Chareteristics of instruments

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Characteristics Characteristics of of

InstrumentInstrument

INSTRUMENTATION CHARACTERISTICS• Shows the performance of instruments to be used.

• Divided into two categories: static and dynamic characteristics.

A)Static characteristics refer to the comparison between steady output and ideal

output when

the input is constant.

B)Dynamic characteristics refer to the comparison between instrument output and ideal

output when the input changes.

A)Static A)Static CharacteristicsCharacteristics

STATIC CHARACTERISTICS

1. ACCURACY

– Accuracy is the ability of an instrument to show the exact reading.

– Always related to the extent of the wrong reading/non accuracy.

– Normally shown in percentage of error which of the full scale reading percentage.

2. PRECISION

• An equipment which is precise is not necessarily accurate.

• Defined as the capability of an instrument to show the same reading when used each time (reproducibility of the instrument).

STATIC CHARACTERISTICS

3. RANGE OF SPAN

• Range The region between the limits within which an instrument is designed to operate for measuring, indicating or recording a physical quantity is called the range of the instrument.

• The range is expressed by stating the lower and upper values.

• Range -100°C to 100°C

STATIC CHARACTERISTICS

• Span • Span represents the algebraic differences

between the upper and lower range values of the instrument.

• An instrument which has a reading range of –100°C to 100 °C span is 200 °C.

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4.Linearity

Most instruments are specified to function over a particular range and the instruments can be said to be linear when incremental changes in the input and output are constant over the specified range.

• Maximum deviation from linear relation between input and output.

• The output of an instrument has to be linearly proportionate to the measured quantity.

• Normally shown in the form of full scale percentage (% fs).

• The graph shows the output reading of an instrument when a few input readings are entered.

• Linearity = maximum deviation from the reading of x and the straight line.

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5.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.

Sensitivity

Variation of the physical variables

Most sensitive

6. DEAD ZONE

• Defined as the range of input reading when there is no change in output (unresponsive system).

Dead Space

OutputReading

MeasuredVariables

- +

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7.Threshold

If the instrument input is very gradually increased from zero there will be a minimum value required to give a detectable output change. This minimum value defines the threshold of the instrument.

input

Output

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8.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.

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9.Drift

Zero drift is 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.

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input

Output

zero drift

input

Output

sensitivity drift

input

Output sensitivity drift

zero drift

10.Reapatability

It is the ability of the measuring instrument to give the same value every time the measurement of given quantity is repeated, under the same conditions

11.RePRODUCIBILITY

B)Dynamic B)Dynamic CharacteristicsCharacteristics

1.Speed of response

It is defined as the rapidity with which an instrument responds to a change in the value of the quantity being measured.

2.Fidelity

Fidelity of an instrumentation system is defined as the degree of closeness with which the system indicates or records the signal which is impressed upon it. It refers to the ability of the system to reproduce the output in the same form as the input.

3.Overshoot.

Because of mass and inertia, a moving part, i.e., the pointer of the instrument does not immediately come to rest in the final deflected position. The pointer goes beyond the steady state i.e., it overshoots

4.Dyanamic Error

The difference between the indicated quantity and the true value of the time varying quantity is the dynamic error, here static error of the instrument is assumed to be zero.

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ErrorError

Error

Error is the difference between the true value of the size being measured and the value found by measurement.

Measurement Error

1.Static Error

E =Vm Vt1.Measured Value2.True Value

2.Relative Error

Error =

Vm Vt

Vt

X 100

Sources of Error

1.Defect in instrument.2.Adjustment of an instrument.3.Imperfection in design of instrument.4.Method of location.5.Environmental effects.6.Error due to properties of object.7. Error due to surface finish of object.8.Observational error.

TYPES OF ERRORS

1.Gross errors.2.Systematic errors.3.Random errors.

Gross errors.

These are basically human errors caused by the operator or person using the instrument. The instrument may be good and may not give any error but still the measurement may go wrong due to the operator. The different types of gross errors are:

1. Observational errors.2. Reading with parallax error.3. Incorrect adjustments of zero and full-scale

adjustments.4. Improper applications of instruments:

Using a 0–100 V voltmeter to measure 0.1 V, etc.

5.Operational error.

2.Systematic Errors

These are divided into two categories:

1)Instrumental errors: Due to shortcomings of the instruments.2)Environmental errors: Due to external conditions affecting the instrument.

A] Instrumental errors.

•  Even if human errors are avoided or proper care is taken to see that such errors do not occur, errors can still occur in measurements due to the instrument. The possible reasons can be as follows:

1) Friction in bearings of various moving components can cause incorrect readings.

2) Irregular spring tension in analog meters.3) Zero setting not adjusted properly.4) Full-scale setting not adjusted properly.5) Faulty display circuit in digital instruments.

B] Environmental errors.

• Ambient parameters such as temperature, pressure, humidity, magnetic and electrostatic fields, dust, and other such external parameters can affect the performance of the instrument.

Improper housing of the instrument also can give wrong readings.

• Such errors can be avoided by air-conditioning, magnetic shielding, cleaning the instruments, and housing the instruments properly depending on the application and type of the instrument.