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SlideShare utilise les cookies pour améliorer les fonctionnalités et les performances, et également pour vous montrer des publicités pertinentes. Si vous continuez à naviguer sur ce site, vous acceptez l’utilisation de cookies. Consultez notre Politique de confidentialité et nos Conditions d’utilisation pour en savoir plus.
Primary sensing element
The quantity under measurement makes its first contact with
primary sensing element of a measurement system here, the
primary sensing element transducer.
This transducer converts measured into an analogous electrical
Variable conversion element
The output of the primary sensing element is the electrical
It may be a voltage a frequency or some other electrical
parameter. But this output is not suitable for this system.
For the instrument to perform the desired function, it may be
necessary to convert this output to some other suitable form
while retaining the original signal.
Consider an example, suppose output is an analog signal form
and the next of system accepts input signal only in digital form
Variable manipulation element
The main function of variable manipulation element is
to manipulation element is to manipulate the signal
presented to it preserving the original nature of the
signal. Here, manipulation means a change in
numerical value of the signal.
Data presentation element:
The information about the quantity under measurement
has to be conveyed to the personal handling the
instrument or system for control or analysis purposes.
The information conveyed must be in the form of
intelligible to the personnel.
The above function is done by data presentation element.
The output or data of the system can be monitored by
using visual display devices may be analog or digital device
like ammeter, digital meter etc.
In case the data to be record, we can use analog or digital
recording equipment. In industries , for control and
analysis purpose we can use computers.
The performance characteristics are divided in to :-
Static characteristics indicate the response of the
instrument for slowly varying data or time invariant
Dynamic characteristics denote the behaviour of the
instrument for the time varying quantities.
The instrument design ,testing and evaluation is
performed based on the parameters.
Instrument: A device or mechanism used to determine
the present value of the quantity under measurement.
Measurement: The process of determining the amount,
degree, or capacity by comparison (direct or indirect)
with the accepted standards of the system units being
The degree of exactness or closeness of a measurement of a
measurement compared to the expected or desired value .
A measure of the consistency or repeatability of measurements
i.e, succesive readings do not differ.
precision is the consistency of the instrument output for a given
value of input.
xn=value of the nth measurement
xnbar=average value of the set of measured values
The ratio of change in output or response of the
instrument to a change of input or measured value.
The reciprocal of sensitivity of is called as deflection factor.
deflection factor = 1/ sensitivity
Resolution is the smallest change in the measured value to which
a instrument can respond .
Calibration is the process of making an adjustment or
marking a scale so that the readings of an instrument agree
with the accepted value and the certified standard.
It is the difference between true value and a measured value.
e = Y n - X n
Where e=absolute errors; Yn=expected value; Xn=measured value;
%error = (absolute value/expected value)*100=(e/Yn)*100
Therefore %error =
It is more frequently expressed as an accuracy rather than error. Therefore
Where A is the relative accuracy
Accuracy is expressed as % accuracy a=100% - %error
a=A*100% (where a=%accuracy)
The errors that may occur in an
i. Gross errors or personal Errors
ii. Systematic errors
iii. Instrumental errors
iv. Environmental errors
v. Observational errors
vi. Random Errors
This class of errors mainly covers human mistakes in reading or using
instruments and in recording and calculating measured values As long
a human beings are involved.
Some gross errors will definitely be committed. Although complete
elimination of gross errors is probably impossible, are should try to
anticipate and correct them.
Some gross errors are easily detected while others may be very
difficult to detect. The experiment may grossly misread the scale.
Great care should be taken in reading and recording the data.
Two , three or even more readings should be taken for the quantity
These readings should be taken preferably by different experimenters
and the readings should be taken at a different reading point to avoid
re-reading with the same error.
Never place complete dependence on one reading but take at least three
separate readings. Preferably under conditions in which instruments are
switched off- on.
These types of errors are divided into three categories such as
instrumental errors, Environmental errors and observational
These errors arise due to inherent short comings in the
instruments misuse of the instruments and loading effects.
These errors are due to conditions external to the measuring
device including conditions in the area surrounding the
These may be effects of temperature, pressure, humidity, dust,
vibrations or of external magnetic or electrostatic fields.
The connective measures employed to eliminate to reduce
these undesirable effects.
This occurs are due to unknown causes and are observed when the
magnitude and polarity of a measurement future in an unpredictable
Some of the more common random errors are:
(i) Rounding error:
This occurs when readings are between scale graduations and the
reading is rounded up or down to the nearest graduation.
(ii) Periodic error:
This occurs when an analog meter reading swings or fluctuates about
the correct reading.
In addition, the meter reading quickly changes in the immediate
vicinity of the corrected value, but changes slowly at the extremes of
Since it could be easier to read the meter when it is slowly changing,
the correct value would be less likely read than an incorrect value.
The other random errors are due to noise backlash and ambient
Random errors cannot normally be predicted or corrected but they can
be minimized by skilled observes using a well maintained quality
It is the degree to which an instrument indicates the
changes in the measured variable without dynamic
error (faithful reproduction).
Comparison between Electronic
meters and Conventional meters
BASIC PRINCIPLE OF ANALOG
This permanent magnet moving coil meter movement
is the basic movement in most analog (meter with a
pointer indicator hand) measuring instruments.
It is commonly called d'Arsonval movement because it
was first employed by the Frenchman d'Arsonval in
making electrical measurements.
Basic Principle Operation Of Permanent-Magnetic Moving-Coil
The permanent magnet moving coil instruments are most
accurate type for direct current measurements.
The action of these instruments is based on the motoring
principle. When a current carrying coil is placed in the
magnetic field produced by permanent magnet, the coil
experiences a force and moves.
As the coil is moving and the magnet is permanent, the
instrument is called permanent magnet moving coil
instrument. This basic principle is called D’Arsonval
The amount of force experienced by the coil is
proportional to the current passing through the coil.
The pointer is carried by the spindle and it moves over a
The pointer has light weight so that it deflects rapidly.
The mirror is placed below the pointer to get the accurate
reading by removing the parallax.
The weight of the instrument is normally counter balanced
by the weights situated diametrically opposite and rapidly
connected to it.
The scale markings of the basic d.c PMMC instruments
are usually linearly spaced as the deflecting torque and
hence the pointer deflections are directly proportional to
the current passing through the coil.
The deflecting torque produced is described below in
Deflecting Torque, T = BINA
B = flux density in Wb/m2 (Tesla)
I = current (A).
N = number of turns of the coils.
A = area ( length X wide), (m2).
A basic d’Arsonval movement can be converted into dc
voltmeter by adding in series resistor multiplier as
shown in fig.
IM = full scale deflection current of the movement (Ifsd)
RM = internal resistance of the movement
RS = multiplier resistance
V = full range voltage of the instrument
The PMMC galvanometer constitutes the basic movement of a dc
ammeter. The coil winding of a basic movement is small and
light, so it can carry only very small currents.
The PMMC can use to build an ammeter with connected the
shunt resistor and meter in parallel.
A low value resistor (shunt resistor) is used in DC ammeter to
measure large current.
Rm = internal resistance of the movement
Rsh = shunt resistance
Ish =shunt current
Im = full scale deflection current of the movement
I = full scale current of the ammeter + shunt (i.e. total current)
R1 R2 R3 R4
Figure 2.3: Multirange Ammeter
Figure 2.4: Aryton Shunt
ARYTON SHUNT AMMETER
Aryton shunt eliminates the possibility of having the
meter in the circuit without a shunt.
Position of the switch:
‘1’: Ra parallel with series combination of Rb, Rc and the
meter movement. Current through the shunt is more than
the current through the meter movement, thereby
protecting the meter movement and reducing its
‘2’: Ra and Rb in parallel with the series combination of
Rc and the meter movement. The current through the
meter is more than the current through the shunt
‘3’: Ra, Rb and Rc in parallel with the meter. Maximum
current flows through the meter movement and very
little through the shunt. This will increase the sensitivity.
The selector switch S, selects the appropriate shunt required to change
the range of the meter.
When the position of the switch is '1' then the resistance R1 is in parallel
with the series combination of R2 , R3 and Rm.
Hence current through the shunt is more than the current through the
meter, thus protecting the basic meter. When the switch is in the position
'2', then the series resistance of R1 and R2 is in parallel with the series
combination of R3 and Rm. The current through the meter is more than
through the shunt in this position. In the position '3', the
resistances R1 , R2 and R3 are in series and acts as the shunt. In this
position, the maximum current flows through the meter.
This increases the sensitivity of the meter.
The voltage drop across the two parallel branches is always equal.
Thus, Ish Rsh = Im Rm
But in position 1, R1 is in parallel with R2 + R3 + Rm
where I1 is the first range required.
In position 2, R1 + R2 is in parallel with R3 + Rm .
where I2 is the second range required.
In position 3, R1 + R2 + R3 is in parallel with Rm .
where I3 is the third range required.
The current range I3 is the minimum while I1 is
maximum range possible. Solving the equations
(1), (2) and (3) the required Ayrton shunt can be
REQUIREMENT OF A SHUNT
1) Minimum Thermo Dielectric Voltage Drop
Soldering of joint should not cause a voltage drop.
- never connect an ammeter across a source of e.m.f
- observe the correct polarity
- when using the multirange meter, first use the
highest current range.
A basic D’Arsonval movement can be converted into a DC voltmeter by
adding a series resistor (multiplier) as shown in Figure 2.3.
Im =full scale deflection current of the movement (Ifsd)
Rm=internal resistance of the movement
Rs =multiplier resistance
V =full range voltage of the instrument
Figure 2.5: Basic DC Voltmeter
From the circuit of Figure 2.5:
2.5: MULTI-RANGE VOLTMETER
A DC voltmeter can be converted into a
multirange voltmeter by connecting a number of
resistors (multipliers) in series with the meter
A practical multi-range DC voltmeter is shown in
Figure 2.6: Multirange voltmeter
R1 R2 R3 R4
A basic D’ Arsonval movement with a full-scale
deflection of 50 uA and internal resistance of
500Ω is used as a DC voltmeter. Determine the
value of the multiplier resistance needed to
measure a voltage range of 0-10V.
Sensitivity and voltmeter range can be used to
calculate the multiplier resistance, Rs of a DC
Rs=(S x Range) - Rm
From example 2.4:
Im= 50uA, Rm=500Ω, Range=10V
So, Rs = (20kΩ/V x 10V) – 500 Ω
= 199.5 kΩ
In order to measure the alternating current with the d’Arsonval
meter movement, we must rectify the alternating current by use of
diode rectifier .
Figure 5.6 is the DC voltmeter circuit modified to measure AC
The diode, assume to be ideal diode, has no effect on the
operation of the circuit .
For example if the 10 V sine-wave input is fed as the source of
the circuit, the voltage across the meter movement is just the
positive half-cycle of the sine wave due to the rectifying effect of
D’Arsonval Meter Movement with
Figure 5.6: DC Voltmeter Circuit
Modified to Measure AC Voltage.
The peak value of 10 Vrms sine wave is,
If the output voltage from the half-wave rectifier is 10V only, a dc
voltmeter will provide an indication of approximately 4.5 V.
From the above equation,
dcac SS 45.0
Example 5.1: D’Arsonval Meter Half-Wave Rectifier.
Compute the value of the multiplier resistor for a 10 Vrms ac range on the
voltmeter shown in Figure 5.7.
Find the sensitivity for a half wave rectifier.
Figure 5.7: AC Voltmeter Using Half-
Commercially produced ac voltmeters that use half-wave
rectification have an additional diode and shunt as shown in
Figure 5.8, which is called instrument rectifier.
.Figure 5.8: Half-Wave Rectification Using an Instrument Rectifier and a Shunt
The full-wave rectifier provide higher sensitivity rating compare to
the half-wave rectifier.
Bridge type rectifier is the most commonly used, Figure 5.9.
5.3 D’Arsonval Meter Movement
with Full-Wave Rectification.
Figure 5.9: Full Wave Bridge Rectifier Used in AC Voltmeter Circuit.
(a) During the positive half cycle (red arrow), currents flows through
diode D2, through the meter movement from positive to negative, and
through diode D3.
- The polarities in circles on the transformer secondary are for the
positive half cycle.
- Since current flows through the meter movement on both half cycles,
we can expect the deflection of the pointer to be greater than with the
half wave cycle.
- If the deflection remains the same, the instrument using full wave
rectification will have a greater sensitivity.
(b) Vise-versa for the negative half cycle (blue arrow).
From the circuit in Figure 5.9, the peak value of the 10 Vrms signal with
the half-wave rectifier is,
The average dc value of the pulsating sine wave is,
Or can be compute as,
The AC voltmeter using full-wave rectification has a sensitivity equal to
90% of the dc sensitivity or twice the sensitivity using half-wave
peakrmsp VEE 14.14*414.1
VEE pave 9636.0
VVEE rmsave 910*9.0*9.0
dcac SS *9.0
The equivalent DC voltage is,
(b) The ac sensitivity,
(c.) The dc sensitivity,
VOLTMETER LOADING EFFECTS
When a voltmeter is used to measure the voltage across a
circuit component, the voltmeter circuit itself is in parallel
with the circuit component.
Total resistance will decrease, so the voltage across
component will also decrease. This is called voltmeter
The resulting error is called a loading error.
The voltmeter loading can be reduced by using a high
How about ammeter??
1. An ohmmeter is an instrument used to measure resistance and
check the continuity of electrical circuits and component. This
resistance reading is indicated through a meter movement.
2. The ohmmeter must then have an internal source of voltage to
create the necessary current to operate the movement, and also
have appropriate ranging resistors to allow desired current to
flow through the movement at any given resistance.
3. Two types of schemes are used to design an ohmmeter – series
type and shunt type.
4. The series type of ohmmeter is used for measuring relatively
high values of resistance, while the shunt type is used for
measuring low values of the resistance.
The purpose of an ohmmeter, of course, is to measure the resistance
placed between its leads. This resistance reading is indicated through a
mechanical meter movement which operates on electric current.
The ohmmeter must have an internal source of voltage to create the
necessary current to operate the movement, and also have appropriate
ranging resistors to allow just the right amount of current through the
movement at any given resistance.
A more accurate type of ohmmeter has an electronic circuit that passes a
constant current (I) through the resistance, and another circuit that
measures the voltage (V) across the resistance. According to the following
equation, derived from Ohm's Law, the value of the resistance (R) is given by
Operation of an Ohmmeter
OHMMETER (Series Type)
Current flowing through meter movements depends on the magnitude
of the unknown resistance.(Fig 4.28 in text book)
The meter deflection is non-linearly related to the value of the unknown
A major drawback – as the internal voltage decreases, reduces the current
and meter will not get zero Ohm.
R2 counteracts the voltage drop to achieve zero ohm. How do you get zero
R1 and R2 are determined by the value of Rx = Rh where Rh = half of full
scale deflection resistance.
The total current of the circuit, It=V/Rh
The shunt current through R2 is I2=It-Ifsd
OHMMETER (Series Type)
From equation (2-8) and (2-9):
Figure 2.7: Measuring circuit resistance with an ohmmeter
2. When RX = 0 (short circuit), R2 is adjusted to get full-
scale current through the movement. Then, I = Ifsd. The
pointer will be deflected to its maximum position on the
scale. Therefore, this full-scale current reading is marked 0
3. When RX = ∞ (open circuit), I = 0. The pointer will read
zero. Therefore, the zero current reading is marked ∞ ohms.
Shunt type ohmmeter
1. Figure shows the basic circuit of the shunt-type ohmmeter where
movement mechanism is connected parallel to the unknown resistance.
In this circuit it is necessary to use a switch, otherwise current will always
flow in the movement mechanism.
2. Resistor ‘Rsh’ is used to bypass excess current.
3. Let the switch be closed. When RX = 0 (short circuit), the
pointer reads zero because full current flows through Rx and no
current flows through the meter and Rsh. Therefore, zero current
reading is marked 0 ohms.
4. When RX = ∞ (open circuit), no current flows through RX.
Resistor R1 is adjusted so that full-scale current flows through
the meter. Therefore, maximum current reading is marked ∞
Digital ohmmeter s are used to measure the
It shows the exact reading of the resistance
It is mostly used to measure the earth resistance
Analog ohmmeters are used to measure
the resistance of the circuit with respect to the
current flowing the circuit
It shows the reading in the analog form
so if there is mistake by the reader in taking
reader there it will cause great difference in the
Many analog ohmmeters will, when
switched to the ohm function, reverse the
polarity of the test leads.