Pressure Measuring Instruments MCQ Quiz - Objective Question with Answer for Pressure Measuring Instruments - Download Free PDF

Explanation:

Bourdon Tube

• A Bourdon tube is a mechanical device used for measuring pressure. It is one of the oldest and most widely used instruments for pressure measurement. The device operates based on the principle of elastic deformation of a curved tube when subjected to pressure. As the pressure changes, the tube either straightens or curls further, and this mechanical movement is translated into a readable value on a calibrated scale.

Working Principle:

• The Bourdon tube is typically a closed, hollow, and curved tube made of an elastic material such as brass, phosphor bronze, or stainless steel. When the fluid (gas or liquid) whose pressure is to be measured enters the tube, the internal pressure causes the tube to deform. The degree of deformation depends on the pressure exerted by the fluid. The curvature of the tube changes (tends to straighten out) as the pressure increases, and it returns to its original shape when the pressure decreases. The mechanical movement of the tube is transferred to a pointer through a linkage mechanism, which displays the pressure on a calibrated scale.

Advantages:

• Simple and robust design.
• Requires no external power source for operation.
• Wide range of pressure measurement capabilities.
• High reliability and durability in industrial applications.

Disadvantages:

• Not suitable for measuring very low pressures due to limited sensitivity.
• Accuracy can be affected by mechanical wear and tear over time.
• Limited to measuring static or slowly changing pressures.

Applications: Bourdon tubes are extensively used in various industrial and commercial applications, such as:

• Pressure measurement in boilers, pipelines, and hydraulic systems.
• Monitoring pressure in refrigeration and air conditioning systems.
• Measuring fuel pressure in engines.
• Pressure monitoring in medical equipment like oxygen cylinders.

Explanation:-

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-tube 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.

Explanation:

Principle of U-tube Manometer Operation

The operation of a U-tube manometer is primarily based on the principle of hydrostatic equilibrium. This principle states that the pressure at any point in a fluid at rest is the same in all directions. In a U-tube manometer, the pressure difference between two points is determined by the height difference of the liquid column in the two arms of the tube.

Analyzing the Given Options

1. "Venturi effect." (Incorrect)

   • The Venturi effect involves the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. This is not the principle that a U-tube manometer operates on.

2. "Pascal's law." (Incorrect)

   • Pascal's law states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid. While relevant to fluid dynamics, it is not the primary principle behind the U-tube manometer.

3. "Hydrostatic equilibrium." (Correct)

   • Hydrostatic equilibrium is the condition in which a fluid is at rest, and the pressure at any point within the fluid is constant. This principle is the basis for how a U-tube manometer measures pressure differences.

4. "Bernoulli's principle." (Incorrect)

   • Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in the fluid's potential energy or pressure. This principle is not directly applied in the operation of a U-tube manometer.

Explanation:

A U-tube manometer is commonly used to measure pressure differences. In a standard U-tube manometer, the manometric liquid (such as mercury) is denser than the fluid in the pipe.

However, in an inverted U-tube manometer, the purpose is to measure small pressure differences for gases or low-density fluids. Here, the manometric liquid should have a lower specific gravity than the liquid flowing in the pipe to ensure proper differential pressure measurement.

Additional Information

• U-tube manometer (normal type): Uses a liquid with a higher specific gravity (e.g., mercury).

• Inverted U-tube manometer: Uses a liquid with a lower specific gravity than the liquid in the pipe.

• The manometric liquid must be lighter so that the pressure difference can be effectively measured.

Explanation:

A manometer is a device used to measure pressure differences using a column of liquid. The accuracy and working of manometers depend on various factors, including fluid density and temperature.

Statement I: The manometer is suitable for low-pressure applications. (True)

• Manometers are not suitable for very high-pressure applications because the liquid column required for measurement would be too large.

• They are primarily used for low to moderate-pressure measurements in laboratory and industrial applications.

Statement II: It has simple operation and construction. (True)

• A manometer is simple in construction as it consists of a U-tube filled with liquid and operates based on the principle of hydrostatic pressure.

• It requires no moving parts and operates based on the difference in liquid levels.

Statement III: The manometric fluids’ density depends on temperature. Hence, errors may result due to the change in the temperature. (True)

• Density of manometric fluid varies with temperature, leading to errors in pressure measurement.

• If the temperature increases, fluid density decreases, affecting the height of the liquid column and leading to incorrect pressure readings.

Explanation:

A manometer is a device that measures pressure by balancing a column of liquid against a column of gas or another liquid. The liquid used in a manometer should have the following characteristics:

• The liquid should stick on the walls: The liquid used in a manometer should stick to the walls of the tube to prevent it from flowing back and forth due to vibration or turbulence.
• Low surface tension: The liquid used in a manometer should have low surface tension to ensure that the meniscus does not significantly affect the pressure reading.
• It should be immiscible: The liquid used in a manometer should be immiscible with the gas or liquid being measured, to prevent the two fluids from mixing and affecting the accuracy of the pressure measurement.
•  High viscosity is NOT a desirable characteristic for a manometer liquid. A highly viscous liquid will not respond quickly to changes in pressure, leading to slow and inaccurate readings. Thus, manometer liquids are typically chosen to have relatively low viscosity.

Explanation:-

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-tube 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.

Concept:

In an open tube manometer

• The pressure at both the open ends is atmospheric.
• The pressure at any point inside the column can be calculated from either side.

Calculation:

Given:

ρ_water = 1000 kg/m3, l = 80 mm and d = 20 mm

So, the pressure at the bottom of the oil column can be equated from either end to find the required value of ρ_oil.

ρ_oil × g × (d + l) = ρ_water × g × l

ρ_oil × (20 + 80) = 1000 × 80

ρ_oil = 800 kg/m3

Hence the required density of oil is 800 kg/m3.

Difficulty in construction: 

• Manometers are relatively simple devices and their construction is not particularly complex. They typically consist of a U-shaped tube filled with a suitable fluid, making them much easier to construct compared to many other pressure-measuring devices.

Need for leveling:

• Manometers must be perfectly level to obtain accurate measurements.
• This is because the measurement is based on the difference in height of the liquid in the two arms of the manometer, and any tilt in the apparatus can skew this difference, leading to inaccurate measurements.

No over-range protection:

• Over-range occurs when the pressure applied to the manometer exceeds its measurement range.
• Manometers don’t have built-in protections against this, and a significant over-range can cause the liquid within the manometer to overflow, leading to faulty readings or damage to the manometer.

Large and bulky size:

• Compared to other pressure measuring devices, such as electronic pressure transducers, manometers can be bulky and take up more space because of the U-shaped tube design.
• Furthermore, the need to visually read the liquid column height can also necessitate a certain minimum size for the manometer.
• These properties can limit their use in locations where space is a constraint.

Explanation:

Given:

ρ1 = Specific gravity of mercury =  13.6, ρ2 =Specific gravity of kerosene =  0.8, h= 40 cm  

Let h₁ on the left side be the level of mercury, before pouring of kerosene

After pouring kerosene, 40 cm height is taken by kerosene, mercury is lowered by h₁ on the left side and mercury on the right side raises by h₁ from its original position, ie when kerosene was not poured 

ρ₁ = 13.6, ρ₂ = 0.8, h= 40 cm 

Writing the pressure equation for the above diagram, according to the final location of the mercury and kerosene, we get

40 × G for kerosene - 2 × h₁ × G for mercury = 0, (pressure at level B and B^' is the same, as they are at the same level)

40 × 0.8 = 2 × h₁ × 13.6

h₁ = 1.17 cm ≈ 1.2 cm

Explanation:

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

Pgauge = 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.

Po + ρAgh1 = Po + ρBgh2

where ρA and ρB 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

ρAh1 = ρBh2

\(\frac{{{{\rm{\rho }}_A}}}{{{{\rm{\rho }}_B}}}\; = \;\frac{{{{\rm{h}}_2}}}{{{{\rm{h}}_1}}}\)

\(\frac{{{{\rm{\rho }}_A}}}{{{{\rm{\rho }}_B}}}\; = \;\frac{{0.3}}{{0.5}} = 0.6\)

Explanation:

U-tube Manometer:

(i) It consists of a glass tube in U shape, one end of which connected to the gauge point and another end open to the atmosphere.

(ii) The tube contains a liquid of specific gravity greater than that of the fluid of which the pressure is to be measured.

(iii) The choice of the manometric liquid depends on the range of pressure to be measured For the low-pressure range, liquid of lower SG is used and for the high range, generally, mercury is used.

(iv) A U-tube manometer is used inverted if the pressure differential is small (density of the manometric fluid is less than fluid) and A U-tube manometer is used upright if the pressure differential is large (density of the manometric fluid is large)

Limitation:

• This method requires reading of fluid level at two or more points since the change in pressure causes a rise in one limb and drop in another.

Explanation:

Bourdon Gauge: 

• If the gauge pressures are very high, then a manometer may not be effective, and so the measurement can be made using Bourdon Gauge.
• This gauge consists of a coiled metal tube that is connected at one end to the vessel where the pressure is to be measured.
• The other end of the tube is closed so that when the pressure in the vessel is increased, the tube begins to uncoil and respond elastically.
• Using the mechanical linkage attached to the end of the tube, the dial on the face of the gauge gives a direct reading of the pressure, which can be calibrated.

Manometer:

• A manometer consists of a transparent tube that is used to determine the gauge pressure in a liquid.
• The simplest type of manometer is called a piezometer.
• The tube is open at one end to the atmosphere, while the other end is inserted into the vessel, where the pressure of a liquid is to be measured.
• It do not work well for measuring large guage pressures, since 'h' would be large.

Pitot tube:

• Pitot tube is used to determine the velocity of a moving liquid in an open channel such as a river.

Important Points

Manometers can be used to measure small pressure in pipes or tanks or differential pressures between points in two pipes.

Explanation:

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.

Important PointsIncreasing order of sensitivity ⇒

Piezometer < U tube Manometer < Single Column Manometer < Inclined single column manometer.

Explanation:

A Bourdon gauge measures the gauge pressure relative to the pressure of the surrounding medium i.e. local atmospheric pressure and not relative to the standard atmospheric pressure. 

P_abs = P_atm + P_gauge

(P_abs)_Q = P_atm + (P_Q)_gauge + (P_P)_gauge

Bourdon Gauge:

• Bourdon Tubes are known for their very high range of differential pressure measurement in the range of almost 100,000 psi (700 MPa). It is an elastic type pressure transducer.
• The basic idea behind the device is that cross-sectional tubing when deformed in any way will tend to regain its circular form under the action of pressure.
• The bourdon pressure gauges used today have a slightly elliptical cross-section and the tube is generally bent into a C-shape or arc length of about 27 degrees.

A detailed diagram of the bourdon tube is shown below.

• The elliptical cross-section ensures that it has a different modulus of elasticity in its cross-section.
• When pressure is experienced inside the tube the deflection is more pronounced across the major axis of the elliptical section hence the deflection is more measurable.
• The deflection is directly proportional to the pressure inside the bourdon tube. This is then calibrated across a dial to indicate the pressure