller, the percent error As the readings get smaller, the percent error will get bigger.will get bigger.

PRECISSION PRECISSION (REPEATABILITY)(REPEATABILITY)

It is the measure of the degree to which successive measurements differs from on to another.

Accuracy versus PrecisionAccuracy versus Precision

True Value of Input

Instrument Reading

Precise but inaccurate

Imprescise and inaccurate

Ideal (precise and accurate)

Precise but not accurate

HysterisisHysterisis

hysteresishysteresis is the measure of having is the measure of having the changes the changes of the input parameter regardless of which of the input parameter regardless of which

direction the change is made; direction the change is made;

is an error caused by when the measured is an error caused by when the measured property reverses direction, but there is some property reverses direction, but there is some finite lag in time for the sensor to respond, finite lag in time for the sensor to respond, creating a different offset error in one direction creating a different offset error in one direction than in the other.than in the other.

Reasons exampleReasons example

OffsetOffset

The offset error is defined as the output that The offset error is defined as the output that will exist when it should be zero or, will exist when it should be zero or, alternatively, the difference between the actual alternatively, the difference between the actual output value and the specified output value output value and the specified output value under some particular set of conditions. under some particular set of conditions.

OffsetOffset

LinearityLinearity

LinearityLinearity