Test: Simple Manometer - Civil Engineering (CE) MCQ

According to Pascal’s law, the local pressure of a fluid is same in all directions. Hence, the pressure won’t vary along the x and y direction. The local pressure will increase with an increase in depth due to the extra weight of water column above that point.

The options may tempt you to subtract the readings, but the concept of barometer and manometer is important. Barometer measures the atmospheric pressure whereas, the manometer reads the gauge pressure. Hence, we need to add the two values.

Since the gauge pressure remains the same ρ*(h₂ – h₁) = constant. The height difference in mercury manometer is 0.30 m and that in a water manometer is 4.08 m. Percent change is thus, 1260%. Be careful about the denominator used for computing percent change.

Pressure along the dotted line will be 50 kPa. Gauge pressure in an inclined manometer is given by
P = ρ.g.h.sin (Ɵ).
Substituting P, ρ and Ɵ,
we get the value of h as 0.75 m.

Single column manometer:
(i) Single column manometer is a modified form of a U-tube manometer in which one side is a large reservoir and the other side is a small tube, open to the atmosphere.
There are two types of single-column manometer:

• Vertical single-column manometer.
• Inclined single-column manometer.

Vertical single column manometer:

Inclined single column manometer:
This manometer is more sensitive. Due to the inclination, the distance moved by the heavy liquid in the right limb will be more.

Single column manometer:
(i) Single column manometer is a modified form of a U-tube manometer in which one side is a large reservoir and the other side is a small tube, open to the atmosphere.
There are two types of single-column manometer:

• Vertical single-column manometer.
• Inclined single-column manometer.

U tube manometers:

• A U-tube manometer is the simplest pressure measurement device.
• Its name comes from the U-shape formed when the two ends of a flexible tube full of liquid are raised to keep the liquid from coming out the ends
• .A U-tube manometer is a ‘liquid’ balance. 

U tube differential manometer:

• A U tube differential manometer is a type of differential manometer which is used to measure the difference of pressure between the two points of the pipes.

Manometer - 
A manometer is an instrument that uses a column of liquid to measure pressure, although the term is currently often used to mean any pressure instrument.
Two types of manometer, such as
1. Simple manometer
A simple manometer consists of a glass tube having one of its ends connected to a point where pressure is to be measured and the other end remains open to the atmosphere. Common types of simple manometers are:

• Piezometer
• U tube manometer
• Single Column manometer

2. Differential manometer
Differential Manometers are devices used for measuring the difference of pressure between two points in a pipe or in two different pipes. A differential manometer consists of a U-tube, containing a heavy liquid, whose two ends are connected to the points, which difference in pressure is to be measured.
The most common types of differential manometers are:

• U-tube differential manometer.
• Inverted U-tube differential manometer

3. Inclined U-tune manometer - 
If the pressure to be measured is very small. It is more accurate than a U-tube manometer.
Then tilting the arm provides a convenient way of obtaining a larger (more easily read) movement of the manometer.
The pressure difference is still given by the height change of the manometric fluid(z₂).

The sensitivity to pressure change can be increased further by a greater inclination of the manometer arm.
An alternative solution to increase sensitivity is to reduce the density of the manometric fluid.

• For amplifying the deflection in a liquid column manometer, liquids with greater density could be used or one of the limbs of the manometer may be inclined.
• Commonly used manometric liquids are mercury, water, or alcohol.
• Some of the important and desirable properties of the manometric liquids are:

1. High chemical stability
2. Low viscosity
3. Low capillary constant
4. Low coefficient of thermal expansion
5. Low volatility
6. Low vapour pressure
7. Low surface tension
8. Sticky to wall
9. Immiscible

• Using Hydrostatic law which states the variation of pressure in the vertical direction in a fluid is equal to the specific weight.

P_gauge = ρgh

• As we move vertically down in a fluid, the pressure increases as +ρgh.

• As we vertically move up in a fluid, the pressure decreases as -ρgh.

P_o + ρ_Agh₁ = P_o + ρ_Bgh₂
where ρ_A and ρ_B are densities of fluid A and B, h₁ and h₂ are heights of fluid in limbs 1 and 2, Po is the atmospheric pressure.
Calculation:
Given: 
h₁ = 0.5 m and h₂ = 0.3 m
ρ_Ah₁ = ρ_Bh₂

A simple manometer consists of a glass tube having one of its ends connected to a point where pressure is to be measured and another end remains open to the atmosphere.
Common types of simple manometers are:

• Piezometer
• U-tube Manometer
• Single Column Manometer

Differential Manometers measure the difference of pressure between two points in a fluid system and cannot measure the actual pressures at any point in the system. It consists of a U-tube, containing a heavy liquid, whose two ends are connected to the points whose difference of pressure is to be measured.

• Upright U-Tube manometer
• Inverted U-Tube manometer
• Inclined Differential manometer
• Micro manometer