Transcript Instrumentation and Measurement

Lecture 02
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The accuracy of measuring instrument is
described with certain percentage (%) .
E.g manufacturer may specify the
instrument to be accurate at 2 % .
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If input measured quantity to the instrument
is steadily increased the output reading varies
in the manner shown in curve A
If the input is then steadily decreased the
output varies in the manner shown in curve B
Non coincidence between the loading and
unloading curves is known as hysteresis
Hysteresis is exhibited by instruments that
contain springs and electrical windings
formed around an iron core
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Instrument System are divided into different
types based on accuracy, performance and
application
Active and Passive Instruments
Null-Type and Deflection-Type Instruments
Analogue and Digital Instruments
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Active Instrument requires external source of
power.
Either a battery or an external voltage source.
Examples of active components include
transistors, LED lights, and opamps or other
IC chips
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Passive Instrument requires no external
source of power
Pressure Gauge
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Pressure Gauge is a Deflection Type
Instrument
Value of quantity measured displayed in
terms of movement of pointer
Calibration of spring is difficult therefore
Deflection type instrument is less accurate
More convenient to use
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Dead Weight Pressure Gauge is a Null Type
Instrument, works on the principle that P=
F/A
Weights are put on top of piston until the
downward force balances the fluid pressure
Weights are added until the piston reaches a
datum level known as the null point
Accuracy depends upon calibration of weights
Calibration of weights is easier therefore null
type instruments are more accurate
Inconvenient to use, for calibration purposes
only
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Analogue Instruments give a continuously
varying output as the quantity being
measured changes
◦ Deflection Type pressure Gauge
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Digital Instruments give output that varies in
discrete quantities
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Standard quantity of any physical quantity
Global standard, the International System of
Units(SI).
History of uniform systems/standards dates
back to Bronze age.
Fundamental units
Derived units
Go through table 2-3 of your book …
Conversion of units
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Standards of measurement
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Electrical standards
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International
Primary
Secondary
Working
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Ampere
Resistance
Voltage
Capacitance
Inductance
brief..
Do not exceed four pages… Be concise and
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Ampere’s Law
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DC
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Electromegnatism…
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Devices use to measure, indicate or both are
called meters…
Why Meters….??
◦ Whilst Troubleshooting, testing, or repairing
equipment you need various meters …
◦ To check for proper circuit voltages, currents,
resistances, and to determine if the wiring is
defective. You may be able to connect these test
instruments to a circuit and take readings.
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A stationary, permanent-magnet, movingcoil meter is the basic meter movement used
in most measuring instruments used for
servicing electrical equipment.
When current flows through the coil, a
resulting magnetic field reacts with the
magnetic field of the permanent magnet and
causes the movable coil to rotate.
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Greater the intensity of current, stronger the
magnetic field.
Stronger the magnetic field , greater the
rotation of the coil.
Example: Galvanometer.
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A galvanometer is an instrument for detecting
and measuring electric current.
The term "galvanometer", in common use by
1836, was derived from the surname of
Italian electricity researcher Luigi Galvani
Galvanometers were the first instruments
used to detect and measure electric currents.
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All galvanometers are based upon the
discovery by Hans C. Oersted that a magnetic
needle is deflected by the presence of an
electric current in a nearby conductor.
The extent to which the needle turns is
dependent upon the strength of the current.
The early moving-magnet form of
galvanometer had the disadvantage that it
was affected by any external magnetic field.
Disadvantage: Mechanical fault in spring
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In the first galvanometers, a freely turning
magnetic needle was hung in a coil of wire; in
later versions the magnet was fixed and the
coil made movable.
Modern galvanometers are of this movablecoil type and are called d'Arsonval
galvanometers (after Arsène d'Arsonval, a
French physicist)
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Further improvement included replacement of
fine wire suspension with a pivot, and
provided restoring torque and electrical
connections through spiral spring.
If a pointer is attached to the moving coil so
that it passes over a suitably calibrated scale,
the galvanometer can be used to measure
quantitatively the current passing through it.
Such calibrated galvanometers are used in
many electrical measuring devices.
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Permanent magnet moving coil and errors
due to PMMC…????
◦ Its part of your assignment 1… brief and simple…
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U-shaped permanent magnet.
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Movable coil.
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A pointer.
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A scale.
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Coil is suspended between the poles of the
magnet.
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Coil twist in response to the interaction of the
applied current through the coil and the
magnetic field of the permanent magnet.
When the driving force of the coil current is
removed, the restoring force provided by the
spring returns the coil to its zero position
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Let B be the strength of the magnetic field, n
be the number of turns in the coil, A be the
area of the coil and I be the current flowing
through the coil.
The Torque will be
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Restoring torque is given by
Where C is the restoring constant and is the
final deflection.
At steady state or equilibrium position,
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Which in turn,
◦ And
◦ Where G is the galvanometer Constant and
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A=
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I = ….?
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B = …?
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T = …?
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= radians
N = ….
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N =100
A = 0.0012
I = 0.02 A
B = 0.05 T
T = ???
 Assignment 1
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Used to measure electric current through the
circuit.
Modified form of galvanometer.
How?
◦ Galvanometer is used for small currents..
◦ What if large value of current???
We add an additional resistor…known as Shunt
Resistor
What does the word shunt means..??
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Shunt is a name given to low
resistance/resistor.
Placed parallel to Galvanometer… why???
Alternate path..
Shunt will be
◦ Small…why?
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Ammeter sensitivity is the amount of current
necessary to cause full scale deflection
(maximum reading) of the ammeter.
The smaller the amount of current, the more
"sensitive" the ammeter…..why???
Ampere or milliampere…??
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How to calculate shunt….?
Let be the maximum current that can be
passes through the galvanometer.
galvanometer resistance =
Shunt Resistance =
Total current =
Current through Shunt resistance =
Current through galvanometer =
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Then voltage drop at galvanometer is
Since shunt resistance is parallel to
galvanometer so
Current through shunt is
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By 1 and 2
Current is larger than
Let n be the factor by which
then…
is greater than
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Putting the above value in 3
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Note:In several books
=
and
=
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Shunt resistance = ?
◦ When
= 1 mA,
= 100 Ω,
= 10mA
◦ Ideally the shunt resistance should have value???
◦ What will happen when you connect the ammeter
parallel to the circuit???
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Several values of shunt resistors and a rotary
switch are used to select the desired range of
current to measure.
An Aryton shunt uses a combination of
resistors.
+
+
R1
R2
R3
R4
Rm
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S
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Multi range Ammeter
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Consider
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Recall equation (5)…
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Using the above equation, we can derive for
the unknown quantities and in general we
have
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Ammeter inserted in series.
Always set the range to the highest scale and
then reduce as needed.
Observe polarity.
Better quality analog meters include a mirror
along the scale. This eliminates parallax
error.
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Besides current, Voltage is also essential
quantity to be measured …
Galvanometer is further altered …
Voltmeter used to measure the potential
difference between any two points in a
circuit…
Connected in parallel to the circuit..
No alteration/drawing the current flowing
through the circuit.
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Current should be negligible through the
voltmeters.
So the resistance of the voltmeter should be
high.
The series resistor is called a "multiplier"
resistor
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To determine the value of ‘multiplier’ resistor
Consider the above figure…
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If is the voltage across the voltmeter,
is
the current through voltmeter, is the
resistance through Galvanometer and is the
multiplier resistance then
Ideally, Multiplier resistance should be….???
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R = 10 Ω
= = 50 mA
Show how it can be adopted to measure
voltage upto 750 V ?
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We can also find the value of multiplier
resistance by determining the sensitivity of
the meter.
Sensitivity is given by S and is determined by
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Also… Sensitivity or
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A DC voltmeter can be converted into a
multi range voltmeter by connecting a
number of resistors (multipliers) in series
with the meter movement.
R1
R2
R3
R4
Im
V1
+
V2
V3
Rm
V4
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Multirange voltmeter
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For extension of simple voltmeter… we utilize
the product of voltmeter sensitivity and the
given range followed by subtraction of
internal/meter resistance from the product …
In other words…