Measurements and Instrumentation

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Measurements and Instrumentation

What is meant by measurement?

Measurement is an act or the result of comparison between the quantity and a predefined standard.

Mention the basic requirements of measurement.

The standard used for comparison purpose must be accurately defined and should be commonly accepted.
The apparatus used and the method adopted must be provable.

What are the 2 methods for measurement?

Direct method and
Indirect method.

Explain the function of measurement system.

The measurement system consists of a transducing element which converts the quantity to be measured in an analogous form. the analogous signal is then processed by some intermediate means and is then fed to the end device which presents the results of the measurement.

Define Instrument.

Instrument is defined as a device for determining the value or magnitude of a quantity or variable.

List the types of instruments.

The 3 types of instruments are
Mechanical Instruments
Electrical Instruments and
Electronic Instruments.

Classify instruments.

Absolute Instruments
Secondary Instruments

What are the 2 modes of operation of secondary Instruments?

Analog mode and
Digital mode

What are analog devices?

Signals that vary in a continuous fashion and take on an infinity of values in any range are called analog signals. The devices that produces these signals are called analog devices

What are digital devices?

Signals that vary in discrete steps and thus take on only finite different values in a given range are called digital signals. The devices that produces these signals are called digital devices.

Mention the functions performed by the measurement system.

Indicating function
Recording function
Controlling function.

Give the applications of measurement systems.

The instruments and measurement systems are sued for
Monitoring of processes and operations.
Control of processes and operations.
Experimental engineering analysis.

List the functional elements of the measurement systems.

Primary sensing element.
Variable conversion element and
Data processing element.

What is signal conditioning?

The performing of non-linear processes like modulation, detection, sampling, filtering, chopping and clipping etc. on the signal to bring it to desired form is called signal conditioning.

What is data transmission element?

When the elements of an instrument are actually physically separated, it becomes necessary to transmit data from one to another. The element that performs this function is called a data transmission element.

Give the function of the variable manipulation element.

The function of the variable manipulation element is to manipulate the signal presented to it preserving the original nature of the signal.

What are the characteristics of the measurement system?

Static characteristics
Dynamic characteristics

Why calibration of instrument is important?

The calibration of all instruments is important since it affords the opportunity to check the instrument against a known standard and subsequently to errors in accuracy.

Explain the calibration procedure.

Calibration procedure involves a comparison of the particular instrument with either.

A primary standard
A secondary standard with a higher accuracy than the instrument to be calibrated or
An instrument of known accuracy.

What are the static characteristics?

Accuracy
Sensitivity
Reproducibility
Drift
Static error and
Dead zone.

Define true value

True value of a quantity to be measured is defined as the average of an infinite number of measured values when the average deviation to the various contributing factors tends to zero.

Define scale range.

The scale range of an instrument is defined as the difference between the largest and smallest reading of the instrument.

What is meant by reproducibility?

Reproducibility is the degree of closeness with which a given value may be repeatedly measured. It may be specified in turns of units for a given period of time.

Define repeatability.

Repeatability is defined as the variation of scale reading and is random in nature.

Define noise.

Noise is defined as any signal that does not convey any useful information.

What is accuracy?

Accuracy is the closeness with which an instrument reading approaches the true value of the quantity being measured.

What is precision?

Precision is measure of the reproducibility of the measurements, i.e. given a fixed value of a quantity; it is a measure of the degree of agreement within a group of measurements.

Mention the characteristics of precision.

Conformity and
Number of significant figures.

UNIT – II
ELECTRICAL & ELECTRONICS INSTRUMENTS

Mention the two main differences between a flux meter and a Ballistic Galvanometer.

Flux meter Ballistic Galvanometer

Controlling torque is very small controlling torque is high
Heavy electromagnetic damping electromagnetic damping is not heavy
Less sensitive more sensitive
Less accurate more accurate

What are the adjustments in energy meter?

Main speed adjustment
Power factor adjustment
Friction adjustment
Creep adjustment

Write about DC tests in magnetic measurements.

These are used to determine B-H curve and hysteresis loop of Ferro-magnetic materials. The direct current is used to have variable m.m.f. and flux meter is used to measure the flux density. A ballistic galvanometer can be used to measure flux density. Such tests are also called ballistic tests.

Give two differences between analog and digital type phase meters.

Analog type                                                       Digital Type
This is called balanced modulator method       this is called flip-flop method
Accuracy is very less                                         high as 0.1o can be achived
Less stable, sensitive & reliable                        high stable, sensitive and reliable
DC meter is used at the output                          display unit is used at the output

What is creeping?

Without any current through current coil, disc rotates due to the supply voltage exciting its pressure coil. This is called creeping.

Write some advantages of instrument transformer.

The normal range ammeters and voltmeters can be used along with these transformers to measure high currents and voltage.
The rating of low range meter can be fixed irrespective of the value of high voltage or current to be measured
These transformers isolate the measurement from high voltage and current circuits. This ensures safety of the operator and makes the handling of the equipments very easy and safe.
These can be used for operating many types of protecting devices such as relays or pilot lights.

Define the terms digitizing and digitizing period.

In the digital voltmeters, it is necessary to convert the analog signal to digital signal. Various techniques used to achieve this conversion. The circuits which are used to achieve such conversion are called digitizing circuits and the process is called digitizing. The time required for this conversion is called digitizing period.

What are the requirements of a shunt?

The temperature coefficient of shunt and the meter should be low and should be as equal as possible.
The shunt resistance should be stable and constant with time.
The shunt resistance should not carry currents which will cause excessive temperature rise.
The soldering joints should not cause a voltage drop.
The values of resistance should not change due to soldering.
The resistance should have low thermal electromotive force with copper.

Mention any two precautions to be taken while using a voltmeter.

It should be always connected across the circuit or component whose voltage is to be measured.
The polarities must be observed correctly.
First use the highest range and then decrease the voltage range until the sufficient deflection is obtained.
Take care of loading effect.

Mention the two main differences between an ammeter and a voltmeter.

Ammeter                                                                      Voltmeter
It is a current measuring device             it is a voltage measuring deveice
Always connected in series with circuit Always connected in parallel with circuit
The resistance is very small                   The resistance is very high

What are the adjustments in energy meter?

Main speed adjustment
Power factor adjustment
Friction adjustment
Creep adjustment

Write about AC tests in magnetic measurements.

When a ferro-magnetic material is subjected to cycle of magnetization and demagnetization then the eddy current and hystereis losses occur. Hence alternating current is used to determine iron losses, having provision of a variable frequency and form factor. Such tests are carried out at power, audio or radio frequencies.

Mention the errors in Moving iron instruments.

Hysteresis error
Temperature error
Stray magnetic field error
Frequency error
Eddy current error

How can we prevent creeping in energy meter?

Two holes are drilled in the aluminum disc 180o opposite to each other. When this holes comes under the shunt magnet pole, it gets acted upon by a torque opposite to the rotation.

Mention the purpose of Schmitt trigger in frequency meter.

To convert the amplified signal from sine wave to square wave with fast rise and fall times.

Define torque to weight ratio in electrodynamometer instrument.

To have reasonable deflecting torque, mmf of the MC must be enough. Thus mmf=NI, hence current through MC should be high or no. Of turns should be large. The current cannot be made high because it may cause excessive heating of spring. Large no. of N hence is the only option but it increases weight of the coil.

What are the requirements of a Multiplier?

Their resistance should not change with time
The change in their resistance with temperature should be small.
They should be non-inductively wound for a.c. meters.

Mention any two precautions to be taken while using an Ammeter.

It should never be connected across any source.
The polarity must be observed correctly.
First use the highest range and then decrease the voltage range until the sufficient deflection is obtained.

UNIT – III
SIGNAL CONDITIONING CIRCUITS

What are the characteristics of a DC amplifier?

It may need balanced differential inputs giving a high common mode rejection ratio (CMRR)
It should have an extremely good thermal and long term stability.

Enumerate the merits and demerits of a dc amplifier.

It is easy to calibrate at low frequencies.
It is able to recover from an overload condition unlike it’s AC counterpart.

Give the purpose of bridge circuits. What are the different type?

The bridge circuits are used in instrumentation systems for the measurement of resistance , inductance and capacitance.
Types:

DC type and
AC type.

What are the 2 types of wheatstone bridge?

Null type bridge
Deflection type bridge.

What are the different types of AC bridges?

AC bridge using push-pull transducers
AC bridge with push-pull inductive transducers
Inductive transducer Blumlein bridge
Capacitive transducer Blumlein bridge

Define slew rate

Slew rate is defined as the maximum output voltage change per unit time.

List the requirements of an instrumentation amplifier

Low drift
High i/p impedance
High linearity
High CMRR
High noise rejection capability

Give few applications of instrumentation amplifier.

The instrumentation amplifier finds increasing application in the amplification of the output signals obtained from thermocouples, strain gauge bridge and biological electrode.

What is a filter?

A filter is often a frequency selective circuit that passes a specified band of frequencies and blocks or attenuated signal of frequencies outside this band.

List the different types of filters.

Analog or digital filters
Passive or active filters
Audio (AF) or radio (RF) filters.

Specify the advantages of an active filter

Gain and frequency adjustment flexibility
No loading problem
Low cost

What is frequency scaling?

The procedure of converting a cutoff frequency to a new cutoff frequency is called frequency scaling.

What is quality factor?

The ratio of resonant frequency to bandwidth is known as the quality factor Q.

What is acquisition time of S/H circuit?

Acquisition time is the time required for the capacitor to charge up to the value of the input voltage after the switch is first started.

What is aperture time of S/H circuit?

The aperture time is the time required for the switch to change from ON state to OFF state.

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UNIT- IV

STORAGE AND DISPLAY DEVICES

List the components of a magnetic tape recorder.

Recording head
Magnetic tape
Reproducing head
Tape transport mechanism
Conditioning devices.

Mention the different methods of magnetic tape recording.

Direct recording
Frequency modulation (FM) recording and
Pulse duration modulation (PM) recording.

What is the purpose of erase head?

In the erase head, a signal of high frequency and level sweeps the magnetic tape thereby completely wiping out the information contained there in. this renders the magnetic tape to be used fresh for smother signal.

What is drop out?

In direct recording, some portions of the tape may not be perfectly recorded owing to dirt or poor manufacturer and this is called drop out.

Mention the 2 factors in FM recording

Percentage deviation and
Deviation ratio

What is the operation of a serial printer?

The serial printer produces a single character at a time, usually moving from left to right across a page. It prints 200 characters per second.

Mention the purpose of line printers.

The line printers are used to print and entire line simultaneously. It points 4000 lines per minute.

Give the operation of pare printers.

The page printer prints in a line at a time mode, but can be stopped and restarted only on the page basis. The top speed is 45,000 lines per minute.

List the classification of printers.

Impact and non-impact printers.
Fully formed character and dot matrix character printer.
Character at a time and a line at a time.

What is daisy wheel printer?

Daisy wheel printer is a fully formed character printer, designed for computer usage and has characters mounted on the periphery of a spinning print head similar to a daisy flow. They are capable of bi-directional printing.

Give short notes on dot-matrix printers.

In dot-matrix printers, the characters are formed by printing a group of dots to form a letter, no. or other symbols. It can print any combination of dots with all available print position in the matrix.

List the important features of CRTs.

Size
Phosphor
Operating voltages
Deflection voltages
Viewing screen .

What is meant by deflection sensitivity in CRT?

The deflection sensitivity of the CRT is usually stated as the DC voltage required for each cm of deflection of the spot on the screen.

What is meant by recurrent sweep in CRT?

When the saw tooth, being an AC voltage alternates rapidly, the display occurs respectively, so that a lasting image is seen by the eye. This repeated operation is known as recurrent sweep.

Mention the methods that are used for generating the 2 electron beams within the CRT.

The methods that are used for generating the 2 electron beams within the CRT are the double gun tube and split beam method.

Explain CRO and its function.

Cathode Ray Oscilloscope (CRO) is a very careful and versatile laboratory instrument used for display measurement and analysis of waveforms and other phenomena in electrical and electronic circuits. CRO is in fact a very fast X-Y plotter displaying an input signal versus another signal or time.

Name the components of a CRO.

CRO consists of a cathode ray tube (CRT) along with electron gun assembly, deflection plate assembly, fluorescent screen, glass envelope and base.

What is an electron gun?

The source of focused and accelerated electron beam is the electron gun. The electron gun which emits electrons and forms them into a beam consists of a heater, a cathode, a grid a pre-accelerating anode, a focusing anode and an accelerating anode.

Name the basic circuitry of CRO.

Vertical (Y) deflection system
Horizontal (X) deflection system
Synchronization
Blanking circuit
Intensity (z-axis) modulation
Positioning controls
Focus control
Intensity control
Calibration control
Astigmatism.

What is LED?

The LED is basically a semiconductor PN junction diode capable of emitting electromagnetic radiation under forward conductions.

Compare common anode and common cathode displays.

Common anode type display require an active low (or current linking) configuration for code converter circuitry, whereas an active high output circuit is necessary for common cathode LED type display.

List the characteristics of LCD.

Light scattering
Can operate in reflective or Transmissive configuration.
Do not actively generate light and depend for their operation on ambient or back light.

On what does the operation of LCD depend?

The utilization of a class of organic materials which remain a regular crystal like structure even when they have melted.

Name the 2 commonly available types of LCDs.

Dynamic scattering and
Field effect type.

What is the purpose of dot matrix displays?

Excellent alphanumeric characters can be displayed by using dot matrix LEDs with an LED at each dot location.

What are the commonly used dot matrices for the display of characters?

The commonly used dot matrices for the display of prominent characters are 5 x 7, 5 x 8 and 7 x 9

What are the 2 writing patterns of dot matrix displays?

Common anode or common cathode connection (uneconomical)
X-Y array connection (economical and can be extended vertically or horizontally using a minimum number of wires)

UNIT – V
TRANSDUCERS

Define transducers.

Transducers are defined as a device which when actuated, transforms energy from one form to another. Generally, any physical parameters is converted into electrical form.

What are the 2 types of transducers?

Electrical and
Mechanical

Name the parameters that dictate the transducer capability

Linearity
Repeatability
Resolution and
Reliability

Define sensitivity

Sensitivity is defines as the electrical output per unit change in the physical parameter. High sensitivity is generally desirable for a transducer.

Classify electrical transducers.

Active
Passive

Name the 2 parts of a transducer

Sensing element
Transduction element

Mention the factors considered while selecting a transducer.

Operating range
Sensitivity
Frequency response & resonant frequency
Environmental compatibility
Minimum sensitivity
Accuracy
Usage and ruggedness
Electrical parameters

What is meant by POT?

POT is a resistive potentiometer used for the purpose of voltage division. It consists for a resistive element provided with a sliding contact called as wiper.

Explain the working principle of a strain gauge.

Strain gauge works on the principal that the resistance of a conductor or a semiconductor changes when strained. This property can be used for measurement of displacement, force and pressure.

Name the different types of strain gauges.

Un-bonded metal strain gauge
Bonded metal wire strain gauge
Bonded metal foil strain gauge
Vacuum deposited thin metal film strain gauge
Sputter deposited thin metal strain gauge
Bonded semiconductor strain gauge
Diffused metal strain gauge

What is meant by rosette?

It is a combination of strain gauges to a single element strain gauge.

Give the 3 working principles of variable inductance transducers

Change of self inductance
Change of mutual inductance
Production of eddy currents.

Write notes on LVDT

It is the linear variable differential transformer which is used to translate the linear motion into electrical signals. It consists of a single primary winding and 2 secondary winding.

List the advantages of LVDT

High range of displacement measurement
Friction & electrical isolation
Immunity from external effects
High I/p and high sensitivity
Ruggedness
Low hysterisis & low power consumption.

What is a digitizer?

It is a digital encoding transducer that enables a linear or rotary displacement to be directly converted into digital form without intermediate forms of analog to digital (A/D) conversion.

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