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# Fluid Mechanics MCQ – Measurement of Pressure, Simple and Differential Manometers

The right limb of a simple U-tube manometer containing mercury is open to the atmosphere while the lift limb is connected to a pipe in which a fluid of specific gravity 0.85 is flowing. The centre of the pipe is 14 cm below the level of mercury in the right limb.Evaluate the pressure of fluid flowing in the pipe if the difference of mercury level in the two limbs is 22 cm.

a) 2.86 N/cm2

b) 5.73 N/cm2

c) 1.43 N/cm2

d) None of the mentioned

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Answer: a

Explanation: Pressure at centre of pipe + Pressure at depth 8 cm in left limb = Pressure at depth 22 cm in right limb

P = 13600×9.81×0.22 – 850×9.81×.08

= 2.86 N/cm2.

A single coloumn manometer is connected to a pipe containing a liquid of specific gravity 0.75. Find the pressure in the pipe if the area of reservoir is 250 times the area of tube for the manometer reading. The difference in mercury level is 40 cm. On the left limb the fluid is upto the height of 20 cm.

a) 10.42 N/cm2

b) 5.21 N/cm2

c) 2.60 N/cm2

d) None of the mentioned

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Answer: b

Explanation: Pressure = a/A height × (density of mercury × 9.81-density of fluid × 9.81)+ height in right limb × density of mercury × 9.81 – height in left limb × density of fluid × 9.81

= 5.21 N/cm2

{ Here a/A = 1/ 250}.

A Differential manometer is connected at the points A and Bat the centre of two pipes. The pipe A(left limb) contains a liquid of specific gravity = 1.5 while pipe B (right limb)contains a liquid of specific gravity 0.85. The pressure at A and B are .5 kgf/cm2 and 1.2 kgf/cm2 respectively. Find the difference in level of mercuru in the differential manometer. A is 2.5m above B and 5 m above the mercury in its own limb. B is 2.5 m above the mercury level in limb A.

a) 12.7 cm

b) 25.5 cm

c) 6.28 cm

d) 10.85 cm

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Answer: a

Explanation: Total pressure at the datum line in limb A = Total pressure at the datum line in limb B\

0.5*9.81*10000 + 5*9.81*1500 + h*9.81*13600 = 1.2*9.81*10000 + (h+2)*9.81*850

After solving,

h=12.7 cm.

An inverted differential manometer is connected to two pipes A and B which covey water. The fluid in manometer is oil of specific gravity 0.75. For the manometer readings, find the pressure difference between A and B. Datum in left limb is 40 cm above point A. Point B is 60 cm below datum line. Difference in level of fluid is 20 cm.

a) 1471 N/m2

b) 2943 N/m2

c) 735.75 N/m2

d) None of the mentioned

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Answer: a

Explanation: Total pressure at the datum line in limb A = Total pressure at the datum line in limb B

Pressure difference between A and B = -0.4*9.81*100 + 0.2*9.81*750 + 0.4*9.81*1000

= 1471 N/m2.

In the inverted U-tube Differential manometer, how is the specific gravity of manometric fluid used relative to the fluid flowing in the pipes

a) Specific gravity is more than that of fluid flowing in pipes

b) Specific gravity is less than that of fluid flowing in pipes

c) Specific gravity is equal to that of fluid flowing in pipes

d) None of the mentioned

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Answer: b

Explanation: In the inverted U-tube Differential manometer, specific gravity of manometric fluid used is less than relative to the fluid flowing in the pipes as the manonmetric fluid is at the top.

Why is large reservoir used in single column manometer?

a) In order to enhance the change in level of liquid in reservoir

b) In order to negate the effects of change in level due to pressure variation

c) In order to reduce the effect due to dynamic pressure variation due to motion

d) None of the mentioned

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Answer: b

Explanation: Single column manometer directly gives the pressure by measuring the height in the other limb and due to large cross sectional area of the reservoir, for any variation in pressure, the change can be neglected.

Manometers are the pressure measuring devices which use the principle of dynamic pressure to measure the pressure difference.

a) True

b) False

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Answer: b

Explanation: Manometers are the pressure measuring devices which use the principle of pressure due to static fluid (i.e the column height) to measure the pressure difference.

The distance moved by liquid will be more in which type of manometer?

a) Inclined Single coloumn manometer

b) Vertical Single coloumn manometer

c) Horizontal Single coloumn manometer

d) None of the mentioned

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Answer: a

Explanation: The distance moved by liquid will be more in Inclined Single column manometer due to its inclination.

Differential manometer gives the pressure reading with respect to atmospheric pressure.

a) True

b) False

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Answer: b

Explanation: Differential manometer gives the pressure difference between the fluid flowing in two pipes with respect to each other.

Which device is popularly used for measuring difference of low pressure?

a) Inverted U-tube Differential Manometer

b) U-tube Differential Manometer

c) Inclined Single column manometer

d) Vertical Single column manometer

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Answer: a

Explanation: Inverted U-tube Differential Manometer has lighter manometric fluid, Hence it is used for measuring the low pressure difference.