Lecture 02. The accuracy of measuring instrument is described with certain percentage (%). E.g...
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Transcript of Lecture 02. The accuracy of measuring instrument is described with certain percentage (%). E.g...
The accuracy of measuring instrument is described with certain percentage (%) .
E.g manufacturer may specify the instrument to be accurate at 2 % .
LIMITING ERROR
EE220 Instrumentation & Measurement
3
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
Hysteresis effect
EE220 Instrumentation & Measurement
5
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
Instrument Types and Performance Characteristics
EE220 Instrumentation & Measurement
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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
Active and Passive Instruments
EE220 Instrumentation & Measurement
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Passive Instrument requires no external source of power
Pressure Gauge
Active and Passive Instruments
EE220 Instrumentation & Measurement
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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
Null & Deflection Type Instruments
EE220 Instrumentation & Measurement
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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
Null & Deflection Type Instruments
EE220 Instrumentation & Measurement
10
Analogue Instruments give a continuously varying output as the quantity being measured changes◦ Deflection Type pressure Gauge
Digital Instruments give output that varies in discrete quantities
Analogue and Digital Instruments
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
Units
Standards of measurement◦ International◦ Primary◦ Secondary◦ Working
Electrical standards◦ Ampere◦ Resistance◦ Voltage◦ Capacitance◦ Inductance
Do not exceed four pages… Be concise and brief..
Assignment 1
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.
Meters
A stationary, permanent-magnet, moving-coil 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.
BASIC METER MOVEMENT
Greater the intensity of current, stronger the magnetic field.
Stronger the magnetic field , greater the rotation of the coil.
Example: Galvanometer.
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.
Galvanometer
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
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 movable-coil type and are called d'Arsonval galvanometers (after Arsène d'Arsonval, a French physicist)
D’Arsonval Galvonometers
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.
Permanent magnet moving coil and errors due to PMMC…????◦ Its part of your assignment 1… brief and simple…
PMMC
U-shaped permanent magnet.
Movable coil.
A pointer.
A scale.
Coil is suspended between the poles of the magnet.
Construction and Working
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
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
Some Maths..
Restoring torque is given by
Where C is the restoring constant and is the final deflection.
At steady state or equilibrium position,
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
ResistorWhat does the word shunt means..??
DC Ammeter
Shunt is a name given to low resistance/resistor.
Placed parallel to Galvanometer… why??? Alternate path.. Shunt will be
◦ Small…why?
Ammeter
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…??
Ammeter Sensitivity
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 =
Calculations
Then voltage drop at galvanometer is
Since shunt resistance is parallel to galvanometer so
Current through shunt is
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???
Question
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.
Multirange Ammeter
Consider
Recall equation (5)…
Using the above equation, we can derive for the unknown quantities and in general we have
Calculations
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.
Reminders…
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.
DC Voltmeter
Current should be negligible through the voltmeters.
So the resistance of the voltmeter should be high.
The series resistor is called a "multiplier" resistor
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….???
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
Voltmeter Sensitivity
A DC voltmeter can be converted into a multi range voltmeter by connecting a number of resistors (multipliers) in series with the meter movement.
53
MULTIRANGE VOLTMETER
Multirange voltmeter
R1 R2 R3 R4
+
_
V1V2
V3
V4
Rm
Im