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 of 17Created : 01 February 2003 Revision No: 01

Last Approved : March 2003 First Published : March 2003

Revised : Owner: Learnership Dept.



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.






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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.






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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:






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






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FIGURE 3






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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

Last Approved : March 2003 First Published : March

2003Revised : Owner: Learnership

Dept.



MODULE - PMM

“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




Dept.



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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.




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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




Dept.



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MODULE – PMM


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?




Dept.



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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.




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Question – 7

State the reason why a manometer should be cleaned.

LEARNER

TRAINING OFFICER

DATE




Dept.



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PMM


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?




Dept.



MODULE - PMM

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.




Dept.

Module - Pmm

Documents

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