II/IV B.Tech Regular Degree Examinations, April 2018

Scheme of Evaluation

Subject code:- 14EC406

Subject Name:- Basic Instrumentation

Faculty: Head of Department

Sk. Idrish Dr. B. Chandra Mohan

Assistant Professor Professor & Head

9866600354

BAPATLA ENGINEERING COLLEGE:: BAPATLA

Department of Electronics and Communication Engineering

II/IV B.Tech Regular Degree Examination

Scheme of Evaluation

Basic Instrumentation (14EC406)

April 2018 Fourth Semester Max Marks:60 Marks

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Answer Question No.1 Compulsorily. (1X12=12M)

Answer one Question from each unit. (4X12=48M)

1. Answer all Questions 1X12=12M

a) Precision : A measure of the reproducibility of the measurements i.e given a fixed value

of a variable.

b) Random errors are due to unknown causes and occur even when all systematic errors

have been accounted for. In well designed experiments, a few random errors usually

occur, but they become important in high accuracy work.

c) Gaussian distribution (also known as normal distribution) is a bell-shaped curve, and it is

assumed that during any measurement values will follow a normal distribution with an

equal number of measurements above and below the mean value.

d) Wein bridge is most commonly used for measurement of audio frequencies.

e) Wagner ground connection is a bridge circuit used to eliminate the effect of stray

capacitance.

f) R1 R4= R2 R3 is the equation for the bridge to be balanced.

g) i) The attenuator probe often called a 10-to-1 probe, provides approximately a 10-to-1

reduction in input capacitance bt also provides reduction in overall oscilloscope

sensitivity.ii) Another useful probe is active probe which achieves a lower capacitance

without attenuation associated with 10-to-1 probe.

h) Specifications Oscilloscope (CRO)

horizontal Axis, vertical Axis, Sweep Time Circuit, trigering, Intensity Modulation,

Power Source.

i) A flood gun is an electromechanical device that provides a steady flow of low-energy

electrons to a desired target or "flood area." Typically, the target is an area on an

insulator or semiconductor where another "writing gun" has just left a net

positivecharge.

j) Resistance Temperature Detector (RTD), as the name implies, is a sensor used to

measure temperature by correlating the resistance with temperature. pt100 is most

commonly used RTD.

k) stress is a type of pressure given by force acting per unit area. Strain is the ratio of

change in length and total length when stress occurs. Ratio of stress and strain gives

Young's Modulus.

l) When a mechanical compression or tension is applied to some crystals like quartz, a

potential difference is developed across the crystal which is proportional to the applied

pressure. This phenomenon is known as Piezo-electric Effect.

UNIT I

2)a) Explain the procedure to extend the range of ammemeter 3M+3M=6M

Any series type or shunt type

(OR)

3)a) PMMC Instrument:-

3)b) Shunt type ohmmeter:-

The basic circuit of the shunt-type ohmmeter is shown where movement mechanism is

connected parallel to the unknown resistance. In this circuit it is necessary to use a switch,

otherwise current will always flow in the movement mechanism.

Resistor Rsh is used to bypass excess current. Let the switch be closed. When RX = 0 (short

circuit), the pointer reads zero because full current flows through Rx and no current flows

through the meter and Rsh.

Therefore, zero current reading is marked 0 ohms. When RX = ∞ (open circuit), no current flows

through RX. Resistor R1 is adjusted so that full-scale current flows through the meter. Therefore,

maximum current reading is marked ∞ ohms.

Analysis : When RX = ∞ the full scale meter current will be Ifsd=E/(R1+Rm)

For any value of Rx connected across the meter terminals,the meter current decreases and is

given by Im={E/R1+[RmRx/(Rm+Rx)]}* Rx/(Rm+Rx)

The meter current for any value of Rx,expressed as a fraction of the full scale current is

S=Im/Ifsd= Rx*(R1+Rm)/[ Rx*(R1+Rm)+R1Rm]

Substituting R1Rm/R1+Rm=Rp

S=Rx/Rx+Rp

At halfscale reading Im=0.5Ifsd

0.5Ifsd=ERh/ [R1Rm+Rh(R1+Rm)]

Where Rh is external resistance causing half scale deflection = R1Rm/ (R1+Rm)

The analysis shows that the half scale resistance is determined by limiting resistor R1 and the

internal resistance of the movement Rm

UNIT-II

4)a) Wheatstones Bridge:-

A Wheatstone bridge is an electrical circuit used to calculate an unknown resistance with the

help of a bridge circuit. For this, the two legs of the bridge circuit are kept balanced and one leg

of it includes the unknown resistance. The Wheatstone bridge principle is similar to the working

of potentiometer.

4)b) Maxwells Bridge:-

It is used to measure unknown inductance in terms of known capacitance

Under balanced condition :

Z1Z4=Z2Z3

Zx=Z2Z3Y1

(Rx+jwLx)=R2R3(1/R1+jwC1)\

Equating real and imaginary terms

RX= R2 R3/R1 Lx=R2 R3C1

(OR)

5)a) Comparison of AC and DC bridges:-

DC & AC Bridge are used to measure resistance, inductance, capacitance and

impedance.

Operate on a null indication principle. This means the indication is independent of the

calibration of the indicating device or any characteristics of it.

Very high degrees of accuracy can be achieved using the bridges .

The differences between a DC bridge and an AC bridge are:

DC bridge

1. DC supply is used. 2. A DC current detector is used. 3. Only resistive components are used. 4. There is no need for Wegner's earthing device. 5. Balancing time is relatively less.

AC bridge

1. AC supply is used. 2. AC current detector is used. 3. Resistive and reactive components are used. 4. Wagner's earthing device is required. 5. Balancing time is high because AC supply is used.

5)b)Successive Approximation type DVM:-

UNIT-III

6)a) Cathode Ray Oscilloscope(CRO):-

In the electron gun of the CRT, electrons are emitted, converted into a sharp beam and

focused upon the fluorescent screen. The signal to be viewed is fed to a vertical amplifier,which

increases the potential of the input signal to a level that will provide a usable deflection of the

electron beam.

To synchronize the horizontal deflection with vertical input a synchronizing or triggering circuit

is used.This circuit is the link between the vertical input and the horizontal time base.

Oscilloscope techniques

Determining frequency

Frequency=1/period

The period is measured between any two positive or any two negative peaks.

Phase angle and time delay measurement

A very simple and effective method is to display the two sine functions as two separate traces on

a dual trace oscilloscope and measure the time delay between the two sine functions.

6)b) Dual Trace CRO:-

The comparison of two or more voltages is very much ,necessary in the analysis and

study of many electronic circuits and systems. This is possible by using more than one

oscilloscope but in such a case it is difficult to trigger the sweep of each oscilloscope precisely at

the same time.

A common and less costly method to solve this problem is to use dual trace or multitrace

oscilloscopes. In this method, the same electron beam is used to generate two traces which can

be deflected from two independent vertical sources. The methods are used to generate two

independent traces which the alternate sweep method and other is chop method.

There are two separate vertical input channels A and B. A separate preamplifier and -

attenuator stage exists for each channel. Hence amplitude of each input can be individually

controlled. After preamplifier stage, both the signals are fed to an electronic switch. The switch

has an ability to pass one channel at a time via delay line to the vertical amplifier.

The time base circuit uses a trigger selector switch 52 which allows the circuit to be

triggered on either A or B channel, on line frequency or on an external signal. The horizontal

amplifier is fed from the sweep generator or the B channel via switch 5! and 51. The X-Y mode

means, the oscilloscope operates from channel A as the vertical signal and the channel B as the

horizontal signal. Thus in this mode very accurate X-Y measurements can be done.

(OR)

7)A) Sampling Oscilloscope:-

The sampling oscilloscope is an advanced version of the digital oscilloscope with some

added features and uses for the special purpose. It is designed to provide a very high-frequency

function by sampling several wave forms successively. Such oscilloscope uses sampling theorem

for fabricating waveform from several input signals. By using strobe light, the fraction of the

motion could be seen, but when the bunch of images are taken, a very fast mechanical motion is

observed. Sampling oscilloscope functions similar to stroboscopic technique and it is used to

observe very fast electrical signals. Approximately, 1000 points are required to create waveform.

Functioning of Sampling Oscilloscope

As it name suggests, it collects samples from several successive waveform and construct a

complete picture of the waveform from the assembled data. The resultant waveform is amplified

with low band pass filter and then shown on the screen. This waveform is made with the joining

of many dots associated with each other to construct the whole shape. Each dot of the wave is the

vertical deflection of the point of the progressive layer in each successive cycle of a staircase

waveform. They are used to monitor high-frequency signals up to 50 GHz or more. The

frequency of the displayed waveform is higher than the sample rate of the scope. It is about 10

pieces per division or more along with large bandwidth of amplifier about 15 GHz. At sampling

stage, signals have low-frequency and to achieve large band-width it combines with an

attenuator.

Storage Oscilloscope:-

In the conventional CRT the persistence of the phosphor ranges from few milliseconds to

several seconds so the event happens will disappear from the screen after a relatively short

period of time.A storage CRT can retain the display much longer .These are mainly classified

into two types

(i)Bistable tubes that produces only one level of image brightness

(ii) Halftone tube that can retain an image for varying lengths of time and at different levels of

image brightness.

When a target is bombarded by a stream of primary electrons,an energy transfer takes place

which seperates other electrons from the surface of the target known as secondary emission.

Secondary emission ratio is defined as the ratio of secondary emission current and primary beam

current.

In this digital storage oscilloscope, the waveform to be stored is digitised ,and then stored

in a digital memory. The conventional cathode ray tube is used in this oscilloscope hence the

cost is less. The power to be applied to memory is small and can be supplied by small battery.

Due to this the stored image can be displayed indefinitely as long ,15 power is supplied to

memory. Once the waveform is digitised then it can be further loaded into thecomputer and can

be ana lysed in detail

7)b) Operation of a Frequency Counter:-

a)The frequency counter operates on the principle of gating the input frequency into the counter

for a predetermined time.

If an unknown frequency were gated into the counter for an exact 1 second,the number of counts

allowed into the counter is the frequency of the input.The term gated means using AND or OR

gate is used to allow the input into the counter to be accumulated.

It is necessary to count these pulses and display the result.If the gate is open for exactly 1s,the

count accumulated is equal to the average frequency of the unknown input in hertz(Hz)

UNIT-IV

8)a) Potentiometric Transducers:-

A potentiometric transducer is a mechanically driven variable resistor. It consists of a

wire-wound fixed resistor and a wiper arm that slides over it and in so doing taps a different

segment of the resistor, as shown diagrammatically in Fig. 5.8a and b, where K represents a

fraction of the resistor that is tapped. The displacement to be measured is linked by a shaft to the

wiper arm, and a measure of the displacement is the fractional resistance KR or the fractional

voltage KV. This is the transduction mechanism. The resolution that one can achieve with this

transducer depends on the gage of the nickel alloy or platinum wire used. For extremely fine

resolution, the wire is replaced by a metallized ceramic or a film resistor.

If the resistance wire is wound on a doughnut-shaped tube, the wiper will measure

angular displacements. The output voltage corresponding to a displacement, force, or pressure is

a fraction of the external voltage V, and therefore it does not need any amplification to activate

external circuitry.

8)b) Difference between active and passive Transducers:-

(OR)

9)a)Thermistor:-

A thermistor is a specialized resistor, intentionally designed to be thermally sensitive and

its primary characteristic is its ability to alter its electrical resistance in response to changes in

case temperature. It can be used to measure temperature, or to sense temperature changes and

compensate for the temperature changes. Thermistor resistance is a function of its absolute

temperature. Thermistors are normally available with accuracy up to ±1o C, however, higher

accuracy devices are available, but are substantially more expensive. A time constant

characteristic is also specified to signify the response rate to a temperature change (i.e., speed of

the thermistor) and is usually expressed in seconds, defined as the time required to change 63.2%

of the total difference between initial and final body temperature, when subjected to a step

function change in temperature, under zero-power conditions.

Characteristics of Thermistors

The relationship governing the characteristics of thermistors is given below as-:

R1 = resistance of thermistor at absolute temperature; T1oK.

R2 = resistance of thermistor at temperature T2oK.

β = constant depending upon material of transducer.

From the above equation it can concluded that relationship between temperature and resistance is

highly non linear. A thermistor exhibits a negative thermal resistance temperature coefficient of

bout 0.05/oC

Advantages of Thermistors:-

Thermistors are well known for their small size and low cost. When compared to thermocouples,

thermistors have a few advantages;

They are more sensitive than other temperature sensors

High sensitivity allows them to work well over a small temperature range

They’re low cost and therefore cheap to replace.

They provide a fast response

Easy to use

Small in size so they can fit into the smallest of spaces.

Options for customisation

Standard two wire connection system means they are compatible with many devices

Easily interfaced to electronic instrumentation

Thermistor Applications:

PTC thermistors were used as timers in the degaussing coil circuit of most CRT displays. A

degaussing circuit using a

PTC thermistor is simple, reliable (for its simplicity), and inexpensive. We can also use PTC

thermistors as heater in automotive industry to provide additional heat inside cabin with diesel

engine or to heat diesel in cold climatic conditions before engine injection. We can use PTC

thermistors as current-limiting devices for circuit protection, as replacements for fuses

. We can also use NTC thermistors to monitor the temperature of an incubator

. Thermistors are also commonly used in modern digital thermostats and to monitor the

temperature of battery packs while

charging. We regularly use NTC thermistors in automotive applications.

NTC thermistors are used in the Food Handling and Processing industry, especially for food

storage systems and food

preparation. Maintaining the correct temperature is critical to prevent food borne illness. NTC

thermistors are used throughout the Consumer Appliance industry for measuring temperature.

Toasters, coffee

makers, refrigerators, freezers, hair dryers, etc. all rely on thermistors for proper temperature

control. We can regularly use the Thermistors in the hot ends of 3D printers; they monitor the

heat produced and allow the

printer’s control circuitry to keep a constant temperature for melting the plastic filament. NTC

thermistors are used as resistance thermometers in low-temperature measurements of the order

of 10 K.

NTC thermistors can be used as inrush-current limiting devices in power supply circuits.

9)b)Digital Data Acquisition system:-

Explanation

Here, the transducer translates physical parameters to electrical signals acceptable by the

acquisition system. The physical parameters include temperature, pressure, acceleration, weight,

displacement, velocity etc. Electrical quantities such as voltage, stance, and frequency may be

measured directly.

The signal conditioner includes the supporting circuitry for the transducer. This circuit

may provide excitation power, balancing circuits and calibration elements and an example of this

is a strain-gauge bridge lance and power supply unit

The scanner or multiplexer accepts multiple analog inputs and sequentially connects them

to one measuring instrument.

The signal converter translates the analog signal to a form acceptable by the analog to

digital converter like an amp1ifier used for amplifying low-level voltages generated by

thermocoup1es or strain gauges. The analog to digital converter (ADC) converts the analog

voltage to its equivalent digital form. The output of the ADC may displayed visually and is also

available as voltage outputs indiscrete steps for further processing or recording on a digital

recorder. The auxiliary section contains instruments for system programming and digital data

processing such as linearizing and limit comparison. These functions may be performed by

individual instruments or by a digital computer.

The digital recorder records digital information on punched cards, perforated paper tape,

magnetic tape, typewritten pages or a combination of these systems. Digital recorder may be

preceded by a coupling unit that translates the digital information to the proper form for entry

into particular digital recorder selected.

Prepared by

Sk.Idrish

Assistant Professor

Department of ECE

9866600354