Learning Outcome - SNS Courseware

M&M/ UNIT- 1 Work piece, Instruments/ K.Chandrasekar,AP/Mech 1/16

Learning Objective:1) Understand difference between Accuracy and

Precision

M&M/ UNIT- 1 Work piece, Instruments/ K.Chandrasekar,AP/Mech 2/16

Learning Outcome:1) Measure the parts and identify accurate or precise

results under repeatable conditions.

Contents

• Persons• Environment• Effect of Accuracy vs Precision

M&M/ UNIT- 1 Work piece, Instruments/ K.Chandrasekar,AP/Mech 3

Persons

• Man is the measure of all things stated byProtagoras.


Environment

• The environment system is indirectly relatedto temperature , Humidity and Pressureconditioning.


1. Accuracy

It is the degree of closeness with which aninstrument reading approaches the true valueof the quantity being measured.

6M&M/ UNIT- 1 Work piece, Instruments/ K.Chandrasekar,AP/Mech

Precision

It is a measure of the reproducibility of themeasurement that is given a fixed value ofvariable.


Precision Measuring instruments


1.Static Characteristics

The static characteristics of an instrumentare required to be considered for theinstruments which measure unvaryingprocess conditions.

The static characteristics are defined for theinstruments which measure quantities whichdo not vary with time.

The static characteristics of an instrumentare required to be considered for theinstruments which measure unvaryingprocess conditions.

The static characteristics are defined for theinstruments which measure quantities whichdo not vary with time.


The main static characteristicsare :-

1. Accuracy2. Sensitivity3. Reproducibility4. Drift5. Static error6. Dead zone7. Precision8. Threshold9. Linearity10. Stability11. Range or Span12. Bias13. Tolerance14. Hysteresis

1. Accuracy2. Sensitivity3. Reproducibility4. Drift5. Static error6. Dead zone7. Precision8. Threshold9. Linearity10. Stability11. Range or Span12. Bias13. Tolerance14. Hysteresis


2.Sensitivity

Sensitivity is the ratio of change in output ofan instrument to the change in input.


Sensitivity Meter:-


Readability:• Readability refers to the ease with which the

readings of a measuring instrument can beread.


Calibration:• The calibration of any measuring instrument is

necessary to measure the quantity in terms ofstandard unit.

• It is carried out by making adjustments suchthat the read out device produces zero outputfor zero input.

• The calibration of any measuring instrument isnecessary to measure the quantity in terms ofstandard unit.

• It is carried out by making adjustments suchthat the read out device produces zero outputfor zero input.


Backlash:

• In backlash, is clearance between matingcomponents, sometimes described as theamount of lost motion due to clearance orslackness when movement is reversed andcontact is re-established.

• In backlash, is clearance between matingcomponents, sometimes described as theamount of lost motion due to clearance orslackness when movement is reversed andcontact is re-established.


Hysteresis:

• It is the difference between the indications ofa measuring instrument when the same valueof measured quantity is reached by increasingor decreasing that quantity.

• It is caused by friction, slack motion in thebearings and gears, elastic deformation,magnetic and thermal effects.

• It is the difference between the indications ofa measuring instrument when the same valueof measured quantity is reached by increasingor decreasing that quantity.

• It is caused by friction, slack motion in thebearings and gears, elastic deformation,magnetic and thermal effects.


Drift:

• It is an undesirable gradual deviation of theinstrument output over a period of time that isunrelated to changes in input operating conditions orload.

• An instrument is said to have no drift if is reproducesthe same readings at different times for samevariation in measured quantity.

• It is caused by wear and tear, high stress developedat some parts etc.

• It is an undesirable gradual deviation of theinstrument output over a period of time that isunrelated to changes in input operating conditions orload.

• An instrument is said to have no drift if is reproducesthe same readings at different times for samevariation in measured quantity.

• It is caused by wear and tear, high stress developedat some parts etc.


Threshold:

• The min. value below which no output changecan be detected when the input of aninstrument is increased gradually from zero iscalled the threshold of the instrument.

• Threshold may be caused by backlash.

• The min. value below which no output changecan be detected when the input of aninstrument is increased gradually from zero iscalled the threshold of the instrument.

• Threshold may be caused by backlash.


Resolution:

• When the input is slowly increased from some non-zero value, it is observed that the output does notchange at all until a certain increment is exceeded;this increment is called resolution.

• It is the min. change in measured variable whichproduces an effective response of the instrument.

• It may be expressed in units of measured variable

• When the input is slowly increased from some non-zero value, it is observed that the output does notchange at all until a certain increment is exceeded;this increment is called resolution.

• It is the min. change in measured variable whichproduces an effective response of the instrument.

• It may be expressed in units of measured variable


Dead zone and Dead Time:

Dead Zone:• The largest change of input quantity for which there

is no change of output of the instrument is termed asdead zone.

• It may occur due to friction in the instrument whichdoes not allow the pointer to move till sufficientdriving force is developed to overcome the frictionloss.

• Dead zone caused by backlash and hysteresis in theinstrument.

Dead Zone:• The largest change of input quantity for which there

is no change of output of the instrument is termed asdead zone.

• It may occur due to friction in the instrument whichdoes not allow the pointer to move till sufficientdriving force is developed to overcome the frictionloss.

• Dead zone caused by backlash and hysteresis in theinstrument.


Continue…

Dead Time:• The time required by a measurement system

to begin to respond to a change in themeasurand is termed as dead time.

• It represents the time before the instrumentbegins to respond after the measuredquantity has been changed.

Dead Time:• The time required by a measurement system

to begin to respond to a change in themeasurand is termed as dead time.

• It represents the time before the instrumentbegins to respond after the measuredquantity has been changed.


RANGE

• Range of accuracy- Accuracy of a measuringsystem is defined as the closeness of theinstrument output to the true value of themeasured quantity

• Range of accuracy- Accuracy of a measuringsystem is defined as the closeness of theinstrument output to the true value of themeasured quantity


Reproducibility

Reproducibility is defined as the degree ofcloseness by which a given value can berepeatedly measured.The reproducibility is specified for a period of

time.Perfect reproducibility signifies that the given

readings that are taken for an input, do notvary with time..

Reproducibility is defined as the degree ofcloseness by which a given value can berepeatedly measured.The reproducibility is specified for a period of

time.Perfect reproducibility signifies that the given

readings that are taken for an input, do notvary with time..


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