Errors in Indicating
Instruments

• Error is Deviation
• The deviation of Measured Quantity from the True Value or Actual
Value is called Error.
• True Value: It is not possible to determine the true of quantity by
experimental means. True value may be defined as the average value
of an infinite number of measured values when average deviation
due to various contributing factor will approach to zero.
• Measured Value: It may be defined as the approximated value of true
value. It can be found out by taking means of several measured
readings during an experiment, by applying suitable approximations
on physical conditions.

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

Gross Errors
Gross Errors mainly covers the human mistakes in
reading instruments and recording and calculating
measurement results.
Example:- Due to oversight, The read of Temperature
as 31.5 ̊ while the actual reading may be 21.5 ̊.

Gross Errors may be of any amount and then
their mathematical analysis is impossible. Then
these are avoided by adopting two means:-
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.

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

Instrumental Errors
These errors arises due to three main reasons.
1. Due to inherent shortcoming in the instrument.
Example:- If the spring used in permanent magnet
instrument has become weak then instrument will
always read high. Errors may caused because of
friction , hysteresis , or even gear backlash.
2. Due to misuse of the instruments.
3. Due to Loading effects of instruments.

Environmental Errors
These errors are due to conditions external to
the measuring Device including conditions of
the surrounding the instrument.
These may be effects of Temperature, Pressure,
Humidity, Dust, Vibrations or of external
magnetic or electrostatic fields.

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.

Random Errors
The quantity being measured is affected by many
happenings in the universe. We are aware for
some of the factors influencing the
measurement, but about the rest we are unaware.
The errors caused by happening or disturbances
about which we are unaware are Random
Errors. Its also known as residual Errors.

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.

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.

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.

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.

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.Due to this, error is
produced in deflection.
➢This can be eliminated by shielding the parts
of the instrument.

• Frequency Error

➢Deflection of moving iron voltmeter depends
upon the current through the coil.
➢Therefore, deflection for a given voltage will
be less at higher frequency than at low
frequency.
➢ A capacitor is connected in parallel with
multiplier resistance.
➢The net reactance, ( X L − XC ) is very small,
when compared to the series resistance.
➢Thus the circuit impedance is made
independent of frequency. This is because of
the circuit is almost resistive.