Errors in Measuring Instruments.ppt

Errors in measuring instruments

1. Errors in Indicating
Instruments

2. •Error is Deviation
•ThedeviationofMeasuredQuantityfromtheTrueValueorActual
ValueiscalledError.
•TrueValue:Itisnotpossibletodeterminethetrueofquantityby
experimentalmeans.Truevaluemaybedefinedastheaveragevalue
ofaninfinitenumberofmeasuredvalueswhenaveragedeviation
duetovariouscontributingfactorwillapproachtozero.
•MeasuredValue:Itmaybedefinedastheapproximatedvalueoftrue
value.Itcanbefoundoutbytakingmeansofseveralmeasured
readingsduringanexperiment,byapplyingsuitableapproximations
onphysicalconditions.

3. Types of Errors
Basically Three types of errors are studied:-
1. Gross Errors
2. Systematic Errors
3. Random Errors

4. Gross Errors
GrossErrorsmainlycoversthehumanmistakesin
readinginstrumentsandrecordingandcalculating
measurementresults.
Example:-Duetooversight,ThereadofTemperature
as31.5̊whiletheactualreadingmaybe21.5̊.

5. GrossErrorsmaybeofanyamountandthen
theirmathematicalanalysisisimpossible.Then
theseareavoidedbyadoptingtwomeans:-
1. Great care is must in reading and recording
the data.
2. Two , Three or even more reading should be
taken for the quantity under measurement.

6. Systematic Errors
Systematic Errors classified into three categories :-
1.Instrumental Errors
2.Environmental Errors
3. Observational Errors

7. Instrumental Errors
These errors arises due to three main reasons.
1.Due to inherent shortcoming in the instrument.
Example:-Ifthespringusedinpermanent
magnetinstrumenthasbecomeweakthen
instrumentwillalwaysreadhigh.Errorsmay
causedbecauseoffriction,hysteresis,oreven
gearbacklash.
2.Duetomisuseoftheinstruments.
3.DuetoLoadingeffectsofinstruments.

8. Environmental Errors
Theseerrorsareduetoconditionsexternalto
themeasuringDeviceincludingconditionsof
thesurroundingtheinstrument.
ThesemaybeeffectsofTemperature,Pressure,
Humidity,Dust,Vibrationsorofexternal
magneticorelectrostaticfields.

9. Observational Errors
There are many sources of observational errors:-
--Parallax, i.e. Apparent displacement when the
line of vision is not normal to the scale.
--Inaccurate estimate of average reading.
--Wrong scale reading and wrong recording the
data.
--Incorrect conversion of units between
consecutive reading.

10. Random Errors
Thequantitybeingmeasuredisaffectedbymany
happeningsintheuniverse.Weareawarefor
someofthefactorsinfluencingthe
measurement,butabouttherestweareunaware.
Theerrorscausedbyhappeningordisturbances
aboutwhichweareunawareareRandom
Errors.ItsalsoknownasresidualErrors.

11. Errors in PMMC
Weakening of Permanent Magnet:
To prevent the weakening of Permanent Magnet
due to ageing and temperature effect, magnets
are aged by heat and vibration treatment during
manufacturing process.
These processes make the magnet to retain its
magnetism over a longer period of time.

12. Errors in PMMC
Weakening of spring:
The weakening of Spring can be reduced by
careful use of material and pre-aging during
manufacturing process.
Change in Resistance of Moving Coil:
When the temperature changes, the resistance of
the coil vary and the spring also produces error in
deflection.
This error can be minimized by using a spring whose
temperature co-efficient is very low.

13. Errors in MI
Temperature error :
•Due to temperature variation, the resistance of the
coil varies.
•This affects the deflection of the instrument.
•The coil should be made of manganin, so that the
resistance is almost constant.

14. Hysteresis error:
•Due to hysteresis affect the reading of the instrument will not
be correct.
•When the current is decreasing, the flux produced will not
decrease suddenly.
•Due to this the meter reads a higher value of current.
•Similarly when the current increases the meter reads a lower
value of current.
•This produces error in deflection.
•This error can be eliminated using small iron parts with
narrow hysteresis loop so that the demagnetization takes
place very quickly.

15. Eddy current error:
The eddy currents induced in the moving iron
affect the deflection.
This error can be reduced by increasing the
resistance of the iron.
Stray field error:
Since the operating field is weak, the effect of
stray field is more.Dueto this, error is
produced in deflection.
This can be eliminated by shielding the parts
of the instrument.

16. •Frequency Error

17. Deflectionofmovingironvoltmeterdepends
uponthecurrentthroughthecoil.
Therefore,deflectionforagivenvoltagewill
belessathigherfrequencythanatlow
frequency.
Acapacitorisconnectedinparallelwith
multiplierresistance.
Thenetreactance,(XL−XC)isverysmall,
whencomparedtotheseriesresistance.
Thusthecircuitimpedanceismade
independentoffrequency.Thisisbecauseof
thecircuitisalmostresistive.