2. Terminologies in measurements
• ACCURACY
• The degree of exactness of a measurement compared to expected
value
• RESOLUTION
• The smallest change in a measured variable to which an instrument
will respond
• PRECISION
• Precision is the consistency of the instrument output for a given value
of input
3. • SENSITIVITY
• The ratio of change in output of the instrument to a change of input
or measured variable
• ERROR
• The deviation of true value from desired value
Errors may be classified as Absolute or as percentage of error
Absolute Error
Absolute error may be defined as difference between the expected
value of the variable and measured value of variable
e=Yn-Xn
Where
e=absolute value,Yn=expected value,Xn=measured value
4. % Error =Absolute value/Expected value *100
% Error=(Yn-Xn/Yn)*100
MOVING COIL GALVANOMETER
A galvanometer is a device that is used to detect small electric current or measure
its magnitude.
The current and its intensity is usually indicated by a magnetic needle’s movement
or that of a coil in a magnetic field that is an important part of a galvanometer.
Moving-coil galvanometers are mainly divided into two types:
Suspended coil galvanometer
Pivoted-coil or Weston galvanometer
5. • Moving Coil Galvanometer Principle
• A current-carrying coil when placed in an external magnetic field experiences magnetic
torque.
• The angle through which the coil is deflected due to the effect of the magnetic torque
is proportional to the magnitude of current in the coil.
• Moving Coil Galvanometer Construction And Diagram
• The moving coil galvanometer is made up of a rectangular coil that has many turns and
it is usually made of thinly insulated or fine copper wire that is wounded on a metallic
frame.
• The coil is free to rotate about a fixed axis.
• A phosphor-bronze strip that is connected to a movable torsion head is used to suspend
the coil in a uniform radial magnetic field.
• Essential properties of the material used for suspension of the coil are conductivity and
a low value of the torsional constant.
6. • A cylindrical soft iron core is symmetrically positioned inside the coil to improve
the strength of the magnetic field and to make the field radial.
• The lower part of the coil is attached to a phosphor-bronze spring having a small
number of turns.
• The other end of the spring is connected to binding screws.
• A cylindrical soft iron core is symmetrically positioned inside the coil to improve
the strength of the magnetic field and to make the field radial.
• The lower part of the coil is attached to a phosphor-bronze spring having a small
number of turns.
• The other end of the spring is connected to binding screws.
7.
8. • Conversion of Galvanometer into Ammeter
• A galvanometer is very sensitive instrument to detect the current.
• It can be easily converted into ammeter and voltmeter.
• Galvanometer to an Ammeter:-
• Ammeter is an instrument used to measure current flowing in the electrical
circuit.
• The ammeter must offer low resistance such that it will not change the current
passing through it. So ammeter is connected in series to measure the circuit
current.
• A galvanometer is converted into an ammeter by connecting a low resistance in
parallel with the galvanometer. This low resistance is called shunt resistance S.
• The scale is now calibrated in ampere and the range of ammeter depends on
the values of the shunt resistance
9.
10. Conversion of Galvanometer into Voltmeter
Galvanometer is a very sensitive instrument, therefore it can not measure
high potential difference.
In order to convert a Galvanometer into voltmeter, a very high resistance
known as "series resistance" is connected in series with the
galvanometer.
11. Let resistance of galvanometer = Rg and resistance Rx (high) is
connected in series to it.
Then combined resistance = (Rg + Rx).
V = Ig (Rg + Rx)
V = IgRg + IgRx
V – IgRg = IgRx
Rx = (V – IgRg)/Ig
12. Multimeter
A multimeter is device that can be used to measure multiple quantities, i.e., when a
single device is used to measure multiple quantities, the device is called multimeter.
On the basis of output representation, there are two types of multimeters −
Analog multimeter
&
Digital multimeter
Analog Multimeter
An analog multimeter is a permanent magnet moving coil (PMMC) meter type
measuring instrument.
It works on the principle of d’Arsonval galvanometer.
The analog multimeter has an analog display that uses the deflection of a pointer on
the scale to indicate the level of measurement being made.
The pointer deflects from its initial position increasingly as the measuring quantity
increases.
13. Working of Analog Multimeter
Since, the analog multimeter is a PMMC types instrument, so when a
current is passed through its coil, the coil moves in a magnetic field
produced by the permanent magnet
A pointer is attached with the coil.
When current flows in the coil, a deflecting torque acts on the coil that will
rotate it by an angle, so the pointer moves over a scale.
A pair of hairsprings is attached to the spindle to provide the controlling
torque.
Analog Multimeter Measuring Quantities
A typical analog multimeter can measure following electrical quantities −
Resistance
DC Voltage
AC Voltage
DC Current
14. • The analog multimeter acts as an ammeter with a low series resistance to
measure direct current.
• For high currents measurement, a shunt resistor is connected in parallel with
the galvanometer.
• With the shunt resistor, an analog multimeter can measure currents in the
ranges of milli-amperes or amperes.
• By adding a battery and a network of resistors, the analog multimeter can work
as an ohmmeter. By changing the value of shunt resistance in resistor network,
different values of resistances can be measured.
• By adding a rectifier unit in the analog multimeter circuit, the AC voltages and
currents can also be measured.
16. Steps for the use of analog multimeter −
*Insert the probes into the correct connections.
*Set switch to the correct measurement types and range for the
measurement to be made.
While selecting the range, ensure that the maximum range is above than
that is expected. The range of multimeter is then optimised for the best
reading.
Advantages of Analog Multimeter
It gives the continuous reading, thus a sudden change in signal can be
detected which is not possible with digital multimeter.
Analog multimeter are very cheap.
All measurement can be made using a single meter only.
17. Disadvantages of Analog Multimeter
• They are bulky and larger sized.
• Multiple scales, these can cause confusion.
• Low input resistance
Digital Multimeter
• A digital multimeter (DMM) is a measuring instrument used to measure various
electrical quantities.
• The standard measurements that are performed by a DMM are current, voltage
and resistance. Apart from these, a digital multimeter can also measure
temperature
19. DMM Controls and Connection Ports
• A typical DMM has a rotary switch, digital display and connecting jacks for the
probes.
• Display − The DMM has an illuminated display screen for better visualisation.
Most DMM have four digit display, the first of which can only be either a 0 or 1
and a + / - indication as well. There may also be some more indicators like AC /
DC etc.
• Connection Ports − There are three or four ports available on the front of the
DMM. However, only two are needed at a time. Typical ports of the DMM are −
• Common − It is used with all measurements. The negative (black) probe is
connected to this.
• VΩmA Port − This port is used for the most measurements and positive
(red)probe is connected to it.
20. • 10A Port − It is used to measure the large currents in the circuits.
• Dial (Selection Knob) − There is a rotary switch to select the types of
measurement to be made and range that is needed.
• Additional Connections − There are some additional connections in
DMM for other measurements like temperature, transistor gains etc.
21. Steps for the use of digital multimeter −
Turn the meter ON.
Insert the probes into the correct connecting ports.
Set the dial (rotary switch) to the correct measurement type and range for
the measurement to be made.
While selecting the range, ensure that the maximum range is above than
that is expected.
Optimise the range for the best reading.
23. • There are two types of digits in digital display: Full digit and Half
Digit.
• A full digit is something which can take any value from 0 to 9.
• Thus it can have a total of 10 different states. So full digit may have
any value 0, 1, 2,….., or 9.
• A half digit is something which can either have a value of 0 or 1.
• This digit is basically the most significant digit and hence has limited
use for displaying any number or reading.
24. 3½DIGIT DISPLAY
• 3.5 digit displays have four digits: one half digit and three full digits.
• As half digit is the most significant digit and can either have value 0 or 1
• Full digit can take any value (0 to 9)
• The range of digital display will be 0 to 1999.
25. • Since negative sign can be displayed by the readout, the range will be -1999
to 1999.
• If we want to display 30 on 3.5 digit display, we need to throw out the half
digit. The three full digits will be used.
• Thus the value displayed will be 300.
• Now we need to place a decimal.
• Decimal point in digital display is just a dot symbol which is manually
displayed after appropriate number of segment to show the complete desired
number.
• This means that the displayed value will be 30.0
•
26. • 4½ DIGIT DISPLAY
• 4.5 digit display have five digits: 1 half digit and 4 full digit.
• The range of the display will be 0 to 19999.
• As negative sign can also be displayed by the instrument, the range
will be -19999 to 19999.
28. A digital frequency meter is an electronic instrument that can measure even
the smaller value of frequency of a sinusoidal wave and displays it on the
counter display.
It counts the frequency periodically and can measure in the range of
frequencies between 104 to 109 hertz
Components of digital frequency meter
Unknown Frequency Source: It is used to measure the unknown value of
input signal frequency.
Amplifier: It amplifies low-level signals to high-level signals.
Schmitt Trigger: The main purpose of the Schmitt trigger is to convert the
analog signal into a digital signal in a pulse train form. It is also known as ADC
and basically acts as a comparator circuit.
29. AND Gate: The generated output from AND gate is obtained only when the
inputs exist at the gate. One of the terminals of the AND gate is connected to
Schmitt Trigger output, and another terminal is connected to a flipflop.
Counter: It operates based on the clock period, which starts from “0”. One
input is taken from the output of the AND gate. The counter is constructed by
cascading many flip flops.
Crystal Oscillator: When a DC supply is given to a crystal oscillator (frequency
of 1MHz) it generates a sinusoidal wave.
30. Time-Based Selector:
Depending on reference the time period of signals can be varied.
It consists of a clock oscillator which gives an accurate value.
The clock oscillator output is given as input to Schmitt trigger which
converts sinusoidal wave into a series of a square wave of the same
frequency.
These continuous pulses are sent to frequency divider decade which is in
series that are connected one after the another, where each divider decade
consists of a counter decade and the frequency is divided by 10.
Each decade frequency divider provides respective output using a selector
switch
31. Working Principle
When an unknown frequency signal is applied to the meter it passes on to
amplifier which amplifies the weak signal.
Now the amplified signal is now applied to Schmitt trigger which can convert
input sinusoidal signal into a square wave.
The oscillator also generates sinusoidal waves at periodic intervals of time,
which is fed to Schmitt trigger.
This trigger converts sin wave into a square wave, which is in the form of
continuous pulses, where one pulse is equal to one positive and one
negative value of a single signal cycle.
The first pulse which is generated is given as input to the gate control flip
flop turning ON AND gate.
32. The output from this AND gate count decimal value.
Similarly, when the second pulse arrives, it disconnects AND gate, and when the
third pulse arrives the AND gate turns ON and the corresponding continuous
pulses for a precise time interval which is the decimal value is displayed on the
counter display.
The frequency of the unknown signal can be calculated by the following formula
F = N / t …………………..(1)
Where
F =frequency of the unknown signal
N = Number of counts displayed by the counter
t = time interval between the start-stop of the gate.
33. Advantages
The following are the advantages of digital frequency meter
Good frequency response
High sensitivity
The production cost is low.
Disadvantages
The following are the disadvantages
It does not measure the exact value.
Digital Frequency Meter Applications
The following are the applications
The equipment’s like radio can be tested using a digital frequency meter
It can measure parameters like pressure, strength, vibrations, etc.
