Sub module 2.8 to 2.12 Pressure and vacuum measurement

Quadrant 2 List of animations/Videos: Pressure measurement (sub-module 2.8):

1. Production of level and pressure measuring instruments is shown http://www.endress.com/eh/home.nsf/contentview/0BCC37441C2E864BC12575F40031FCAF?Open&popup

2. Assembly Inspection http://www.endress.com/eh/home.nsf/contentview/E4E39C1350892808C125761600350550?Open&popup

3. Pressure measurement is shown http://www.youtube.com/watch?v=iMIzApq1CQ0

4. Bellow pressure gauge details are shown http://www.youtube.com/watch?v=0uDbYvQ18B0&feature=player_detailpage

5. How pressure gauges are calibrated? http://www.youtube.com/watch?v=10Seo_TVJSs

6. How pressure gauges are calibrated? http://www.youtube.com/watch?v=HgU88a7jSaE

7. Capacitive pressure sensor working is explained http://www.youtube.com/watch?v=SeWdfXsnKKE

8. Working of MEMS pressure sensor is shown http://www.youtube.com/watch?v=juf4d3sgOJw

9. LVDT working is demonstrated http://www.youtube.com/watch?v=G_q9jF4EscU&feature=fvwp

10. Hydraulic pressure and flow test – precautions and preparation http://www.youtube.com/watch?v=ctb_rLuswmk

11. How pressure is created in a hydraulic system? http://www.youtube.com/watch?v=P7h3oBSe2zU More details visit: http://www.techtrixinfo.com/

U-Tube Manometer (Sub Module 2.9) 12. Introduction to manometers – part 1

http://www.youtube.com/watch?v=5QEAHhXAu3M 13. Introduction to manometers – part 2

http://www.youtube.com/watch?v=SG-nz_Ibp5k 14. Solving a simple U tube manometer problem

http://www.youtube.com/watch?v=wRaWfXeVFkc 15. Example problem- manometer equation

http://www.youtube.com/watch?v=NcGsC2FpOvE 16. Manometer problem

http://www.youtube.com/watch?v=HwIvfvDChRs

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Bourdon gauge (sub module 2.10):

17. How pressure gauges are made? http://www.youtube.com/watch?v=IxhNLtP8jpI

18. How does a dead weight tester works? http://www.youtube.com/watch?v=_ErO0aos5Zw&feature=endscreen

19. How pressure gauges are calibrated? http://www.youtube.com/watch?v=e_s0A0o-t_8&feature=endscreen

Pressure transducers (sub module 2.11):

20. What is a transducer? http://www.youtube.com/watch?v=4468GWn33UU

21. How to choose a pressure transducer? http://www.youtube.com/watch?v=YnhlNkfnCr4

22. Transducers and its types – thermocouples, RTD, thermistors , LVDT, strain gauge, photo transducers http://www.youtube.com/watch?v=_2ZQ1mKYK3Q

Vacuum measurement (sub-module 2.12):

23. Working of ion vacuum gauge is shown www.youtube.com/watch?v=ls6kfQLQWPk

24. How to measure pressure and vacuum using FLUKE PV 350 http://www.youtube.com/watch?v=Zcfq7s_53qc

Illustrations: 1. Boudon Gauges are illustrated here:

http://www.google.co.in/search?q=bordon+gauges&tbm=isch&hl=en-IN&gbv=2&oq=bordon+gauges&gs_l=img.12...18875.25828.0.28031.21.20.0.1.0.0.281.1795.16j2j1.19.0....0...1ac.1.34.img..21.0.0.GFdE8I7L84E

2. U-tube manometers are illustrated: http://www.google.co.in/search?q=u-tube+manometer&tbm=isch&hl=en-IN&gbv=2&oq=u-tube+manometer&gs_l=img.1.0.0j0i24l9.139687.144312.0.147578.16.10.0.6.6.0.282.1236.3j6j1.10.0....0...1ac.1.34.img..0.16.1312.bAxpn3xZr9U

3. Pressure transducers are illustrated: http://www.google.co.in/search?q=pressure+transducres&tbm=isch&hl=en-IN&gbv=2&oq=pressure+transducres&gs_l=img.12...68375.74015.0.76547.20.12.0.8.6.0.156.1138.11j1.12.0....0...1ac.1.34.img..3.17.1168.aJPzQY3bw4k

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List of questions (FAQ): Basics of pressure (sub-module 2.8):

1. Why pressure measurement is important? 2. Classify the most common pressure-measuring systems 3. Define pressure. Mention the units of pressure 4. Explain the terms :absolute pressure, gauge pressure, vacuum 5. Sketch the pressure term relationships 6. List the various pressure measuring devices and the corresponding ranges. 7. List the useful units and conversion factors related to pressure measurement. 8. Sketch the relationship between vacuum, absolute pressure, atmospheric pressure, absolute

zero pressure, gauge pressure.

U-tube Manometers (sub-module 2.9): 9. What is a manometer? 10. Briefly explain pressure measurement using U-tube manometer 11. List the different types of manometers. 12. Illustrate an inclined tube manometer 13. With a neat schematic diagram, explain well type manometer 14. Discuss U tube manometer 15. Explain dynamic response of a U-tube manometer 16. Illustrate the response of U tube manometer to step input 17. What are piezometers? 18. Discuss U-tube manometers for (a) positive gage pressure (b) negative gage pressure 19. What are differential manometers

Bourdon tube (sub-module 2.10): 20. With a neat sketch explain Bourdon pressure gauge 21. Explain the construction and working of a dead weight pressure gauge tester 22. Discuss a ring balance type pressure gauge 23. What is a Bourdon tube? Mention its types 24. Explain with a neat diagram the construction and working of a bourdon tube pressure gauge 25. Discuss the following types of Bourdon tubes:

a. Spiral tube b. Helical tube c. Twisted tube

Pressure transducers (sub-module 2.11): 26. What are pressure transducers? 27. Classify and explain the pressure transducers. 28. Explain strain gage theory and strain gage constructional details 29. Explain with a neat diagram a pressure tube with bonded strain gage

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30. Mention typical strain gage materials and gage factors 31. Illustrate bridge circuits for use with strain gages 32. Explain 3 wire arrangement for lead wire compensation 33. Illustrate a diaphragm pressure gage 34. Explain with schematic diagrams the diaphragm and bellows type pressure gages that use

LVDT for displacement measurement 35. Discuss a diaphragm gage with strain gage for displacement measurement 36. Discuss a typical LVDT signal conditioner 37. Mention the common specifications for commercially available LVDT 38. Discuss a differential pressure cell 39. Discuss a pressure transducer system that employs a displacement sensor to produce an

output voltage. 40. Explain capacitance diaphragm gauge for pressure measurement. Also explain bridge circuit

for capacitance pressure gage. 41. Write a note on measurement of transient pressures. 42. Explain pressure measurement in liquid and gas system 43. Explain transient response of a bellow type pressure transducer 44. Explain force balancing element for pressure measurement

Vacuum measurement (sub-module 2.12): 45. What is vacuum? How is it measured? 46. Explain measurement of vacuum by a U tube manometer 47. Illustrate the working principle of the McLeod gage with a neat sketch 48. List the various types of thermal conductivity pressure gages. 49. Discuss the working principle of ionization pressure gage. 50. Explain Pirani gauge, ionization gauge and Alphatron gauge

Quadrant 3 Wiki links/reference links/other university web courses Pressure measurement (Sub-module 2.8 to 2.10):

1. http://en.wikipedia.org/wiki/Pressure_measurement 2. Boyes, Walt (2008). Instrumentation Reference Book, Fourth Edition. Butterworth-

Heinemann. pp. 1312. 3. Beckwith, Thomas G.; Roy D. Marangoni and John H. Lienhard V (1993). "Measurement of

Low Pressures". Mechanical Measurements (Fifth ed.). Reading, MA: Addison-Wesley. pp. 591–595. ISBN 0-201-56947-7.

4. http://www.vgscienta.com/_resources/File/Catalogue_Sections/VGScienta_Pressure_Measurement.pdf

5. http://www.omega.com/literature/transactions/volume3/high3.html (pressure gauges) 6. The pressure, strain and force handbook, omega press LLC 1996 7. Process/Industrial Instruments and Controls Handbook, 4th Edition, Douglas M. Considine,

McGraw-Hill, 1993.

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8. Pressure gauge calibraters www.adarshpressuregauge.com

9. http://www.bctech.com.tw/index.php/products (pressure and vacuum gages) 10. http://www.efunda.com/designstandards/sensors/bourdon_tubes/bourdon_intro.cfm 11. http://www.efunda.com/DesignStandards/sensors/u_tubes/u_tubes_intro.cfm (u-tube) 12. http://www.efunda.com/DesignStandards/sensors/diaphragm/diaphragm_intro.cfm

(diaphragm pressure gauge) 13. http://www.efunda.com/DesignStandards/sensors/diaphragm/diaphragm_theory.cfm

(diaphragm theory) 14. http://www.omega.com/literature/transactions/volume3/pressure.html 15. http://www.sensorland.com/HowPage018.html 16. Relationship between vacuum, gage pressure, absolute pressure

Refer ‘ Principles of industrial instrumentation and control systems by Chennakesava R. Alavala, Cengage Learning, 2009, page 179.

Pressure transducers (sub module 2.11): 17. Keeping up with pressure sesnsors, R. Merrit, I & CS, April, 1982 18. Bellow type pressure transducers -images

https://www.google.co.in/search?q=bellow+type+pressure+transducer&tbm=isch&tbo=u&source=univ&sa=X&ei=exc9UoP5MMmKrQeftoDQCA&ved=0CEEQsAQ&biw=1280&bih=677&dpr=1

19. Pressure detectors –resistance type, inductance type, capacitive type, circuitry, bellow type, bourdon type http://www.techtransfer.com/resources/wiki/entry/3390/

20. Pressure gauges and switches http://www.omega.com/literature/transactions/volume3/pressure.html

Vacuum measurement (sub-module 2.12): 21. http://www.efunda.com/DesignStandards/sensors/mcleod/mcleod_intro.cfm (McLeod Gauge) 22. Jone’s instrument technology, vol. 1, mechanical measurements, editor- B E Noltingk,

Butterworths, IV Edn., page 145 (vacuum measurement) 23. http://philiphofmann.net/ultrahighvacuum/ind_iongauge.html (ion gauge construction) 24. http://www.vgscienta.com/productlist.aspx?MID=227 (ion gauge working principle and photo) 25. http://www.bctech.com.tw/index.php/products (pressure and vacuum gages) 26. http://www.instrumentationtoday.com/alphatron-vacuum-gauge/2011/10/ (alphatron gage) 27. http://en.wikipedia.org/wiki/Hot-filament_ionization_gauge (ionization gauge) 28. http://en.wikipedia.org/wiki/McLeod_gauge (McLeod gauge) 29. http://www.globalspec.com/learnmore/manufacturing_process_equipment/vacuum_equipment

/vacuum_gauges (vacuum gauges) 30. http://www.efunda.com/DesignStandards/sensors/mcleod/mcleod_theory.cfm

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Quadrant 4 Numerical problems: Problem on U-tube manometer:

1. A U-tube manometer uses a special oil having a specific gravity of 0.82 for the manometer fluid. One side of the manometer is open to the atmospheric pressure of 750 mm Hg and the difference in column height (h) is measured as 25 cm when exposed to an air source at 25 oC. Determine the system pressure in Pa. Also find the influence of uncertainty, if the column height measurement is 1 % uncertain. Solution: ρm = ρspecial oil = specific gravity of special oil x density of water at 25 oC

= 0.82 x 996 = 816.7 kg/m3 The local atmospheric pressure in Pa = (750/760) x 1.013 x 105 = 99967.1 Pa

Air density = ρa = ρf = Pa/(RgT) Where Rg = gas constant = 287 J/KgK T = air temperature = 273 + 25 = 298 K ρa = ρf = 99967.1/(287 x 298) = 1.17 kg/m3 The column height = h = 25 cm = 0.25 m The pressure differential = P-Pa = ( ρm – ρf ) gh = ( 816.7 - 1.17 ) x 9.8 x 0.25

= 1998 Pa System pressure = P = 1998 + Pa = 1998 + 99967 = 101965 = 1.019x 10 5 Pa

The error in the measured pressure difference due to 1 % measurement uncertainty is = ∆ (P-Pa) = ± (1/100)x (P-Pa) = ± 0.01x1998 = ± 19.98 ~ ± 20 Pa

2. The pressure difference (P-Pa) measured using a U-tube manometer, employing an oil having a specific gravity of 0.82, is 1600 Pa. Determine the difference in column height in cm, when one end is exposed to local atmospheric pressure of 10x104 Pa. Standard acceleration of gravity exists. The other end of manometer is connected to an air pressure source at 25 oC. Solution: The manometer fluid has a density of 82 % of that of water at 25 oC Manometer fluid density = ρm =0.82 x 996 = 816.7 kg/m3 Density of air at 298 K = ρa = ρf = Pa/(RgT) = 10 x104 / (287x298) = 1.17 kg/m3 Now, ( ρm – ρf ) gh = P-Pa = 1600 Pa h = 1600/ ( ρm – ρf ) g = 1600/(816.7-1.17)9.8 h = 0.2002 m = 20.02 cm

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3. A U-tube manometer is used to measure the pressure of water in a pipe line which is greater than atmospheric pressure. The right limb of the manometer contains mercury and is open to atmosphere. The mercury contacts water in the left limb. Determine (i) the pressure in the pipeline if the difference in the level of mercury in the limbs of U-tube is 12 cm and the free surface of mercury is in level with the pipeline center. (ii) If the pressure changes to 1200 N/m2 , calculate the new difference in the mercury level. Solution: (i) P x = gauge pressure of water in the pipeline

h = difference in the mercury level = 12 cm Px + ρ w g h 1 = ρ m g h 2 where ρ w = density of water ρ m = density of mercury h 1 = h 2 =12 cm = h = 0.12 m P x = ( ρ m - ρ w )gh = (1000x13.6 – 1000)9.81x0.12 = 14832.72 N/m2

(ii) New difference in mercury level when pressure changes to 12000 N/m2 The new equation is: Px + ρ w g (12-x)/100 = ρ m g (12-2x)/100 12000+1000x9.81 (12-x)/100 = 13.6x1000x9.81(12-2x)/100 After simplification, x = 1.102 cm Therefore, new difference in mercury level is = h = 12-2x = 9.8 cm

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4. Water is flowing in a pipeline. A U –tube manometer is used to measure the pressure drop for flow through the pipe. The difference in level of manometric fluid (CCl 4 ) in the two limbs is found to be 22 cm. Density of CCl 4 is 1.596 g/cm3. If mercury is used as manometric fluid, what will be the difference in level ? Density of mercury is 13.6 g/cm3. Comment on the new difference in level.

Solution: h ccl4 = 22 cm = 0.22 m

ρ ccl4 = 1.596 g/cm3 = 1.596 x 103 kg/m3 ρ Hg = 13.6 x 103 kg/ m3 Pressure drop = P = ρ ccl4 x g x h ccl4 = 1.596 x 103 x 9.81 x 0.22 N/m2

= 3444.5 N/m2 = 3.44 kN/m2 If mercury is used as manometric fluid, Then, h Hg = h ccl4 x ρ ccl4 / ρ Hg = 0.22 x 1.596 x 103 /(13.6 x 103) = 0.0258 m = 2.58 cm

Comment: The new difference in level is less, since denser fluid (ie Hg) is used in the manometer.

Problem on diaphragm pressure gauge: 5. A diaphragm pressure gauge is to be fabricated using a spring steel diaphragm of

diameter 40 mm. It is to be designed to measure a maximum pressure of 2 MPa. (i) Calculate the thickness of the diaphragm needed, if the maximum deflection is limited to 1/3 of its thickness. (ii) What is the natural frequency of this diaphragm? The edges of the diaphragm are fixed at its perimeter and it is uniformly loaded over entire surface. (iii) Also, sketch the diaphragm arrangement. Solution: E= Modulus of elasticity = 2 x 1011 N/m2 µ = Poisson’s ratio = 0.3 ρ = density of the material = 7800 kg/m3 a = radius of diaphragm = 0.02 m t = thickness of diaphragm = ? f = natural frequency = ? (i) Diaphragm thickness (t):

Y max = t/3 = 3∆p x a4 x (1- µ2)/(16Et3) t4 = 9 x 2 x 106 x 0.024 (1-0.32) / (16 x 2 x 1011) t = 9.513 x 10 -4 m = 0.951 mm

(ii) Natural frequency ( for steel diaphragm): f = 4.912 x 104 x t/(π x a2) = 4.912 x 104 x 9.513 x 10 -4 /( π x 0.022) = 37203 Hz = 37.2 kHz

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6. An LVDT – diaphragm combination is used to measure low pressure. The sensitivity

of the LVDT is 3 nm and the diaphragm is fabricated using steel (E = 2x1011 Pa and µ = 0.3). The diameter of the diaphragm is 10 cm. Calculate the thickness of the diaphragm if the maximum deflection does not exceed 1/3 of its thickness. What is the lowest pressure sensed by this arrangement? Also, calculate the natural frequency of this diaphragm. Solution: Use the procedure given in the problem No. 5 and solve it.

Problem on McLeod gage: 7. A McLeod gage is used for low pressure measurement. It has a volume of 160 cm3,

and a capillary diameter of 0.4 mm. Calculate the gage reading for a pressure of 25 µm. Solution: V = volume of gauge = 160 cm3 = 160x10-6 m3 P = pressure indicated using approximate formula = h2 Ac / V = 25 x 10-6 m Ac = capillary area = π d2/4 = π x 0.00042 / 4 m2 h = capillary reading = gage reading = ? h2 = PV/Ac = 25 x 10-6 x 160 x 10-6 / (π x 0.00042 / 4 ) = 0.032 h = 0.1784 m = 17.84 cm

8. A McLeod gage has a capillary diameter of 0.35 mm and a volume of 130 cm3. What gage reading will result for a pressure of 25 µm ? Solution: Follow the procedure presented in the previous problem and solve it.

9. A McLeod gauge is used to measure vacuum. It has a bulb of 90 x 10-6 m3 and its capillary diameter is 1 mm. Calculate the pressure when the capillary reading is 30 mm. Solution: V = volume of bulb = 90 x 10-6 m3

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d = capillary diameter = 0.001 m Ac = capillary area = π d2/4 = 0.785 x 10-6 m2 h = capillary reading = 30 mm = 0.03 m P = pressure using approx formula = h2 Ac / V = 7.85 x 10-6 m = 7.85 µm P = pressure using exact formula = h2 Ac / (V-Ac x h) = 7.852 x 10-6 m Error = 7.852-7.850 = 0.002 µm

Multiple choice questions 1. When pressure is expressed in terms of equivalent column height of a liquid, the

general term used is a. Barometric pressure b. absolute pressure

c. Gage pressure d. head

2. A bourdon tube is a key element of a. a piezoelectric pressure transducer

b. a dial type mechanical pressure gage c. an inclined U-tube manometer d. a barometer

3. Which of the following device is not suitable for measurement of negative pressure a. Piezometer

b. Pirani gage c. U-tube manometer d. Bourdon gage

4. In which type of bourdon tube tip travel is small?

a. C-type b. Spiral type c. Helical type d. None of these

5. Piezometer is suitable for measurement of a. gauge pressure b. vacuum c. absolute pressure d. pressure difference between two points 6. Manometers measure pressure by a. measuring liquid heights b. measuring liquid column height c. balancing unknown force formed by liquid pressure against a known force d. recording the pointer deflection

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7. Manometric liquid should a. have high viscousity b. have high co-efficient of thermal expansion c. have low vapour pressure d. be corrosive 8. Manometer has the following advantages a. good accuracy and repeatability b. delicateness c. ability to work in unsafe atmosphere d. work at all altitudes 9. Pressure is defined as a. force divide by unit area

b. unit area divided by force c. force X unit area d. all of the above

10. Dead weight pressure tester is used for a. testing dead weights b. measuring process pressures c. developing high vacuum d. calibrating pressure gauges

11. 1 mm Hg is equal to a. 1 torr b. 1 Pa c. 1 millibar d. 1 micron

12. Pirani gauge is a type of a. ionization gauge b. bellow gauge c. thermal conductivity gauge d. diaphragm pressure gauge

13. The basic element in a manometer is a. capillary b. bellow c. diaphragm d. u-tube

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Answer table 1. d 2. b 3. a 4. a 5. a 6. c 7. c

8. a 9. a 10. d 11. a 12. c 13. d

Fill in the blanks with most appropriate word(s):

14. -------------is the output quantity of the mechanical primary detector transducers 15. The output signal of elastic members is ----------- 16. A manometer is a device whose -------- is proportional to pressure 17. Orifice and venture devices offer flow information in the form of -------- change as a

result of energy transformation 18. Pitot tubes are used to find out pressures resulting from ---------- rate than change of

rate 19. ----------is the input for electrical primary detector transducers 20. Manometers measure ------------pressures 21. Piezometers measure ----------- pressure 22. The cross-section of the c-type Bourdon tube is ---------- in shape 23. Mcleod gage works on the basis of ------- 24. Thermal conductivity pressure gage is normally used for ------measurement 25. Most liquid pressure gauges use ----------------- as a zero point 26. A pressure lesser than the atmospheric pressure is called --------- 27. Piezoelectric pressure transducer cannot measure ---------- pressure 28. To measure pressure inside the gasoline engine cylinder --------- is normally used 29. 1 N/sqmm = -------------- Pa 30. Pirani gauges are used to measure pressure from about--------------to -------- 31. Thermocouple gauges are used to measure pressure from about---------- to ------ 32. An ionization gauge measures the ------------- of a gas 33. Thermal conductivity gauges are used to measure pressures from about ------ to 34. A barometer is a well type absolute pressure gauge whose operating range is from

zero absolute to ------------ pressure 35. When manometer is used to measure high pressures, --------- is used as the

manometer fluid

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Answer table 14. Displacement 15. Displacement 16. Deflection 17. Pressure 18. Total flow 19. Displacement 20. Differential 21. Gage 22. Elliptical or oval 23. Boyle’s law 24. vacuum

25. atmospheric pressure – 760 mm Hg 26. vacuum 27. static 28. piezoelectric pressure transducer 29. 1 30. 10-4 to 1 torr 31. 10-4 to 1 torr 32. Density 33. 1 torr to 10-4 torr 34. Atmospheric 35. mercury

Say True/false

36. The basic dimensions of pressure are similar to the basic dimensions of stress. 37. The basic purpose of making a U-tube manometer inclined is to improve its

resolution 38. To measure the deflection of the diaphragm of a pressure transducer

capacitance transducer is used 39. Pressures of the order of 1mmHg or less are usually treated as very low pressures

40. A resistance strain gauge type pressure transducer is not suitable for measurement

of dynamic pressure 41. A piezo-electric pressure transducer can be used for measuring the dynamic

pressure 42. Ionization gauge is used for measurement of very low pressures of the order of

1 micron and less 43. McLeod gauge is used to measure pressure between 0.01 and 1000 microns 44. The diaphragm in the diaphragm pressure transducer can be in the flat plate form 45. Piezoelectric pressure transducers can be used to measure very high pressures that

change swiftly 46. Bellow type pressure gauges can be used to measure vacuum also 47. Pirani gauge measures pressure by detecting change in the thermal conductivity of a

gas 48. A micromanometer is used to measure extremely small pressure differences

accurately 49. U tube manometer is used to measure liquid flow rate 50. Bourdon tube has circular cross section

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Answer table

36. T 37. T 38. T 39. T 40. F 41. T 42. T 43. T 44. T

45. T 46. T 47. T 48. T 49. F 50. F

Assignment questions and solutions: Pressure measurement:

1. Mention the advantages and limitations of bellow type pressure gauges Advantages:

a. Simple and rugged construction b. Can be used for low to moderate pressures c. Suitable for gauge, differential and absolute pressure measurements d. Moderate cost

Limitations: e. Hysteresis and zero shift exists f. Due to large relative movement and mass, these are not suitable for transient

pressures g. Springs are required for accurate characterization h. Compensation is required for ambient temperature changes

2. Mention the advantages and limitations of diaphragm type pressure gauge

Advantages: a. Comparatively smaller size b. Moderate cost c. Withstands high overpressures d. Has good linearity over a wide range e. Absolute and differential pressure measurements possible f. Hysteresis is minimum

Limitations: g. Needs protection from shock and vibration h. Repair is difficult

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3. Mention some of the piezoelectric materials used in piezoelectric pressure transducers.

Ans: Barium titanate, quartz crystal, tourmaline, Rochelle salt

4. Discuss the errors in manometers Ans: 1. Effect of temperature: Due to increase in temperature, fluid expands, hence

error creeps in. 2. Capillary effect: Capillary effect can be reduced, by using large diameter tubes

(above 10 mm diameter) 3. Effect of meniscus

5. Mention the manometer fluids

Ans: The most common manometer fluids are water, red oil and mercury

Vacuum measurement: 6. What is vacuum? Mention the unit of vaccum.

Ans: It is a space entirely devoid of matter. A space from which air has been removed by an air pump. Vacuum is measured in terms of Torr. 1 Torr is pressure offered by 1 mm of mercury column at standard atmospheric conditions.

7. Mention a few engineering applications of vacuum. Ans: 1. In the fabrication of x-ray tubes

2. In the manufacture and preservation of pharmaceutical products 3. To maintain required furnace conditions in metal industries

8. Write the classification of vacuum measuring instruments Ans: (a) Direct method based on the displacement caused by the act of force: Examples are Bourdon gauge, manometer, diaphragms, capsules. (b)Indirect method based on measurement of certain other pressure controlled properties such as volume, thermal conductivity etc.: Examples are McLeod gauge, Pirani gauge, Thermocouple gauge, Ionisation gauge

Self answered questions and answers

1. Why pressure measurement is important? Ans: Visit the website nptel.iitm.ac.in/courses/IIT-MADRAS/Mechanical _Measurements (sub-module 2.8)

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2. Explain the following with the help of schematic diagrams a. Diaphragm type pressure gauge which uses LVDT as a displacement

transducer b. Bellow type pressure gauge which uses LVDT as a displacement transducer

Ans: Visit the website nptel.iitm.ac.in/courses/IIT-MADRAS/Mechanical _Measurements (sub module 2.11)

3. Sketch the relationship between vacuum, absolute pressure, atmospheric pressure,

absolute zero pressure and gage pressure. Ans: Please visit http://www.engineeringarchives.com/les_physics_absgagevacatmpres.html

4. Illustrate a bridge circuit for capacitance pressure gage. Ans: Visit website nptel.iitm.ac.in/courses/IIT-MADRAS/Mechanical _Measurements (sub module 2.11)

5. Mention the various units of pressure measurement and also their relationship/conversion Ans: Please visit http://en.wikipedia.org/wiki/Pressure (Pressure units table)

6. Discuss a typical Bourdon gage capable of measuring vacuum. Ans: Visit website nptel.iitm.ac.in/courses/IIT-MADRAS/Mechanical _Measurements (sub module 2.12)

7. Discuss the working of a McLeod gage with a schematic diagram Ans: Visit website nptel.iitm.ac.in/courses/IIT-MADRAS/Mechanical _Measurements (sub module 2.12)

8. Discuss the construction and working of the following types of vacuum gauges. Also mention their range of measurement: a. Ionization gauge b. Pirani gauge c. Alphatron gauge d. McLeod gage Ans : Visit web site nptel.iitm.ac.in/courses/IIT-MADRAS/Mechanical _Measurements (sub module 2.12)

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Test your skills/knowledge:

1. A pressure measurement is made at a certain place which records a vacuum of 80,000 Pa. Assuming the local atmospheric pressure of 85,000 Pa, find the absolute pressure. Solution: P absolute = P atms – P vacuum = 85000-80000 = 5000 Pa

2. List the parameters to investigate when assessing a measuring instrument proposal (assessment checklist) Answer: 1. Does the device overload the observed condition? 2. Is the device compatible to other devices/system? 3. What are the interfacing requirements? 4. Is the sensitivity adequate? 5. Is it fast enough? 6. What is the cost? What are installation, calibration, freight and spares charges? 7. What is the calibration interval? 8. What is the arrangement of calibration? 9. Is the transduction principle sound and effective? 10. Is it robust and reliable? 11. What is feedback from other users? 12. Is the documentation proper for its lifelong requirements? 13. What other equipment/support equipment are needed? 14. What are the maintenance needs? 15. What is availability of spares? 16. Are the makers and suppliers reputed? 17. Is the technology old or advanced? 18. What is the lead time of supply?

3. List the commonly used manometric fluids along with their specific gravity values.

Answer: For high pressures mercury is used ( Sp. Gravity = 13.6) For low pressures, CCl 4 (Sp. Gravity = 1.59) and acetylene tetrabromide (Sp. Gravity = 2.59) are used.

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4. Identify the following apparatus:

( a )

( b )

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( c )

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