Module - Pmm
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MODULE - PMM
PMM
PRESSURE MEASUREMENT USINGMANOMETERS
OBJECTIVE:
Without the use of references, you must be able to:
a. Name three types of manometers used.
b. State the type of pressure that the manometer is used to measure.
c. State what type of manometer is used to measure very low pressures.
d. State typical pressure ranges that the “U” tube manometer can measure.
e. State the maximum pressure that an inclined manometer can measure.
At the end of the module you will be required to write a criterion test in which all of the
questions must be answered before proceeding with the next module.
RESOURCES
1. Your Training Officer
2. Module PMM
3. Manometers in the workshop
Instrumentation : Document No. 10 Page 1 of 17Created : 01 February 2003 Revision No: 01
Last Approved : March 2003 First Published : March 2003
Revised : Owner: Learnership Dept.
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MODULE - PMM
MODULE – PMM
PRESSURE MEASUREMENT USING MANOMETERS
INTRODUCTION
The measuring instrument, which utilizes the principle of pressure measurement by
means of balancing against a column of liquid of known density, is called
manometer.
Where the calibration of instruments or pressure sensors is required, the most
common measuring instrument used, is the manometer. This is due to the
simplicity and also the inherent accuracy of the manometer instrument.
This module will introduce you to the various types of manometers in use and
describe the typical application of these instruments.
SIMPLE “U” TUBE MANOMETER
A column of liquid exerts a pressure proportional to its height and is equal to h.G.
per unit area, h being the height displacement of the liquid and
G being the S.G of the liquid.
The operation of the “U” tube manometer and also the more advanced monometer
is based upon this fact. Figure – 1 shows an elementary form of manometer,
consisting of a glass tube bent into the shape of a “U” and then partly filled with
liquid of known density. A scale similar to a ruler is fixed between the limbs of the
glass tube.
Instrumentation : Document No. 10 Page 2 of 17Created : 01 February 2003 Revision No: 01
Last Approved : March 2003 First Published : March 2003
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THE PRINCIPLE OF A SIMPLE “U” TUBE MANOMETER
The manometer is a differential pressure instrument in that it responds to a
difference in the pressure exerted on the liquid limbs A and B. The pressure
should be regarded as absolute pressures. If the higher pressure P1 is applied to
limb A and the lower pressure P2 is applied to limb B, the liquid in limb A will be
forced down and consequently, the liquid level in limb B will rise. This action will
continue, until the pressure P1 is balanced by the sum of pressure P@ and that
pressure due to the column of liquid h between the two levels.
P1 - P2 = G cm
Therefore h = P1 – P2 cm
G
The simple “U” tube manometer is normally used to measure pressure between 0
– 70 cm of water and if the manometer is filled with mercury, it can be used to
measure pressure between 0 – 950 cm of water.
Instrumentation : Document No. 10 Page 3 of 17Created : 01 February 2003 Revision No: 01
Last Approved : March 2003 First Published : March 2003
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CISTERN TYPE MANOMETER
FIGURE – 2
If a differential pressure P1 – P2 is applied to the cistern manometer and column
A1 is the area of the cistern, A2 the area of the narrow tube, and h is the rise of the
liquid from zero level in this tube then:
Instrumentation : Document No. 10 Page 4 of 17Created : 01 February 2003 Revision No: 01
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MODULE - PMM
Volume 1 = Volume 2
Therefore A1. d = A2. h
And Sin α = opposite = dl
Hypotenuse d
Therefore dl = Sin α d
Substituting we get:
(A2 )
P1 – P2 = (A1 + Sin α D) G
Therefore P1 – P2 =Gd (A1 + Sin α)
NOTE: G = S.G. of liquid
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THE INCLINED MANOMETER
Instrumentation : Document No. 10 Page 6 of 17Created : 01 February 2003 Revision No: 01
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MODULE - PMM
FIGURE 3
Instrumentation : Document No. 10 Page 7 of 17Created : 01 February 2003 Revision No: 01
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READING OF MANOMETER
In order to achieve consistent results when reading a manometer, it is
necessary that the fluid meniscus as shown in figure – 4 and figure – 5
always be observed in the same way. A meniscus surface should always
be read at its center, this would be at the bottom of the meniscus in the
case of water or Meriam fluid and at the top or the meniscus in the case of
Mercury.
Figure –4 figure – 5
ADJUSTING THE ZERO OF TH MANOMETER
To have a accurate reading on the manometer, the zero of the manometer
must be checked and adjusted if required each time before using the
manometer. If there is a variation of temperature, then there will be a
variation in the reading of the manometer, due to the expansion or
contraction of the liquid due to the temperature. The zero of the
manometer is adjusted by moving the scale on the manometer either up or
down. The scale is adjusted by turning the zero adjustment knob as
shown in figure – 6 either clockwise, or anti-clockwise until the line marked
Instrumentation : Document No. 10 Page 9 of 17Created : 01 February 2003 Revision No: 01
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“0” on the scale is in line with the center of the fluid meniscus used in the
manometer. It is important that the manometer be vented to atmosphere
at the high-pressure connection during this procedure, shown in figure – 7
FIGURE – 6 FIGURE – 7
CLEANING THE MANOMETER
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Some manometer indicating fluid become oxidized with use, or over a long
period of time react with various gasses, leaving a deposit on the surface
of the fluid, which in turn clings to the glass tube. A clean manometer is
essential for accuracy and readability.
Cleaning the manometer is done by removing the fitting on top of the
manometer shown in figure – 8 and inserting a cleaning brush shown in
figure – 9 from the top of the manometer. The brush must be moved up
and down the full length of the manometer, until the glass tube is clean.
When the glass tube is clean, remove the cleaning brush and replace the
fitting.
FIGURE – 8 FIGURE – 9
FILLING THE MANOMETER
WARNING !! Mercury and Meriam fluid are poisonous. Handle these
fluids with care.
Instrumentation : Document No. 10 Page 11 of 17Created : 01 February 2003 Revision No: 01
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Remove the fill plug on the cistern or well as shown in figure – 10. Ensure
that the manometer is properly vented on the low-pressure side, by
removing the fitting on the top of the manometer. Adjust the scale to the
mid position of the available adjustment. Slowly our the manometer fluid
into the well until the fluid level is approximately at the zero mark on the
scale. Make sure that all of the air bubbles are eliminated. Replace the fill
plug tightly. Replace the top fitting. Adjust the scale for the correct zero
position in relation to the fluid meniscus.
FIGURE – 10
Instrumentation : Document No. 10 Page 12 of 17Created : 01 February 2003 Revision No: 01
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MODULE – PMM
PRESSURE MEASUREMENT USING MANOMETERS
SELF TEST
Question – 1
Name three types of manometers commonly used for pressure
measurement
Question – 2
What type of pressure do the three monometers measure?
Question – 3
What type of manometer will you use to measure very low pressure?
Instrumentation : Document No. 10 Page 13 of 17Created : 01 February 2003 Revision No: 01
Last Approved : March 2003 First Published : March
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Question – 4
What are the two typical ranges that the simple “U” tube manometer is
used to measure?
Question – 5
Why must the zero of a manometer be checked?
Question – 6
Wit the aid of sketches; show the type of meniscus for Meriam fluid and
Mercury. Also show where the meniscus is read on each type of fluid.
Instrumentation : Document No. 10 Page 14 of 17Created : 01 February 2003 Revision No: 01
Last Approved : March 2003 First Published : March
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Question – 7
State the reason why a manometer should be cleaned.
LEARNER
TRAINING OFFICER
DATE
Instrumentation : Document No. 10 Page 15 of 17Created : 01 February 2003 Revision No: 01
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PMM
PRESSURE MEASUREMENT USING MANOMETERS
CRITERION TEST
LEARNER
LEARNER SIGNATURE
AGREEMENT NUMBER
ASSESSOR
SIGNATURE
DATE
RESULT COMPETENT / NOT YET COMPETENT
Question – 1
Name three types of manometers commonly used for pressure
measurement
Question – 2
What type of pressure do the three monometers measure?
Instrumentation : Document No. 10 Page 16 of 17Created : 01 February 2003 Revision No: 01
Last Approved : March 2003 First Published : March
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Question – 3
What type of manometer will you use to measure very low pressure?
Question – 4
Why must the zero of a manometer be checked?
Question – 5
Wit the aid of sketches; show the type of meniscus for Meriam fluid and
Mercury. Indicate how the reading must be taken.
Instrumentation : Document No. 10 Page 17 of 17Created : 01 February 2003 Revision No: 01
Last Approved : March 2003 First Published : March
2003Revised : Owner: Learnership
Dept.