Measurement System Practical

Measurement System Experiment - 1 DEAD WEIGHT PRESSURE GAUGE TESTER AIM - CALIBRATION OF PRESSURE GAUGE (BOURDON TUBE)

The dead weight free piston gauge has been used for precise determination of steady pressures for nearly eighty years. The gauge (Fig 1.1) consists of accurately machined piston of known weight, which is inserted in to close fitting cylinder, both of known cross sectional area. A number of masses of known weight are first loaded on one end of free piston and fluid pressure is then applied to the other end until enough force is developed to lift the piston - weight combination. When the piston is floating fully within the cylinder the piston is in equilibrium with the system pressure. Therefore P(dead weight pressure) = Fc\Ac. Where Fe - is the equivalent force of the piston Weight combination and Ac is the equivalent area of the piston cylinder combination.

Chamber filled with oil. Fig 1.1 Observation Table Equivalent pressure of weight 1kgf/cm2

Reading of pressure gauge Test 1kg/cm2

2kgf/cm2

2kg/cm2

3kgf/cm2

3kg/cm2

5kgf/cm2

5kg/cm2

10kgf/cm2

10 kg/cm2

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Measurement System Experiment No.2 AIM :- LEVEL MEASUREMENT WITH CAPACITANCE GAUGE.

A capacitance transducer can be used to measure the level of liquid in a tank. Fig.(2.1) shows a schematic arrangement of a suitable system. A metal electrode is placed inside the tank and insulated from it. The tank itself is earthed and forms one of the plates. The transducer therefore consist of two concentric metal cylinders and a change in liquid level alters the dielectric constant and hence capacitance.

The capacitance transducer is connected to electronic evaluator.

Capacitance transducer can be used to measure levels from a few millimeters to hundreds of meters. The method may be used for corrosive liquids provided a suitable metal electrode is used i.e. stainless steel.

Cast aluminium housing 1 1/2 BSP Rod Full insulation (b)

(a) Electronic insert

Probe

Digital converter

Digital output

power supply

Silo Fig 2.1

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Measurement System The meter is set at 0 to 100% of level for certain level of water. Observation: Level in cm

Meter Reading %

1.

80cm

79.9

2.

75cm

74.5

3.

70 cm

68.5

4.

65 cm

66

5.

60 cm

58.5

6.

50 cm

48.5

Salient points:1. Remote sensing is possible 2. 0 & 100 % of level can be adjusted for particular water level in the tank. 3. 0-5V output corresponding to 0 - 100 % level change is available for connecting to Blind limit controller (electronic) to obtain alarm/control function. 4. Can be used for corrosive liquids as probe is fully sealed.

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Measurement System Experiment No.3 AIM :- CALIBRATION OF RESISTANCE WIEN STRAIN GAUGE METER FOR LOAD AND STRAIN

Resistance wien strain gauges (two) of 120Ω are used on the overhang beam as shown in fig 3.1. 17 cm S 41

b=3cm t = 0.5cm cross section x-x

S 42

3 wiens ( one common)

X junction box

123

Load ( p) pan weights Fig. 3.1 wien strain gauge 120Ω gauge F=2 Front Panel controls R Output

B

0 5

Fine Bal.

5

Course Bal.

R 1 B 4

B 2

R

R 3 Connect to Junction Box

meter Input

B

Strain Gauge bridge output connected to meter input

Fig. 3.2 Input - Terminals 1,2,3 to be connected to terminals 1,2,3 on strain gauge transducer, Bridge is completed by internally connected resistance across terminals 1 and 4, 4 and 3 Fig 3.2 Front panel is shown in the figure 3.2, which shows various connections and meter. A wheat stone bridge and block diagram of electronic circuit is shown is fig 33.

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Measurement System 1 120 Ω

Potentiometer for Bridge Balance 2.2k

SG

stable D.C.

excitation

4

2

Ampl.

120Ω

R Output to meter

1k SG

2.2k

3

B 3 2 1

Fig 3.3

Operating instructions: 1. Ensure all connections and turn on 2. Balance instruments 3. Select position (3 - lowest, 2 - medium, 1 - high) for sensitivity of meter. 4. Now apply a gentle pressure to the cantilever beam. The meter pointer will deflect right or left according to direction of pressure. 5. Take readings Observation: Sr. No. 1 2 3 4 5

Applied load P

Meter Reading Kg 1 kg 1.5 kg 2 2.5 3

1 1.5 2 2.5 3

Calculated Strain = G = p x17 1/6x3x0.52 2x106

Cm 3 0.5 0.6 0.9 1.1

6.8x10-5 6x10-6 2.12x10-6 1.7x10-4 2.04x10-4

Plot two graphs one for (p) load - kg reading and another for strain (G)-Cm readings Remarks - Meter reading in kg should be corrected as shown in graph for load Kg scale reading. Another graph is also plotted for Strain - cm readings. For measurement of strain Load Vs Kg reading 3.5

Applied Load

3 2.5 2 1.5 1 0.5 0 1

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2

Kg

3

5

4

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Measurement System Experiment No - 4 AIM: - MEASUREMENT OF LINEAR DEFLECTION BY LINEARLY VARIABLE DIFFERENTIAL TRANSFORMER L V D T

Ferrous Rod

-ve

+ve

54321012345

secondary coil

primary coil

scale

(Fig 4.1) L.V.D.T. transducer is as shown in fig 4.1 Block diagram of electronic circuit used for display of output of L V D T is as shown in fig 4.2 Signal generator AMP primary ferrous rod - ve X

+ve X

+ phase detector

AMP

R AMP _

B

output to meter

Secondaries

3

2 1

( Fig. 4.2)

range selector

Front panel of instrument is as shown in fig 4.3 Output of LVDT

R

B Bal.

R

Primary

0 5

5

B R

R Secondary

B

Connection for LVDT

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B Meter input terminals

Connected to meter Fig 4.3

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Measurement System Circuit Operation - The primary winding of L V D T is excited by means of 0.3 KHz power source. The Wien Bridge Oscillator circuit generates a suitable a.c. excitation at fixed frequency. The output of this signal generator amplified and fed to primary coil of L V D T. The output from secondaries is amplified and converted in to perfect square wave form. Input and output signals are compared for their phase difference. This difference signal obtained from phase detector is rectified and amplified and fed to meter to obtain deflection. (Readings). It is possible to display the core displacements in the range of 0±5 mm, 0to 10 mm and 0 to ±20 mm. Operating Instructions: 1. Ensure connections 2. Keep range selector switch 3 (lowest sensitivity) and then select proper position of selector 3. Turn on and obtain readings Sr.No. 1. 2. 3. 4. 5. 6. 7.

Input +x displacement 5 5.5 10 13 15 17 20

Meter reading 4.5 5.6 10.2 13.1 18.2 17.1 26.2

Input displacement -vc x -5.9 -5.5 -10 -13 -15 -17 -20

Plot a graph +x & -ve x Vs deflection of meter

Meter Reading -4.9 -5.9 -9.8 12.8 14.9 16.8 19.3

Remark : - Study the graph.

Angular Displacement Vs Meter reading 30

Meter reading

20 10 0 -25

-20

-15

-10

-5

0

5

10

15

20

25

-10 -20 -30 Displacement

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Measurement System Experiment No.5 AIM: - ANGULAR MEASUREMENT BY CAPACITIVE DISPLACEMENT.

The capacitive transducer is based on the principle of variation of effective area of the conductors, other parameter i.e. separation distance and dielectric strength being kept constant. A two ganged condenser normally used in radio receivers is used for creation of variable capacitance. Electronic block diagram for capacitive transducer set up is as shown in fig 5.1 Two ganged Condenser

Wien bridge

Amplifier

Schmidt Trigger

Mono Stable

Meter Bucking voltage

(a) Meter R

movable plates

B

Max. Fixed plates Min. R Input

B Ganged condenser (c)

(b) Fig. 5.1

Circuit operation :- The basis of the angular displacement measurement with the help of capacitive transducer is frequency modulation system. The two sets of identical condensers of the ganged condenser form a part of wien bridge oscillator for which frequency f = 1 So as C is varied typically between (550 PR to 50 pF) 2 πRC a frequency variation in the range 1:10 is obtained. The block diagram for electronic circuit is as shown in fig 5.1. The meter circuit is connected to special bucking circuit so that for zero angular displacement meter circuit can be adjusted to zero

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Measurement System Operating instructions 1. Ensure connections Turn on 2. Obtain readings Observations: Sr.No

Angular displacement

Meter Reading

Volt(v)

1. 2. 3. 4. 5. 6. 7. 8. 9.

90 100 110 120 130 140 150 160 170

0 6 11 17 24 29 36 44 51

0 0.3 0.6 0.9 1.3 1.6 2.0 2.4 2.8

Plot a graph Inputs displacement in degrees to output in degrees on meter

Angular Displacement Vs meter reading 180 Angular displacement

160 140 120 100 80 60 40 20 0 0

10

20

30 Meter reading

40

50

60

Remark - Study graph.

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Measurement System Experiment No.6 AIM :- DISPLACEMENT MEASUREMENT BY INDUCTIVE PICK-UP. The inductance is determined by number of turns, geometrical con figuration and effective permeability and variation in any one of there usually caused by displacement, alters the inductance L by ∆L. Circuit operation - The inductive pick has three distinct stations as shown in fig 6.1

1) bridge 2) Excitation Source 3) Bridge output amplifier and indication Input displacement Stable A.C. Excitation (wienbridge oscillation)

Induction Bridge

A.C. amplifier & Rectifier

D.C. amp.

output to meter

Balance Network

transformer

Fig 6.1

Block diagram of Inductive pickup.

A sinusoidal excitation of 1 Kh3 is obtained in wien bridge oscillator. The output of excitation is impressed cuross the transformer primary and secondary of transformer excites inductive pick. Two arm of bridge are inductive and two arms are resistive as shown in fig 6.2 O/P of Wien bridge

active L1 L2

to Amp. rectifier & meter

Fig 6.2 From panel for connection is as shown in fig 6.3.

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Measurement System Output R

B

Max. Min. R B Meter input Input

Fig 6.3

Input - connect single way threaded socket with cable from inductive pick up. Operation instructions: i)

Ensure connections and turn on

ii)

Obtain reading

Sr.No.

1 2 3 4 5 6

Input Displacement

5 10 15 -5 -10 -15

Meter Reading

4.5 9.8 15 -5.1 -10.2 -15.9

Increasing Increasing Increasing Decreasing Decreasing Decreasing

Plot a graph displacement Vs meter reading Remark - Study graph

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Measurement System Experiment No.7 AIM: - MEASUREMENT OF SPEED BY VARIABLE RELUCTANCE METHOD.

The basis for variable reluctance transducer is an electromagnetic circuit whose reluctance varies as the shaft rotates - because of periodic changes in air gap. Variation in reluctance causes variation in flux, which in turn causes induced emf. In the output coil. The output voltage is fairly sinusoidal and peak to peak value is proportional to shaft speed ( n rev/min ). Gear teeth=20 Photo source Piezoelectric pickup Magnetic Pick up To front panel control Fig. 7.1

In this set up, a toothed gear wheel is mounted on the motor shaft. The no. of teeth of the gear is 20. The gear wheel is of ferromagnetic material. The pick up consists of a coil wound around - a permanent magnet. The magnetic field surrounding the coil is distorted by the passing of a tooth causing a pulse of out put voltage in the coil. The r.m.s. Value of output voltage increases with reduction in clearance between rotor and pick up, with an increase in tooth size, with an increase in rotor speed. The frequency of the output pulses is dependent on no. of teeth and the rotor speed. For this set up when motor runs at 1500 rpm or 25 rps are produced and hence number of pulses is 500 per second. Thus pick up works as a selfgenerating and variable frequency generator transducer Circuit Description - The circuit includes 2 stages of A.C. amplification 1 giving a very high overall gain. The resultant output is fed to Schmitt trigger circuit. The Schmitt trigger in turn trigger circuit of the monostable which generator instant width, constant height pulses. There variable frequency pulses are given to the meter for final indication. When the motor is running at 1500 rpm. The pick up produces 500 pulses/s. A separate signal generator with a stable frequency of 500 H3 is provided, Called a calibration source. When the output of this calibration is connected to

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Measurement System input of the amplifier stage, and by using the potentiometer marked Max, when F.S.D. of meter is adjusted then the set up is said to be calibrated for 1500 rpm. Now meter directly reads rpm of the motor. Electronic circuit is shown in fig 7.2 1.magnetic 2. Calibration Source 3. Photo electric picks up. Output

1520 rpm

R

B

Max. photoelectric pickup switch R magnetic pickup

B

meter input

1 A.C. amp.

2

Schmidt trigger

mono stable

O/P to meter circuit

3 1) Magnetic

2) Calibration Source

3) Photoelectric Pickup

Fig 7.2

Operating instruction 1) Ensure connections and turn on 2) Connect magnetic pick up and Adjust 3) Take readings Sr.No.

True reading

Meter reading

1

80

150

2

190

300

3

250

450

4

840

600

5

870

900

Plot a graph of actual rpm Vs meter reading. Remark - Study graph

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Measurement System Experiment No.8 AIM: - MEASUREMENT OF SPEED BY PHOTOELECTRIC PICK UP. The principle of measurement is based on photoelectric effect. The set up is designed to produce pulses proportional to rpm of shaft using phototransistor as a sensing element. A disc with 20 holes is mounted on the motor shaft and when photo - transistor and light source are properly fitted every passage of a hole across them produces a voltage pulse of high amplitude. Circuit description - The circuit includes 2 stages of A.C. amplification : giving very high overall gain. The resultant output is fed to schemitt trigger circuit - The Schmitt trigger in turn trigger circuit of the monostable, which generates constant width constant height pulses. These variable frequency pulses are given to the meter for final incation as shown in fig 8.1 R

Output

B

Max Photoelectric

pickup Switch Magnetic pickup Input (a)

Switch - To change from Magnetic to photoelectric and vise versa Max - Pot to adjust up to 1500 rpm 1 A.C. amp.

2

Schmidt trigger

mono stable

O/P to meter circuit

3 1) Magnetic

2) Calibration Source

3) Photoelectric Pickup

Fig 8.1 (b) When the motor is running at 1500 rpm, the pick-ups produce 500 pulses per second.

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Measurement System No 2 in fig 8.1 is a Calibration source of constant frequency 500H2 and FSD adjusted according to this source and then meter is said to be Calibration for 1500 rpm. Operating Instructions - (1) Ensure connection and put on (ii) Take readings Sr.No

Meter

Meter Reading of rpm

1 2 3 4 5

80 190 250 850 870

100 200 260 700 900

Plot a graph

Meter reading of rpm

Meter Vs Meter reading of rpm 1000 900 800 700 600 500 400 300 200 100 0 0

200

400

Meter

600

800

1000

Remark - Discuss graph

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Measurement System Experiment No.9 AIM: - TEMPERATURE MEASUREMENT BY A THERMOCOUPLE Thermocouple, transducer based on See back effect, is the most commonly and widely used single device for temperature measurement in industrial applications, for the range 00 - 40000F. (0-22000C.).

Thermocouple is a self-

generating transducer and basically is a pair of dissimilar metallic conductor joined so as to produce an emf.

When three junctions are at different temperature.

Magnitude of emf depends upon magnitude of temperature difference and materials of conductors.

Combinations used for base metal thermocouple are copper

constant. (-1500C - 3150C), Iron constant (-1500C - 8.600C), Chromal - Alumal etc. Thermocouples are low in cost, reliable in service, are easily used cover wide range of temperature measurement and have very good time response Characteristics (because of low thermal mass). But they care not perfectly linear over entire range, require cold junction compensation if ice-bath is to be avoided. Circuit description - Thermocouple output is connected to the non -inverting terminal of the operation amplifier and gain of the amplifier is seen to be 10. To the inverting terminal of operational amplifier output from a wheat stone bridge is supplied (Fig 9-1). This bridge is excited from highly stabilised D.C. supply. As the combient temperature goes changing the changing the R.T.D`s resistance also changes and small output is fed to the inverting input to the amplifier, with higher ambient temperature, thermocouples transducer tends to produce lower output voltage.

The R.T.D. Bridge thus compensates for ambient temperature and

climinates requirement of cold junction. The output of bridge is connected to the meter.

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Measurement System R Output

B

0

Max

S

S

Min. R

B

R

Input

Min.-- to adjust Ice bath temp Max -- is used to Adjust boiling water temp.

B meter input

(a) Control panel front RTP

+ ve

max. R

O.P Amp.

to meter

Compensation bridge

B - ve

Thermo-couple Input

(b) Fig.. 9.1

Operating Instruction - (1) Ensure connection and start (2) Take readings Observation Sr.No. 1 2 3 4 5

Actual Temp.

Meter reading

Plot a graph Remark - Discuss the graph

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Measurement System

Experiment No.10

Measurement of Temperature Measurement of the temperature of a body depends upon the establishment of thermodynamic equilibrium between the body and the device used to sense temperature. This condition is rarely achieved as establishment of instanteneous equilibrium depends on the sensors, its size and shape, it`s thermal capacitance and variation in parameters of recording instruments and compensation provided. Mechanism Of Heat Transfer Conduction :- Some substances are good conductors, Like metals; others are bad conductors like glass, plastics, oxide. Conductivities of metals is approximately of the order of one thousand times those of other solids or liquids.

Mercury is

exception as it is a liquid metal. Conductivities of liquids are times those of gases. It is desirable that heat be conducted as rapidly as possible to the temperature sensor which should be directly immersed in the heated medium or if it is not possible; a protective pocket should be provided and the sensor immersed in a good conductor such as mercury or aluminium powder. Convection :-Liquid and gasses transfer heat by convection. Convection is either natural (like boiling water) or forced (directing air over hot element by fan, blower etc). Radiation :- Radiant heat (Intra - red radiation), like light is considered to take the form of electromagnetic wave (speed - 3,00,000 km/s in vacuum and air). It can be focused, reflected, transmitted, absorbed and radiated by materials. It`s essential differences from light are a) It is not visible. b) It has a longer wave length ranging from 0.75µm to about 100µm. c) When it falls on skin it produces the sensation of warmth. All substances radiate heat at all temperatures above absolute zero; thus if an attempt is made to measure the temperature of a hot gas surrounded by

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Measurement System cooler walls using a thermocouple it is found that the temperature recorded is lower than that of the gas due to exchange of heat between couple and cool walls by radiation. Absolute thermodynamic scale :The absolute thermodynamic scale was originally established in 1854. The term thermodynamic means that temperature scale conforms to the laws of thermodynamics and is independent of any thermometric substance. Widely used method to establish this scale is constant volume gas thermometer obeying gas law pv = RT. The unit degree on the scale was originally defined as one hundredth part of the temperature interval between the freezing and boiling point of pure water. The present thermodynamic scale is called Kelvin scale and its sole defining point is tripple point of water 273.160 K (0.010C) as shown in fig.10.1. This scales matches with earlier 0C scale. Establishment of the thermodynamic scale by gas thermometer is a difficult technique requiring great skill, and in view of this International practical temperature scale was established which confirms closely to the absolute scale using instruments for fixed point determination which conveniently and accurately reproduce temperature within the scale range.

(IPTS) International Practical Temperature Scale 1968 The primary fixed points on the IPTS are shown in Table 10.1. These are determined at one standard atmosphere (101 325 N/m2). The methods used to establish the scale are show in Table 10.2. A good number of secondary points are also given in Table 10.3 for establishing fixed points for instruments.

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Measurement System Table - 10.1 Defining fixed points of I.P.T.S. 0

Equilibrium

0

K

Equilibrium state between

C

solid, 13.81

-259.34

liquid and vapour phases of equilibrium hydrogen (Tripple point of equilibrium hydrogen) Equilibrium between the liquid 17.042 and

vapour

equilibrium

phases

hydrogen

-256.108

of at

a

pressure of 33.330.6 N/m2 (25/76 of standard atmospheric pressure) Boiling point of equilibrium 20.28

-252.87

hydrogen Boiling point of neon

27.102

-246.048

Tripple point of oxygen

54.361

-218.789

Boiling point of oxygen

90.186

-182.962

Tripple point of water

273.16

0.01

Boiling point of water

373.15

100

Freezing point of Zinc

692.73

419.58

Freezing point of silver

1235.08

961.93

Freezing point of gold

1337.58

1064.43

Fig 10.2 Methods to establish IPTS Range

Method

Output measured

13.8 K to the antimony Platinium point 630.740c

resistance Electrical Resistance

thermometer

630.740c to the gold point Pt/Pt+10%Rh 1064.630c

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Electromotive force

thermocouple

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Measurement System Above (1064.430C.) 1337.580C is defined by planck`s law. Table 10.3 IPTS Secondary points Substance

solid/liquid Tem. T 0K

Temp 0C

equilibrium Mercury

234.288

-38.862

Water (ice point)

273.15

0

Bismuth

544.592

271.442

Lead

600.652

327.502

Antimony

903.89

630.74

Nickel

1728

1455

Platinum

2045

1772

Rhodium

2236

1963

Tridium

2720

2447

Tungsten

3660

3387

There is no IPTS from absolute zero to 13.810K. But temperature between range 00K to 200K are measured by magnetic, vapour pressure and ultrasonic (noise) thermometers. Temperature Measuring Instruments Temperature measuring instruments can be divided into 'two groups'. Non electrical methods. 1) Liquid, Vapour pressure and gas thermometer 2) Thermocouple pyrometer 3) (a) total radiation pyrometer. (b)photoelectric pyrometer. (c) Optical pyrometer. Liquid in glass thermometer Liquids like Mercury (-350c to 5100c), Alcohol (-800c to +700c); Toluene (-800c to 1000c) Pentone (-200 to 300c) and Creosole (-5 to 2000c) are used for these thermometers.

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Measurement System

Essential characteristics of thermometer. (1) Inexpensive (2) Simple (3) Easily portable (4) Fragile (5) No requirement of indication instruments (6) Stem readings should be visible (7) High heat capacitance (8)Slow response to dynamic temperature measurements (9)Distant reading not possible (10)Not suitable for surface temperature measurement (11) Used as immersed in mercury/aluminium powder container for solids. (12) Used by applying correction for exposed portion in liquids. (13) Used in gas by providing bright metal shield around and forced gas flow. Vapour condensation must be avoided. Measurement of Temperature Temperature measuring instruments can be divided in to two groups.

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Measurement System Non electrical Methods. (i)

liquid, vapour pressure and gas thermometer

(ii)

Bimetal strip thermometer

(iii)

Refractory cones, paints and crayons

Electrical Methods (i)

Electrical Resistance pyrometer

(ii)

Thermocouple pyrometer

(iii)

(a) total radiation pyrometer (b)photoelectric pyrometer (c) Optical pyrometer

Liquid in glass thermometer Mercury is usually used in liquid in glass thermometers although other liquids, such as alcohol and pentane, which have lower freezing temperatures than mercury do not cause contamination through breakage, are also used. LIQUIDS USED LIQUID

RANGE 0C

Mercury Alcohol Toluene Pentane Creosote

-35 to +510 -80 to +70 -80 to +100 -200 to +30 -5 to +200

0

K

238 to 783 193 to 343 193 to 373 73 to 303 268 to 573

Increase in temperature causes the liquid to expand and rise up the stem. When measuring tempertures above the boiling point of mercury (3570C at atmospheric pressure) the spare above the liquid is filled with nitrogen under pressure, thus raising the boiling point and allowing temperatures up to 5100C to be measured. Thermometers are classified as chemical or Industrial.

Chemical

thermometers acts as standards for industrial thermometers and are used in laboratories and are calibrated regularly against standard instruments when in use. Industrial thermometers may have long stem. They may of registering type to indicate maximum and minimum temperatures. Small range thermometer

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Measurement System usually 50C and one degree interval 50 mm long, can give estimated values of the order of 0.01 to 0.0020C when used in conjugation with a law temperature telescope. Essential characteristics of liquid in glass thermometer 1) Inexpensive 2) Simple 3) Easily portable 4) Fragile 5) Additional Indication instruments are not required. 6) Can be used only when stem readings are visible. 7) Shows temperature lag due to relative high capacity in dynamic temperature measurements. 8) Not suitable for remote measurements. 9) Not suitable for surface temperate measurements. Applications - Solids -- The thermometer may be immersed in a mercury or aluminium powder filled hole in the solids to ensure rapid heat conduction. Steady state condition is necessary. Liquids - Immersed in liquid.

Correction can be applied to apart of column

exposed to atmosphere. Gases - are poor conductors of heat. Radiation losses occur. Bright metal shields round the thermometer and forced gas flow is used to improve accuracy of reading. Liquid in Metal thermometer These instruments are of the bulb; capillary Bourdon tube type, filled with liquid under pressure, measuring change of volume of the liquid. It has temperature lag characteristic but it is quite robust. Liq.

RangeoC

Mercury

-39 to + 650

Xylene

-40 to + 400

Alcohol

-46 to + 150

Ether

+ 20 to +90

The main advantages of this thermometer. 1)linear Scale 2) Wide temperature range 3) Ample power to operate pointer 4) Liquid can be under high pressure thus reducing head error.

5) no effect of

burrometric pressure variation 6) long capillary allow large differences in bulb and indicator levels. 7) Remote sensing. Accuracy - ï ½ % of FSD

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Measurement System Constant volume gas expansion thermometer The main advantages of this instrument is its rapid response to temperature changes as gas has lower heat capacity than liquid and solids gases have much higher coefficient of expansion e.g. N2 3.644 x 10-3 & Ne2 3.6617 x 10-3 as compare to 0.181x10-3 that of mercury. Accuracy - ± 1/2 % of FSD Vapour pressure thermometers Ether is introduced in evacuated chamber. There is in pressure change corresponding to temperature change. Liquid used - Methyl chloride 0 - 50 Diethyl ether

60 - 160

Ethyl alcohol

90 - 1700c

Water

120 - 2200c

Toluene

150 - 2200c

Characteristic - (1) non linear scale (2) small scale range (up to 1000c) (3) fast response (4) Distant reading (up to 65 m ) (5) Range - 0 - 1500c Instrument - Bourden capillary and bulb

Fig 10.4 a) Bimetal strip thermometer b) Compensated liquid in metal thermometer

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Measurement System Bimetallic strip thermometer Characteristics - (1) Inexpensive 2) Compact 3) Robust 4) Linear 5) Ranges - 30 to 2000c and 0 to 5500c 6) Accuracy - 2 % of FSD Temperature Indication by change of state of Solids Pyrometric Cones :- Material - China, Clay tale feldspar Range - 600 to 20000c. A minimum of three cones mounted having various melting points are mounted on to a refractory plate. The required temperature is indicated when the selected cone reaches its end point i.e its tip just touches the plate. Change of colour Paints and Crayons :- These are useful for small scale heating of steel parts, weldments, for stress relief and preheating. The paint or Crayon changes colour or Appearance at fixed temperature. Cones, bars, recorders and rings are used in heat treatment kilns for ceramic wear grinding wheels, bricks, refractories, electrical porcelain, earthware sanitary ware files and china. They are often used in conjunction with a pyrometer. Accuracy - ± 10 %. Thermoelectricity (1) Two dissimilar in contact with each other exhibit potential difference (2) A potential gradient exists even in a single metal if there is a temperature gradient. Secback effect - Emf generated depends on material of thermocouple as well as temperature gradient (T1 - T2). Cold junction

Hot junction

T1 J1

T2 J2

Peltier effect - Current flowing in thermocouple heats cold (due to heat generation) and cools hot junction (due to heat absorption). Practically this effect is negligible.

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Measurement System Thomson effect :- Junction emf may be slightly altered if there exist temperature gradient along thermocouple wien. This effect is also practically negligible. E=aT+bT2 Where E - emf a, b - constants T = T1 - T2 Materials - A) Base materials thermocouples- Copper Constant, range - 3 - 6730c Accuracy ± ½ % Sensitivity - 0.05 mv/k Iron constantant - range - 63 - 14730c, Accuracy ± 1 % Sensitivity - 0.05 mv/0K B) Rare - Metals - Platinum - platinum/10 % sodium

range 233 - 20330c

Accuracy ± ½ %, , sensitivity 0.01 mv/0K Application of thermocouple - Thermocouple is welded or soldered to surface for surface temperature measurements. Thermopiles - Series of thermocouple attached in series are called thermopiles. Total output is the sum of emf output of all the thermocouples. Principal :- Energy received by thermocouple E = e σ K4 K = true temperature of the body. Fery total radiation pyrometer - The main features of this pyrometer are as follows

(a) A blackened tube (T) open at one end to receive radiation and carrying an adjustable eyepiece E at the other. (b) A thermocouple C shielded from in coming radiation and carrying a blackened copper target disc. (c) A concave mirror M - adjustable by rack and pinion arrangement (d) Two small flat mirror for adjustment of focus on the thermocouple.

x

10

x

9

8 witness

Set x-x T.Y.B.E. ( Production )

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signal

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Measurement System incoming radiation

Major

Thermopile Dome

Thermocouple

Total radiation pyrometer Mirror if focused properly it appears as shown at ii otherwise as shown at I through eyepiece E Faster pyrometer - This is a fixed focus thermocouple instrument arranged in such a way that provided the cone of radiation fills the tube the distance of the pyrometer from the source is unimportant. Land surface pyrometer :- This pyrometer is specially designed for measuring the temperature of surfaces in open. It is also used to measure total emissibilty of the surfaces. Optical pyrometers, photo electrical pyrometers are the fen other pyrometers used for measurement of temperature.

Also electrical resistance thermometer,

thermisters, glass probe and metal probe thermometers are used for temperature measurement.

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Measurement System Experiment no 11

MEASUREMENT OF SOLID AND LIQUID LEVEL Liquid level measurement Liquid level measurement and control is essential in modern industrial plants which use large quantities of water, solvents, chemicals and other liquids which are required for processing materials and products. The instruments used for liquid level measurement in storage tanks may be broadly classified under the following headings: (i) Direct and indirect mechanical methods (ii) Pneumatic methods (iii) Electrical methods (iv) Ultrasonic systems (v) Nucleonic gauges. The choice of instrument to be used in a particular application will depend on several factors such as the liquid level range, the nature of the liquid, the cost involved and the operating pressures. Direct and indirect mechanical methods Dip-sticks The ordinary dip-stick marked in units of length is the simplest of all level measuring devices. Common applications are the measurement of oil level in the car engine or the height of fuel oil in a uniformly shaped storage tank. Accurate level measurement using dip-sticks is achieved by the Customs and Excise Department in both the brewing and the petroleum industry. A refinement of the simple rod-type dip-stick is the bob and tape where the bob weight is lowered to the bottom of the tank containing the liquid and the level is found by measuring the point on the tape reached by the liquid surface. It is obviously important to keep the tape vertical and taut when a reading is taken.. Hook gauges Hook gauges are generally used for measurement of small changes in level in very large diameter storage tanks. A typical schematic arrangement of such a gauge is

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Measurement System shown in Fig. 10.1. In practice the gauge is fixed at a datum or reference level. Small changes in level with respect to the datum may then be measured by adjusting the position of the hook until the tip just Hook gauges Hook gauges are generally used for measurement of small changes in level in very large diameter storage tanks. A typical schematic arrangement of such a gauge is shown in Fig.10.1. In practice the gauge is fixed at a datum or reference level. Small changes in level with respect to the datum may then be measured by adjusting the position of the hook until the tip just breaks the liquid surface. Screw knob Vernier Scale Liquid level

Tip of Hook

Sight glasses The sight glass is normally a graduated glass tube mounted on the side of the tank as shown in Fig. 10.2. This method is very simple and gives direct reading of level at the sight tube. Corrections may have to be made owing to variations in density if the temperature in the storage tank is much higher than the temperature surrounding the glass sight tube. Buoyant floats Many kinds of float-operated devices are available for continuous level measurement. The primary element is the float which, because of its buoyancy, will follow the changing liquid level. The movement of the float is then relayed to a pointer or recorder by using some form of transducer or converting device. The mechanical float operated level controller, the ordinary ball-cock, is one of the

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Measurement System simplest and most elegant of all proportional control systems. A float level gauge using a counterweight is shown in Fig.10.3. In this case the float is coupled directly to the indicating element but other systems are available where the float movement is used to modulate an external source of power, hydraulic, electrical or pneumatic.

An example of a float operated method equipped for electrical transmission is shown in Fig.10.4. In this system the movement of the float produces an angular rotation of the take-up drum which is connected via suitable gearing to a rotary potentiometer type displacement transducer.

The output voltage from the

potentiometer is proportional to the angular movement of the drum and hence the linear float movement.

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Measurement System Indirect mechanical methods Consider a tank of uniform cross-sectional area A which contains a liquid of density p. the level of the liquid to be measured being indicated by the height h above the bottom of the tank; Absolute pressure at the bottom of the tank = pgh + atmospheric pressure Now

Gauge pressure = absolute pressure - atmospheric pressure

Or

Gauge pressure = pgh

As the gauge pressure is proportional to the height h, a meter is required which will measure gauge pressure. A typical Bourdon-type pressure gauge as described in Chapter 9 can therefore be used to measure gauge pressure at the base of the tank. A suitable system is shown in Fig. 10.5 where the scale of the instrument is calibrated directly in level measurement units. Strictly speaking the indicating gauge should be mounted at exactly the same height as the bottom of the tank in order to indicate the level correctly. Figure 10.6 shows the gauge with the connecting pipe full of liquid. The total head of liquid acting on the gauge is the height of liquid above the gauge and it can be seen that the readings will be subjec to a 'zero error'. If the zero error is not too large, however, this could possibly be calibrated out on the gauge scale.

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Measurement System Pneumatic methods The principle of a bubbler level gauge is illustrated in Fig. 10.7. The air pressure in the bubbler tube is adjusted until bubbles can be seen slowly leaving the bottom of the tube. The pressure gauge then measures the air pressure required to overcome the pressure of the liquid head above the bottom of the tube. Normally the gauge is calibrated directly in head units but, provided the cross-sectional area of the tank is constant, volume units may be used.

Electrical methods The variable capacitance transducer is the most widely used electrical method for liquid level measurement. A simple capacitor consists of two electrode plates separated by a material called the dielectric. The capacitance of a parallel plate capacitor can be expressed in the following form C = KA ε d where C is the capacitance, K is a constant, A is the overlapping area of the plates, e is the dielectric constant and d is the distance between the plates A capacitance transducer can be used to measure the level of liquid in tank and Fig. 10.8 shows a schematic arrangement of a suitable system. A metal electrode is placed inside the tank and insulated from it. The tank itself is earthed and forms one of the plates. The transducer therefore consists of two concentric metal cylinders and a change in liquid level alters the dielectric constant and hence the capacitance. The capacitance transducer is connected to one arm of a Wheatstone bridge circuit and changes in capacitance will alter the output voltage from the bridge. The bridge output voltage can therefore be calibrated directly in terms of liquid level

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Measurement System Capacitance transducers can be used to measure levels from a few millimeters to hundreds of meters. The method may be used for corrosive liquids provided a suitable metal electrode is used, e.g. stainless steel.

Ultrasonic systems Ultrasonic systems use an ultrasonic signal source and a matched receiver. Figure 10.9 illustrates the principle of ultrasonic level indication where the ultrasonic transmitter and receiver are placed above the 'full' level of the tank. In this case two echoes are received, one from the surface of the liquid and one from the bottom of the tank. The time separation between receiving the two echoes is a measure of the liquid level in the tank and the echoes may be displayed on a suitable analogue device such as a cathode ray oscilloscope. This method of level measurement is very expensive but can be used for 'difficult' liquids, i.e. corrosive or radioactive, as none of the equipment is in contact with the liquid to be measured.

Nucleonic gauges Owing to the ready availability of radioactive materials nuclear techniques can now be employed for the extension of some of the more conventional methods of level measurement. Nuclear gauges have the advantage that they can operate entirely from outside the containing vessel. These systems may be designed to provide

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Measurement System on/off control at a fixed level in the vessel, or to provide continuous indication of level over a given range. Nucleonic-type measuring units consist of a radioactive source, a radioactive detector and a rate meter to detect changes in radiation intensity received by the detector. A simple on/off level control device is shown Fig. and consists of a radioactive source, mounted in a suitable shield to provide good collimation, and a radiation detector. The source and detector are mounted on opposite sides of the tank at the critical level. When the contents of the tank rise above the critical level gamma radiation is absorbed and the detector output is reduced. This reduction in detector output is used to operate the control relay, thus closing the valve a stopping the flow of fluid into the tank. Similarly, when the vessel is being emptied, the increase in signal strength as the contents fall below critical level may be used to operate the control valve.

When continuous level measurement is required a strip gamma-ray source and a long tubular detector can be used. The liquid, upon rising and falling inside the tank, absorbs radiation, and the change in intensity received by the detector is a function of liquid level. A typical level gauge installation is shown in Fig.10.11.

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Measurement System Long tubular detectors are not a commercial item and if they have to be specially manufactured, then such installations can prove expensive. Perhaps the most popular continuous level gauge is the moving source and detector system. This system is essentially the on/off system discussed previously in which the collimated source and detector are arranged to traverse vertically together. The traversing gear is normally driven by an electric motor which is controlled by the relay current. The source and detector can therefore follow any change in level and are always positioned in line with the liquid surface. Solid level measurement Many industrial processes require continuous level indication of the levels of solid substances in storage tanks, typical examples being the measurement of the level of flour and grain. Of the methods already described for liquid level measurement the capacitor probe, the nucleonic gauge and the ultrasonic method can also be applied to the measurement of solid levels. The most popular method used for solids is the indirect method of weighing the material in a tank or storage bin. Provided the cross-sectional area of the storage vessel is constant then the level will be linearly related to the weight. The storage tanks may be weighed on mechanical scales or electrically using strain gauge load cells (Fig. 10.12). This technique of level measurement will only be accurate provided the density and the particle size of the material are uniform. The moisture content should also remain fairly uniform or errors can occur.

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Measurement System Experiment no 12 AIM: SPEED MEASUREMENT BY USING STROBOSCOPE Use of Stroboscope: When speed measurements are to made by stroboscopic methods, we generally work with a single distinguishing mark and proceed to find the highest flash frequency at which a true stationary image is seen. This approach stems from the fact that if the frequency of the flashing light is twice the shaft speed, a single mark on the rotating shaft appears to be two standing marks 180 apart. Accordingly the flash frequency is gradually increased from a low value until the rotating member appears to be stationary. The flash frequency is noted and then increased to twice its value. If there is still only one apparent stationary image the flash frequency is doubled again.

This procedure is

continued until two images appear 180° apart. When two images are observed for the first time, the flash frequency is twice the speed rotation. Consider a stroboscopic light, flashing 3600 times per minute, focused upon the end of a rotating shaft with a single keyway in it. In case, there appear to be four keyways 90° apart, then the shift is rotating at 900 rpm. Further, if the keyways appear to be slowly rotating under this light, then the shaft speed is either slightly more or slightly less than 900 rpm.

The apparent revolutions of keyways are then counted per unit time and the

relative rotational speed, called slip, is determined.

If the keyways are observed to be

revolving once in 12 seconds, then slip equals 5 rpm. The possible shaft speed is then determined by adding or subtracting slip from the basic 900-rpm synchronous frequency. If keyways appear to be rotating in a direction opposite to the direction of shaft rotation, then slip is negative and it must be subtracted from the synchronous 900 rpm. For exact speed measurement, the flashing rate is adjusted and synchronism is attained (appearance of a single line stationary image) for the higher rate of flashing. The flashing rate is then gradually reduced and synchronism is observed at reduced flash rate. If synchronism occurs at n different flashing rates f1, f2,……….fn then the actual Shaft speed in calculated from the relation: fr=

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f 1 f n (n − 1) f1 − f n

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Measurement System Where f1 is the lowest flashing frequency, fn is the highest flashing frequency and n is the number of flashing frequencies. These flashing frequencies refer to the frequencies at which single-line images are obtained. In addition to checking and measuring speeds of rotation of shafts and other parts of machinery, stroboscopes are also used for high speed photography and apparently slowing down periodically repetitive motions and thus enable those to be observed more conveniently. The device is especially valuable where it is inconvenient to make a connection with the rotating shaft or for low-powered machinery where any load to drive a tachometer would affect the operation of the machine. Commercial stroboscopes are available to read angular velocities between 600 and 20,000 rpm. The device, however, can- not be used where the ambient light is above a certain value; the stroboscope requires a subdued surrounding light for its efficient operation. Example

The speed of a turbocharger was measured by a stroboscope and for that a

radial mark was made on the rotating shaft .The synchronism was attained for the highest rate of flashing and subsequently the flashing rate was reduced and a single image observed at reduced flash rates.

Calculate the speed of the turbocharger if

synchronizations achieved for stroboscopic settings of 13600, 1800, 1200,900 and 720 rpm. Solution:- The actual speed is given by fr=

f 1 f n (n − 1) f1 − f n

Where fn=highest flashing frequency =3600 rpm fi=lowest flashing frequency=720 rpm n==number of flashing frequencies=5 fr=

720 × 3600 × ( 5 − 1 ) = 3600 rpm 3600 − 720

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