Enrol to start learning
Reading is open to everyone. Enrolling is free, and it is what unlocks the audio lessons, practice tests and progress tracking.
3.1. Barometers and Manometers
Learn content
Interactive Audio Lesson
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Today, we’ll start with barometers. Can anyone tell me what a barometer measures?
It measures atmospheric pressure.
Correct! A barometer helps us understand atmospheric conditions. It mainly uses mercury in a tube to show pressure changes. When the pressure increases, the mercury rises, and when it decreases, it falls. This is often measured in millimeters of mercury, or mmHg. Can someone explain why we use mercury instead of water?
Because mercury is denser than water, so it doesn’t need as much height to measure the same pressure.
Exactly! The density is crucial. A common value for mercury is 13.6 times denser than water. This is why a barometer can show pressure in a more compact column. A handy formula to remember is: Pressure (P) = Density (S) * Height (R). Let’s break that down.
So, if we have 750 mmHg, how do we calculate the pressure?
Good question! If R is 750 mmHg, you multiply by the density of mercury and gravitational acceleration, which gives you the pressure in Pascals. This kind of relationship helps us see atmospheric pressure changes. Let’s move on to how we measure pressure variations.
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Now let's dive into pressure variations in compressible fluids. Who can tell me something about isothermal processes?
Isothermal means constant temperature?
Exactly! During an isothermal process, the temperature remains unchanged. This leads us to the ideal gas law, but before we get into that, let’s imagine an experiment at home where you can fill a balloon with air and observe. When you heat it, what happens?
It expands!
Right! Heat increases pressure within the balloon if volume is constant. Now back to our formula: In isothermal processes for perfect gases, we can derive that... pressure varies according to this formula: p2 = p1e^[-(Mg)/(R * Ts)(Z2-Z1)]. Can anyone remember where these variables come from?
M is the molecular mass, right?
Exactly! Keep this equation in mind as we explore how it applies in practical situations.
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Let’s switch gears to manometers. What types of manometers do you remember?
There's the standard manometer and the differential manometer!
Exactly! Standard manometers measure pressure relative to the atmosphere, while differential manometers measure pressure differences between two points. Can anyone explain how we would calculate the pressure in our water distribution system?
We’d use the height of the water column in relation to the pressure in the system.
Great! If our pressure is 500 kPa in a water system, what height does water rise in the manometer? Remember the density of water is 9800 N/m³. Let’s compute...
It would be height h equals pressure divided by density, right? That’s 500,000 over 9800, which is about 51 m.
Exactly! And that’s why we often use mercury, as its height would be much lower. Keep these applications in mind as they relate heavily to real-world practice.
Overview
Short Summary
This section explores the principles of barometers and manometers, focusing on their functions and applications in measuring pressure.
Medium Summary
The section explains how barometers measure atmospheric pressure and how manometers measure pressure relative to atmospheric pressure or the difference between two points. Key equations and practical applications such as pressure calculations in liquid systems are also discussed.
Detailed Summary
Barometers and Manometers
This section focuses on two crucial devices used in fluid mechanics: barometers and manometers. A barometer measures local atmospheric pressure, often represented in millimeters of mercury (Hg). It operates based on the principle of piezometric head and helps calculate pressures using specific equations. For example, if R equals 750 millimeters of Hg, the atmospheric pressure at a corresponding point—using known fluid properties (density) and gravitational forces—can be derived, yielding approximately 100,000 Pascals.
Additionally, pressure variations in compressible fluids are also briefly examined, highlighting the differences in isothermal processes versus those with temperature gradients. A detailed formula for pressure variation in a perfect gas under isothermal conditions is derived.
The section transitions into discussing standard and differential manometers, which measure pressure relative to the atmosphere and the pressure difference between two points, respectively. By using examples, the concepts are further clarified, emphasizing the practical implications of pressure measurements in fluid systems.
Overall, understanding barometers and manometers is essential for accurately measuring pressure in various applications in fluid mechanics.
Audio Book
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountSo, this is the barometer. This is the principle of working of barometers. Now, a simple question is, what is the local atmospheric pressure when R is 750 millimeters of Hg. This is R we are just going to see we have given the =13.6 then we have assumed incompressible fluid constant, we are going to see how it works.
Detailed Explanation
A barometer is a device that measures atmospheric pressure, which is the weight of the air above us. When we mention 'R is 750 millimeters of Hg', we refer to the height of mercury in the barometer column. The density value (13.6) indicates that mercury is much denser than water (1,000 kg/m³). Hence, a shorter column of mercury can exert the same pressure as a taller column of water.
Examples & Analogies
Imagine a water bottle filled to the brim. If you press down on the water inside, the water will rise in the small opening of the bottle. Similarly, in a barometer, changes in atmospheric pressure push against the mercury, causing it to rise or fall in the tube.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountSo, going to can be written as a S * .. So, pressure will be P = S * R, on calculation it is going to give almost 100,000 Pascal. So, this is one example as well of calculating the pressure p1 but this is also indicating how this barometer system works.
Detailed Explanation
The formula P = S * R indicates that the pressure (P) can be calculated by multiplying the density of the fluid (S) by the height of the fluid column (R). In this case, calculating gives us approximately 100,000 Pascal, which reflects the standard atmospheric pressure at sea level.
Examples & Analogies
Think of a stack of books. The pressure you feel at the bottom book is the weight of all the books pressing down on it. In a similar way, atmospheric pressure is the result of the weight of the air's 'column' above you, measured by how high mercury can rise in a barometer.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountNow, we must also be a little aware about the pressure variations in a compressible fluid. So, there are 2 processes, one is perfect gas at constant temperature isothermal that we have been seeing till now. Secondly perfect gas with constant temperature gradient.
Detailed Explanation
In fluids, especially gases, pressure can change based on temperature and volume. An isothermal process maintains a constant temperature, meaning that if you compress a gas (reducing its volume), the pressure increases. The second process involves a temperature gradient, where temperature may change as the gas is compressed or expanded.
Examples & Analogies
Consider a can of soda. When you shake the can, the gas inside compresses, resulting in higher pressure. Upon opening, the pressure is released, and the temperature changes as the gas cools when it expands into the larger volume of the open air.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountPressure measurement devices, we have discussed one already barometers, there are manometers, a standard manometer or a differential manometer and the pressure transducers. Barometers measure the atmospheric pressure. Manometers, this standard manometer measures the pressure relative to the atmosphere differential the pressure it measures the pressure difference between 2 points.
Detailed Explanation
A manometer is another device used to measure pressure, especially in liquids. A standard manometer measures the pressure of a fluid relative to atmospheric pressure, while a differential manometer compares pressures between two points, providing important information in various scientific applications.
Examples & Analogies
Think of a manometer like a balance scale. Just as a scale can show you the difference in weight between two objects, a manometer shows you the pressure difference in fluids, helping engineers ensure systems operate safely.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountSo, how high would the water rise in a manometer connected to a pipe containing water at 500 kPa? … So, h can be written as p is 500,000 Pascal divided by is 9800 N/m3. So, this will give the height 51 m of water will rise in the manometer.
Detailed Explanation
In this calculation, we determine how high water will rise in a manometer based on a pressure of 500 kPa. By knowing the density of water (or any fluid), we can find the corresponding height (h) using the equation h = pressure/density, resulting in a rise of 51 m, which is very high.
Examples & Analogies
When you use a straw to drink a thick smoothie, you notice it’s hard to suck up the smoothie higher than your mouth. It’s similar with manometers; they reveal how pressure can relate to the height of a fluid, highlighting why we often use less dense fluids like mercury for these measurements to keep heights manageable.
--
Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Barometer:
A tool for measuring atmospheric pressure, utilizing mercury.
- Manometer:
Measures pressure differences between two points or relative to the atmosphere.
- Isothermal Process:
A process that occurs at a constant temperature and affects gas pressure.
- Piezometric Head:
Indicates the elevation and associated potential energy of a fluid.
Examples
Memory aids
Imagine you're at sea level with a glass tube filled with mercury. As the storm approaches, the mercury rises, and you know pressure is dropping—this helps you prepare to stay safe!
Remember 'BARO' for Barometer: B for Barometric pressure, A for Atmospheric pressures it finds, R for Reading mercury levels, O for Observing weather changes.
Flash Cards
Glossary
Barometer
An instrument measuring atmospheric pressure.
Manometer
A device measuring pressure of a fluid relative to atmospheric pressure or the difference between two fluid points.
Isothermal
A process occurring at constant temperature.
Piezometric Head
The potential energy of a fluid due to its elevation.