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SPHOORTHY ENGINEERING COLLEGEApproved by AICTE, Permanently Affiliated to JNTUH,
UNIT-IBLOCK SCHEMATICS OF MEASURING SYSTEMS
C.Lokanath ReddyAssistant Professor
Dept of ECE
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Contents
• Performance Characteristics
• Measuring Instruments: DC Voltmeters
• D’ Arsonval Movement
• DC Current Meters
• AC Voltmeters and Current Meters
• Ohmmeters
• Multi meters
• True RMS Responding Voltmeters
• Specifications of Instruments.
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Performance characteristics
• The treatment of instrument & measurement system
characteristics can be divided into two distinct
categories.
1. Static characteristics
2. Dynamic characteristics
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Static characteristics
1. Accuracy: Accuracy is the closeness with which an instrument
reading approaches the true value of the variable being
measured.
2. Precision: It is the measure of order or degree to which a
particular parameter is measured.
3. Resolution: Resolution is the smallest change in the measured
value to which the instrument can respond. It is the smallest
change the instrument can measure.
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Static characteristics
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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Three targets with three arrows each to shoot.
Can you hit the bull's-eye?
Both accurate and precise
Precise but not accurate
Neither accurate nor precise
How do they compare?
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Dynamic characteristics
1. Fidelity: It is the degree to which an instrument indicates the
changes in the measured variable without dynamic error
(faithful reproduction).
2. Lag: It is the retardation or delay in the response of an
instrument to changes in the measured variable.
3. Repeatability: This is defined as the variation of scale reading
when the input is randomly applied.It is the closeness value of
same output for same input under same operating condition.
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Dynamic characteristics
4. Reproducibility: This is a scale reading over a given period of
time when the input is continuously connected to the
instrument.It is the closeness value of same output for same
input under same operating condition over a period of time.
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Errors
• It is the numerical difference between the true value of a
quantity & its value as obtained by measurement.
• Error may be expressed as absolute or as percentage error.
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Types of Errors:
• Gross Errors: These errors are mainly due to human mistakes
in reading or in using instruments or errors in recording
observations.
• Systematic Errors: These errors occur due to shortcomings of,
the instrument, such as defective or worn parts, or ageing or
effects of the environment on the instrument.
1. Instrumental Errors
2. Environmental Errors
3. Observational Errors
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Types of Errors:
• Random errors: These are errors that remain after gross &
systematic errors have been substantially reduced. No specific
reason can be assigned.
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Problem:
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Statistical Analysis:
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Gaussian Error :
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Root Sum Squares formula:
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Problems:
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Problems:
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Problems:
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D’ Arsonval Movement :
• The motor action is produced by the flow of a small current
through a moving coil which is positioned in the field of a
permanent magnet. This basic moving coil system is often
called the D’Arsonval galvanometer.
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D’ Arsonval Movement :
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Permanent Magnetic Moving Coil Movement:
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Permanent Magnetic Moving Coil Movement:• In this instrument we have a coil suspended in the magnetic
field of a permanent magnet in the shape of a horse shoe. Thecoil is suspended so that it can rotate freely in the magneticfield.
• When current flows in the coil, the developed(electromagnetic) torque causes the coil to rotate.
• The electromagnetic torque is counter balanced by amechanical torque of control springs attached to the movablecoil.
• The balance of torque, and therefore the angular position ofmovable coil is indicated by a pointer against a fixed referencecalled a scale.
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Permanent Magnetic Moving Coil Movement:
• The equation for the developed torque, derived from the basic
law for electromagnetic torque is
• The above equation shows that developed torque is
proportional to the flux density of the field in which coil
rotates, the current coil constants.
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B A I N
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Permanent Magnetic Moving Coil Movement:
• Since both flux density & coil constants are fixed for a given
instrument, the developed torque is direct indication of the
current in the coil. The pointer deflection can therefore be used
to measure current.
• The spring control provides a restoring torque
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Problem:
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Problem:
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Indicators:
• Different instrument forms may be obtained by starting with
the basic meter movement & adding various elements as
follows
i) DC current by adding a shunt resistance, forming a micro
ammeter or a milliammeter.
ii) DC voltage by adding a multiplier resistance, forming a
millivoltmeter or a voltmeter.
iii) Resistance, by adding a battery & resistive network, forming
an ohmmeter.
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Indicators:
• The basic meter movement becomes an AC instrument,
measuring
i) AC voltage or current by adding a rectifier, forming a rectifier
type meter for power.
ii) RF voltage or current, by adding a thermocouple type meter
for RF.
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Indicators:
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Types of Instruments:
• PMMC
• Moving Iron
• Electrodynamometer
• Hotwire
• Thermocouple
• Induction type
• Electro static
• Rectifier
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DC voltmeter:
• A basic D’Arsonval movement can be converted into a DC
voltmeter by adding a series resistor (multiplier) as shown in
Figure.
• Rs = multiplier resistance
• Rm = internal resistance of the movement
• V = full range voltage of the instrument
• Im= full scale deflection current of the movement (Ifsd)
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DC voltmeter:
• From the circuit of above figure
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m
m
s
m
mm
mms
msm
RI
VR
RI
V
I
RIVR
RRIV
)(
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Problem:
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Problem:
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Multirange voltmeter:
• The range of the basic DC voltmeter can be extended by using
number of multipliers and a selector switch. Such a meter is
called multirange voltmeter and is shown in the Fig. 1.
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Practical multirange voltmeter:
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Practical multirange voltmeter:
• The multiplier resistances can be calculated as :
• In position V4,
• In position V3, the multiplier is R3 + R4.
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Practical multirange voltmeter:
• In position V2, the multiplier is R2 + R3 + R4. In position V4,
• In position V1, the multiplier is R1 + R2 + R3 + R4.
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Extending Voltmeter Ranges:
• The range of a voltmeter can be extended to measure high
voltages by using a high voltage probe or by using multipier
resistors as shown in figure. However, great care must be used
not to exceed the voltage drop required for full scale deflection
of the basic movement.
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Precautions to be taken while using a voltmeter :
• The voltmeter resistance is very high and it should always be
connected across the circuit or component whose voltage is to
be measured.
• The polarities must be observed correctly.
• While using the multi Range voltmeter. First use the highest
range and then decrease the voltage range until the sufficient
deflections is obtained
• Take care of the loading effect.
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DC voltmeter :
Globe Analogue Voltmeters
• Analogue Voltmeter- 0 to 30Vdc Measuring Range Accuracy2.5%
• Resolution- 0.5V• Moving Coil,• Grade B, 7.5cm Scale,• Easy to Read,• Inclined Display,• Fully Enclosed Plastic Case, Overload Protection, External
Screw Adjustment to Zero,• 135 Length, 100 Width, 85mm Height
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DC ammeter:
• 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)
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DC ammeter:
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mmsh
msh
mmshsh
II
RIR
III
RIRI
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DC ammeter:
Properties of Shunt Resistor
• Resistance of the shunt should not vary with time
• Temperature co-efficient of shunt and instrument should be
low and should be same
Multirange Ammeter
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Problem:
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Problem:
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AC voltmeter Block Diagram:
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RectifierVoltage divider
DC AmplifierPMMC meter
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AC voltmeter :
• Basic Rectifier Type AC Voltmeter
AC Voltmeter using Half Wave Rectifier
• The diode D conducts only during positive half cycle.
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AC voltmeter :
AC Voltmeter using full Wave Rectifier
• The full-wave rectifier provides higher sensitivity rating
compare to the half-wave rectifier.
• Bridge type rectifier is the most commonly used
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AC voltmeter :
65 Moving Iron Instruments AC Voltmeter
• Type SD-65 AC V : Accuracy: Class 2.5
• Specification: 30V 50V 75V 100V 120V 150V 200V 250V 450V 500V 600V, if Over 600V, meters can be connected potential transformer with secondary Voltage 100V.
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AC voltmeter :
Average responding voltmeter
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AC voltmeter :
Average responding voltmeter
• The AC input applied is first amplified with the help of high gain
stabilized amplifier, up to a required level. This voltage is the
rectified using the diodes D1 and D2. The rectified voltage is fed
to DC milliammeter used as a measuring meter.
• The blocking capacitor used at the input side blocks any DC
component in the input voltage. Large amount of negative
feedback is used for the amplifier to ensure stability for
measurement accuracy.
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AC voltmeter :
Peak responding voltmeters
• As the name indicates, this type of meter responds to the peak
vale of the AC input signal
The two types of peak responding voltmeters are
1. D.C coupled peak responding voltmeter
2. A.C coupled peak responding voltmeter
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AC voltmeter :
Peak responding voltmeters
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Peak responding voltmeters
Advantages• When AC input is strong, the pre amplification of AC input is
not necessary. In such case, the capacitor and diode can betaken out of the instrument and placed in a probe. Themeasured AC signal in this cased, travel no further than diode.The loading effect is minimized.
• When capacitor and diode are placed in the probe, thefrequencies up to hundreds of MHZ can be measured.
Disadvantages• The harmonic distortion in the input causes the errors.• The instrument has limited sensitivity due to imperfect and
nonlinear diode characteristics.• The error is introduced if input waveform is not symmetrical.
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AC ammeter :
Thermocouple type RF ammeter
• Thermocouple consists of a junction of two dissimilar wires, so
chosen that a voltage is generated by heating the junction.
• The output of thermocouple is delivered to a sensitive dc
micrometer.
• Thermocouple instruments are standard means for measuring
currents at radio frequencies.
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AC ammeter :
Thermocouple type RF ammeter
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AC ammeter :
Thermocouple type RF ammeter
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AC ammeter :Thermocouple type RF ammeterAdvantages:• Thermocouples are cheaper than the resistance thermometers.• These instruments are not affected by stray magnetic fields.• These instruments are free from frequency errors.• These instruments have high sensitivity.• These are very useful when used as transfer instruments to
calibrate dc instruments by potentiometer.
Disadvantages:
• They have lower accuracy & cannot be used for precision work.• To ensure long life of thermocouples in their operating
environments, they should be protected in an open or closedend metal protecting tube.
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True RMS responding voltmeter:
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True RMS responding voltmeter:
• The heating power of a given input signal is proportional to the
square of the rms value of the voltage. Sensing this power, this
meter produces a deflection using a thermocouple.
Thermocouple outputs are non-linear, in general.
• Therefore, DC current is directly proportional to the effective
or rms value of the input voltage.
• This is indicated on the meter movement in the output circuit
of the DC amplifier. Thus, the true rms value of the AC input can
be measured irrespective of the shape of the input.
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True RMS responding voltmeter:
Advantages:
• These instruments are not affected by stray magnetic fields.• These instruments are free from frequency errors.• These instruments have high sensitivity.• These are very useful when used as transfer instruments to
calibrate dc instruments by potentiometer.• Thermocouples are cheaper than the resistance thermometers.
Disadvantages:
• They have lower accuracy & cannot be used for precision work.• To ensure long life of thermocouples in their operating
environments, they should be protected in an open or closedend metal protecting tube.
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True RMS responding voltmeter:
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Series type Ohmmeter :
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Fig. Series type Ohmmeter Dial of series ohmmeter
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Series type Ohmmeter :
• Values of R1 and R2 can be determined from the value of Rx,
which gives half the full scale deflection.
• The total current of the circuit,
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mmh
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2
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Series type Ohmmeter :
• The shunt current through R2 is I2=It-Ifsd
• The voltage across the shunt, Vsh= Vm
• So, I2 R2=Ifsd Rm
• Since I2=It-Ifsd
• Then,
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mfsd
II
RIR
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Series type Ohmmeter :
• Since It=V/Rh
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hmfsd
RIV
RRIR
2
V
RRIRR
hmfsd
h 1
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Shunt type Ohmmeter :
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Fig. Shunt type Ohmmeter Dial of series ohmmeter
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Shunt type Ohmmeter :
• From fig, Rx=∞, the full scale meter current is
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Shunt type Ohmmeter :
• With any resistance Rx to be measured;
• The meter current
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Multimeter :
• A multimeter or a multitester, also known as a VOM (Volt-Ohm
meter), is an electronic measuring instrument that combines
several measurement functions in one unit.
• A multimeter is basically a PMMC meter. To measure dc current
the meter acts as an ammeter with a low resistance.
• Range changing is accomplished by shunts in such a way that
the current passing through the meter does not exceed the
maximum rated value.
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Multimeter :
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Meter Protection:
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Extension of Range:
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Extension of Range:
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Extension of Range:
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Extension of Range:
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Digital Voltmeter:Ramp type DVM
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Digital Voltmeter:Successive Approximation type DVM
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Digital Voltmeter:Successive Approximation type DVM• At the start of conversion cycle both the control register & the
ring counter are set with 1 in the MSB and 0 in all bits of lesssignificance.
• Thus the ring counter shows that the cycle has started & theprocess is in its first phase.
• The control register shows 1000(4 bit converter) & this causesan output voltage at D/A converter section of one half ofreference voltage supply.
• At the same time a pulse enters the time delay circuitry. By thetime that the D/A converter & comparator have settled thisdelay pulse is gated with the comparator output.
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Digital Voltmeter:Dual Slope Integrating type DVM
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Digital Voltmeter:Dual Slope Integrating type DVM
• In this technique, an integrator is used to integrate an accurate
reference voltage for a fixed period of time. The same
integrator is then used to integrate with the reverse slope, the
input voltage. The time required to return to the starting
voltage is measured.
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