Test: Barometer & Piezometer - Civil Engineering (CE) MCQ

Barometer: Long ago in the mid-17th century, an Italian scientist by the name of Evangelista Torricelli decided to create a device to measure atmospheric pressure. This device is called the barometer.

• The function of a barometer is to let you know whether or not atmospheric pressure is rising or falling. 

Mercury: It is the only common metal that is liquid at ordinary temperatures.

• Mercury is sometimes called quicksilver. It is a heavy, silvery-white liquid metal. It is a rather poor conductor of heat if compared with other metals but it is a fair conductor of electricity,
• It alloys easily with many metals, such as gold, silver, and tin. These alloys are called amalgams
• Mercury has a melting point of -38.9°C, a boiling point of 356.7°C. and is the only metal to remain in liquid form at room temperature.
• Droplets of liquid mercury are shiny and silver-white with high surface tension.

Mercury is used in Barometer for the following reasons:

• Mercury is denser. Its relative density is 13.534 times that of water. So to measure the atmospheric pressure, which is 760 mm of mercury you need a barometer, with mercury of length slightly more than 0.76 meters, say one meter to cover the special cases of some higher times the length of mercury barometer, which may be more than 11 meters in length.
• Further mercury, being metal has a shining quality which highlights it's reading clear.
• Also, mercury, having comparatively lower specific heat and a good conductor of heat (low melting point), could come to the same temperature of the atmosphere more quickly.
• Hence option 4 is the correct option.

At base P_B = 750 mm of Hg
At top, P_A = 600 mm of Hg
∴ Pressure difference = 150 mm of Hg = 150 × 10^-3 × 13.6 × 103 × 9.81
(ΔP) = hρg

= 2040m
⇒ Height of mountain = 2040 m

• A hydrometer is used to find the specific gravity or relative density of liquids.
• A barometer is used to measure local atmospheric pressure.
• U - tube manometer is used to find pressure difference across two points.
• Bourdon gauge is used to measure gauge pressure.

A pressure gauge is a mechanical instrument designed to measure the internal pressure and vacuum of a system. Types of pressure gauge:

• Dead weight pressure gauge: A dead weight pressure gauge tester apparatus applies pressure to a fluid using known traceable weights to check the accuracy of pressure gauge readings. A dead weight tester (DWT) is a calibration standard method that involves placing a load on a piston-cylinder and applying pressure beneath the piston to achieve equilibrium. In other words, a deadweight pressure gauge is used for calibration of the other pressure measuring gauges.
• Bourdon tube pressure gauge: This is used for the measurement of relative pressures from 0.6 - 7,000 bar. They are classified as mechanical pressure measuring instruments and thus operate without any electrical power. The Bourdon pressure gauge uses the principle that a flattened tube tends to straighten or regain its circular form in cross-section when pressurized.
• Diaphragm pressure gauge: It is a device that uses the deflection of a flexible membrane that separates regions of different pressure. The amount of deflection is repeatable for known pressure so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to the atmosphere to measure gauge pressure, open to the second port to measure differential pressure and or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure.
• Manometer gauge: 
• It is the device used for the difference of pressure between two points in a pipe or two different pipes. The most common type of differential manometers are the U-tube differential manometer and the Inverted U-tube differential manometer.

• Piezometer is the simplest form of manometer used for measuring gauge pressure
• One end of this manometer is connected to the point where the pressure is to be measured and the other end is open to the atmosphere
• As one end is open to atmosphere so it can’t be used for measuring the gas pressure
• Gas pressure cannot be measured by means of piezometers because a gas forms no free atmosphere surface and it can’t be used when large pressures in the lighter liquid are to be measured

Concept:

• Pressure: The force per unit area is called pressure. It is denoted by P.
• Piezometer: A device that is used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity or a device that measures the pressure (more precisely, the piezometric head) of groundwater at a specific point is called piezometer.

Limitations of Piezometer

1. Piezometer cannot be used to measure the very high pressure of a liquid, because in that case a long glass tube will be required. A long glass tube is inconvenient to be used.
   • Mathematically, we know
   • Pressure : p = ρgh
   • or p ∝ h
   • Thus, at the same liquid, pressure head: h increases with an increase in pressure; p.
2. It cannot be used to measure the gauge pressure of a gas i.e. leakage of gas through a piezometer.
3. It cannot measure the vacuum pressure. Instead of a column of liquid going up in the tube, the atmospheric air will enter into the pipe through the glass tube. Hence, it can neither measure the high pressure nor the vacuum pressure of a liquid.

• Piezometer is the simplest form of manometer used for measuring gauge pressure
• One end of this manometer is connected to the point where the pressure is to be measured and the other end is open to the atmosphere
• As one end is open to atmosphere so it can’t be used for measuring the gas pressure
• Gas pressure cannot be measured by means of piezometers because a gas forms no free atmosphere surface and it can’t be used when large pressures in the lighter liquid are to be measured

Concept:
Applying pressure equation
P + (ρwgh) - (ρmgh) = 0 
Calculation:
Given:
ρw = 1000 kg/m³, ρm = 13.6 × 103 kg/m³, h = 30 cm = 0.3 m 
⇒ P + 0.30 × 9810 – 0.30 × 13.6 × 9810 = 0
⇒ P = 3.78 m of water
P= 3.78 × 10 × 1000 = 37.8 kPa

• Piezometer is the simplest form of manometer used for measuring gauge pressure
• One end of this manometer is connected to the point where the pressure is to be measured and the other end is open to the atmosphere
• As one end is open to the atmosphere so it can’t be used for measuring the gas pressure
• Gas pressure cannot be measured by means of piezometers because a gas forms no free atmosphere surface and it can’t be used when large pressures in the lighter liquid are to be measured

Velocity of a fluid flowing through the pipe is measured by

• Current and turbine meter
• Wire anemometer
• Pitot tube

Piezometer: 

• A device that is used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity or a device that measures the pressure (more precisely, the piezometric head) of groundwater at a specific point is called piezometer.

• One end of the piezometer is connected to the point where the pressure is to be measured and the other end is open to the atmosphere.