rm unit 1 instrunents

8/13/2019 RM Unit 1 Instrunents

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Instrumentation Basics

Instrumentationis a technology of measurement whichserves sciences, engineering, medicine and etc.

Measurementis the process of determining theamount, degree or capacity by comparison with theaccepted standards of the system units being used.

Instrument is a device for determining the value ormagnitude of a quantity or variable.

Mechanical instrumentis based on mechanicalprinciples for its measurement functions

Electronic instrumentis based on electrical orelectronic principles for its measurement functions


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Instrumentation Schemes/Functions

The 3 basic functions of instrumentation :-

Indicating visualize the process/operation

Recording observe and save the measurement reading

Controlling to control measurement and process


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Basic components in a measurement system

Basic components in a measurement system are shown below:

It is also important to mention that a power supply is an important

element for the entire system.

Amplification and Conditioning


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PERFORMANCE CHARACTERISTICS

Performance Characteristics - characteristics that show theperformance of an instrument.

Eg: accuracy, precision, resolution, sensitivity.

Allows users to select the most suitable instrument for a specificmeasuring jobs.

Two basic characteristics : Static measuring a constant process condition.

Static characteristics refer to the comparisonbetween steady output and ideal output when theinput is constant.

Dynamic - measuring a varying process condition.

Dynamic characteristics refer to the comparisonbetween instrument output and ideal output when theinput changes.


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Static Characteristics

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.

the degree of exactness (closeness) of

measurement compared to the expected(desired) value.


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Precision

An equipment which is precise is notnecessarily accurate.

Defined as the capability of an instrumentto show the same reading when used eachtime (reproducibility of the instrument).

a measure of consistency or repeatability of

measurement, i.e successive reading do notdiffer.


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Example :X : result

Centre circle : true value

XXX

XXXX

XXX

XXX

XXX

X X

X

xxHigh accuracy, high precision

Low accuracy, high precision

Low accuracy, low precision


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Tolerance

Defined as the maximum error expectedin an instrument.

Closely related to accuracy of anequipment where the accuracy of anequipment is sometimes referred to inthe form of tolerance limit.

Explains the maximum deviation of anoutput component at a certain value.


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Range of Span

Defined as the range of reading betweenminimum value and maximum value for themeasurement of an instrument.

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


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Bias Constant error which occurs during the

measurement of an instrument.

This error is usually rectified through calibration.

Example :

A weighing scale always gives a bias reading. Thisequipment always gives a reading of 1 kg evenwithout any load applied. Therefore, if A with aweight of 70 kg weighs himself, the given reading

would be 71 kg. This would indicate that there is aconstant bias of 1 kg to be corrected.


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Sensitivity

ratio of change in the output (response) ofinstrument to a change of input or measuredvariable.

Sensitivity (K) = i: change in output; i : change in input

Example 1:

The resistance value of a Platinum ResistanceThermometer changes when the temperature increases.

Therefore, the unit of sensitivity for this equipment isOhm/C.


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Linearity 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 instrumentwhen a few input readings are entered.

Linearity= maximum deviation from the reading of xand the straight line.


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Linearity

Output

Readings

Measured Quantity


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Expected value the design value or the most probable value

that expect to obtain.

Error the deviation of the true value from the desired value.

Threshold The minimum limit of the input reading is

threshold. When the reading of an input is increased from zero, the

input reading will reach a certain value before change

occurs in the output.


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Dynamic Characteristics

Dynamic measuring a varying process condition.

Instruments rarely respond instantaneously to changes in themeasured variables due to such things as mass, thermalcapacitance, fluid capacitance or electrical capacitance.

Pure delay in time is often encountered where the instrument

waits for some reactionto take place. Such industrial instruments are nearly always used for

measuring quantities that fluctuate with time.

Therefore, the dynamic and transient behavior of theinstrument is important.


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The dynamic behavior of an instrument isdetermined by subjecting its primary element(sensing element) to some unknown andpredetermined variations in the measured quantity.

The three most common variations in the measuredquantity: Step change

Linear change

Sinusoidal change



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Step change-in which the primary element is subjected to an

instantaneous and finite change in measured variable.

Linear change-in which the primary element is following the

measured variable, changing linearly with time.

Sinusoidal change-in which the primary element follows ameasured variable, the magnitude of which changes in

accordance with a sinusoidal function of constant amplitude.



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DYNAMIC CHARACTERISTICS

Step Input

Sudden change in input signal from steadystate.

The output signal for this kind of input isknown as transient response.

Input

Time


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Ramp Input The signal changes linearly.

The output signal for ramp input is

ramp response.

Input

Time


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Sine- wave Input

The signal is harmonic.

The output signal is frequency response.

Input

Time


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The dynamic performance characteristics of aninstrument are:

Speed of response- The rapiditywith which aninstrument responds changes in measured quantity.

Dynamic error-The difference between the true andmeasured value with no static error.

Lag delay in the response of an instrument tochanges in the measured variable.

Fidelity the degree to which an instrument indicatesthe changesin the measured variable without dynamicerror (faithful reproduction).


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Fidelity of Instruments

Fidelity- truthfulness

Error-Actual Vs Expected

Types of Errors and Reasons

Data Collection using Computer Systems

Role of DSP in Correction


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TYPES OF STATIC ERROR

Types of static error

1) Gross error/human error

2) Systematic Error3) Random Error


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1) Gross Error cause by human mistakes in reading/using instruments

may also occur due to incorrect adjustment of the instrument and thecomputational mistakes

cannot be treated mathematically

cannot eliminate but can minimize

Eg: Improper use of an instrument.

This error can be minimized by taking proper care in reading andrecording measurement parameter.

In general, indicating instruments change ambient conditionsto someextent when connected into a complete circuit.

Therefore, several readings (at threereadings) must be taken to minimize

the effect of ambient condition changes.

TYPES OF STATIC ERROR


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TYPES OF STATIC ERROR (cont)

2) Systematic Error

- due to shortcomings of the instrument (such as

defective or worn parts, ageing or effects of the

environment on the instrument)

In general, systematic errors can be subdivided into static anddynamic errors.

Static caused by limitationsof the measuring device or the

physical laws governing its behavior.

Dynamic caused by the instrument not responding very fast

enough to follow the changes in a measured variable.


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- 3 types of systematic error :-

(i) Instrumental error

(ii) Environmental error

(iii) Observational error



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(i) Instrumental error

- inherent while measuring instrument because of

their mechanical structure (eg: in a DArsonval meter,

friction in the bearings of various moving component,irregular spring tension, stretching of spring, etc)

- error can be avoid by:

(a) selecting a suitable instrument for the particular

measurement application

(b) apply correction factor by determining

instrumental error(c) calibrate the instrument against standard


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(ii) Environmental error- due to external condition effecting the

measurement including surrounding area conditionsuch as change in temperature, humidity,

barometer pressure, etc- to avoid the error :-

(a) use air conditioner(b) sealing certain component in the instruments(c) use magnetic shields

(iii) Observational error

- introduce by the observer

- most common : parallax error and estimation error (while reading

the scale)

- Eg: an observer who tend to hold his head too far to the leftwhile reading the position of the needle on the scale.


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3) Random error

- due to unknown causes, occur when all systematic

error has accounted

- accumulation of small effect, require at high degree of

accuracy- can be avoid by

(a) increasing number of reading

(b) use statistical means to obtain best approximation

of true value

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