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/0BCC37441C2E864BC12575F40031FCA
F?Open&popup
2. Assembly Inspection
http://www.endress.com/eh/home.nsf/contentview/E4E39C1350892808C12576160035055
0?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
Page 1 of 19
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=enIN&gbv=2&oq=bordon+gauges&gs_l=img.12...18875.25828.0.28031.21.20.0.1.0.0.28
1.1795.16j2j1.19.0....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=enIN&gbv=2&oq=utube+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=enIN&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
Page 2 of 19
List of questions (FAQ):
Basics of pressure (sub-module 2.8):
1.
2.
3.
4.
5.
6.
7.
8.
Why pressure measurement is important?
Classify the most common pressure-measuring systems
Define pressure. Mention the units of pressure
Explain the terms :absolute pressure, gauge pressure, vacuum
Sketch the pressure term relationships
List the various pressure measuring devices and the corresponding ranges.
List the useful units and conversion factors related to pressure measurement.
Sketch the relationship between vacuum, absolute pressure, atmospheric pressure, absolute
zero pressure, gauge pressure.
U-tube Manometers (sub-module 2.9):
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
What is a manometer?
Briefly explain pressure measurement using U-tube manometer
List the different types of manometers.
Illustrate an inclined tube manometer
With a neat schematic diagram, explain well type manometer
Discuss U tube manometer
Explain dynamic response of a U-tube manometer
Illustrate the response of U tube manometer to step input
What are piezometers?
Discuss U-tube manometers for (a) positive gage pressure (b) negative gage pressure
What are differential manometers
Bourdon tube (sub-module 2.10):
20. With a neat sketch explain Bourdon pressure gauge
21.
22.
23.
24.
25.
Explain the construction and working of a dead weight pressure gauge tester
Discuss a ring balance type pressure gauge
What is a Bourdon tube? Mention its types
Explain with a neat diagram the construction and working of a bourdon tube pressure gauge
Discuss the following types of Bourdon tubes:
a. Spiral tube
b. Helical tube
c. Twisted tube
Pressure transducers (sub-module 2.11):
26.
27.
28.
29.
What are pressure transducers?
Classify and explain the pressure transducers.
Explain strain gage theory and strain gage constructional details
Explain with a neat diagram a pressure tube with bonded strain gage
Page 3 of 19
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
Mention typical strain gage materials and gage factors
Illustrate bridge circuits for use with strain gages
Explain 3 wire arrangement for lead wire compensation
Illustrate a diaphragm pressure gage
Explain with schematic diagrams the diaphragm and bellows type pressure gages that use
LVDT for displacement measurement
Discuss a diaphragm gage with strain gage for displacement measurement
Discuss a typical LVDT signal conditioner
Mention the common specifications for commercially available LVDT
Discuss a differential pressure cell
Discuss a pressure transducer system that employs a displacement sensor to produce an
output voltage.
Explain capacitance diaphragm gauge for pressure measurement. Also explain bridge circuit
for capacitance pressure gage.
Write a note on measurement of transient pressures.
Explain pressure measurement in liquid and gas system
Explain transient response of a bellow type pressure transducer
Explain force balancing element for pressure measurement
Vacuum measurement (sub-module 2.12):
45.
46.
47.
48.
49.
50.
What is vacuum? How is it measured?
Explain measurement of vacuum by a U tube manometer
Illustrate the working principle of the McLeod gage with a neat sketch
List the various types of thermal conductivity pressure gages.
Discuss the working principle of ionization pressure gage.
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. ButterworthHeinemann. 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_Measu
rement.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.
Page 4 of 19
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&t
bo=u&source=univ&sa=X&ei=exc9UoP5MMmKrQeftoDQCA&ved=0CEEQsAQ&biw=1
280&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
Page 5 of 19
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
Page 6 of 19
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
Page 7 of 19
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:
Comment:
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
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
Page 8 of 19
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
Page 9 of 19
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
Page 10 of 19
7. Manometric
a.
b.
c.
d.
liquid should
have high viscousity
have high co-efficient of thermal expansion
have low vapour pressure
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
Page 11 of 19
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
Page 12 of 19
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
Page 13 of 19
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
Page 14 of 19
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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