11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement
3 min read•june 18, 2024
Pressure is a fundamental concept in physics, crucial for understanding fluid behavior and many everyday phenomena. From tire gauges to weather forecasts, pressure measurements are all around us.
Gauge and , along with various measurement devices like manometers and barometers, help us quantify and compare pressures. Understanding pressure units and conversions is essential for practical applications in engineering and daily life.
Pressure Concepts
Gauge vs absolute pressure
(Pg) measures pressure relative to ambient
Can be positive (above atmospheric pressure) or negative (below atmospheric pressure)
Commonly used for measuring tire pressure, blood pressure, and pressure in sealed systems (hydraulic, pneumatic)
Absolute pressure (Pa) measures pressure relative to a perfect vacuum (zero pressure)
Always positive because it includes atmospheric pressure
Related to by the equation: Pa=Pg+P[atm](https://www.fiveableKeyTerm:atm), where Patm is the atmospheric pressure
Used in calculations involving the ideal gas law and thermodynamic processes (Boyle's law, Charles's law)
Pressure measurement devices
Manometers measure pressure difference between two points using a U-shaped tube partially filled with liquid (mercury, water)
Pressure difference is calculated using the formula: ΔP=ρgh
ρ represents the density of the liquid
g is the acceleration due to gravity (9.81 m/s²)
h is the height difference between the two liquid columns
Open-tube manometers have one end open to the atmosphere, measuring gauge pressure
Closed-tube manometers have both ends connected to sealed systems, measuring differential pressure
Barometers measure atmospheric pressure
Mercury barometers consist of a glass tube filled with mercury, inverted in a mercury reservoir
Vacuum exists at the top of the sealed tube
Height of the mercury column is proportional to atmospheric pressure (760 at sea level, 0°C)
Aneroid barometers use a sealed, flexible metal chamber (aneroid capsule) that deforms with changes in pressure
Deformation of the capsule is mechanically amplified and displayed on a calibrated scale
Commonly used in portable weather stations and altimeters (aircraft, hiking)
convert pressure into an electrical signal for digital measurement and monitoring
Pressure unit conversions
(Pa) is the SI unit of pressure, defined as 1 Newton per square meter (1 N/m²)
Kilopascal () is commonly used, equal to 1,000 Pa
Atmosphere (atm) is a non-SI unit of pressure, defined as the average atmospheric pressure at sea level
1 atm = 101,325 Pa = 101.325 kPa
1 atm = 760 mmHg at 0°C
Millimeters of mercury (mmHg) is a non-SI unit of pressure based on the height of a mercury column
1 mmHg ≈ 133.322 Pa
760 mmHg = 1 atm at 0°C
To convert between pressure units, use conversion factors
Example: Convert 1,520 mmHg to atm
1,520 mmHg × (1 atm / 760 mmHg) = 2 atm
Example: Convert 3 atm to kPa
3 atm × (101.325 kPa / 1 atm) = 303.975 kPa
Fluid Statics and Pressure
occurs when fluid is at rest and pressure forces are balanced
refers to the height of a fluid column that would create a given pressure
affects how pressure changes with depth in a fluid
Key Terms to Review (30)
Absolute Pressure: Absolute pressure is the total pressure exerted on a surface, including both the atmospheric pressure and any additional pressure from a force or fluid. It represents the actual pressure at a given point, rather than the pressure relative to the surrounding atmosphere.
Aneroid Barometer: An aneroid barometer is a type of pressure-measuring instrument that determines atmospheric pressure without the use of liquids. It relies on the measurement of the expansion and contraction of a sealed metal capsule in response to changes in air pressure, providing a direct reading of the pressure value.
Archimedes' Principle: Archimedes' Principle states that the buoyant force exerted on an object immersed in a fluid is equal to the weight of the fluid displaced by the object. This principle has important applications in understanding the behavior of objects in fluids, including density, pressure, and convection.
Atm: atm, short for atmosphere, is a unit of pressure that is commonly used to measure and describe the pressure exerted by the Earth's atmosphere. It is a fundamental concept in the study of physics, particularly in the topics of pressure, gauge pressure, absolute pressure, and pressure measurement.
Atmospheric Pressure: Atmospheric pressure is the force exerted by the weight of the Earth's atmosphere on the surface of the planet. It is a fundamental concept in physics that is closely related to the study of fluids and their behavior.
Bar: A bar is a unit of pressure defined as exactly 100,000 pascals, commonly used in various scientific and engineering contexts. This unit is particularly relevant when discussing the measurement of pressure, as it helps differentiate between absolute and gauge pressures. Understanding the concept of a bar is essential for interpreting pressure readings in applications ranging from meteorology to engineering.
Barometer: A barometer is an instrument used to measure atmospheric pressure, which is the force exerted by the weight of the air above a given surface area. Barometers play a crucial role in understanding and predicting weather patterns, as changes in atmospheric pressure are closely linked to weather conditions.
Blaise Pascal: Blaise Pascal was a renowned 17th century French mathematician, physicist, inventor, and philosopher. He is best known for his contributions to the fields of hydrostatics and fluid mechanics, which laid the foundation for important principles and concepts related to pressure and its applications.
Closed-Tube Manometer: A closed-tube manometer is a device used to measure pressure in a system by comparing the pressure of the system to a known reference pressure. It consists of a U-shaped tube filled with a liquid, typically mercury or water, and is used to determine the absolute or gauge pressure of a fluid or gas.
Compressibility: Compressibility is a measure of how much a fluid or material can be reduced in volume when subjected to an external pressure. It is a fundamental property that describes the ability of a substance to decrease in volume when a force is applied to it.
Diastolic pressure: Diastolic pressure is the minimum arterial pressure during the relaxation phase of the heart cycle when the heart ventricles are filled with blood. It's typically measured in millimeters of mercury (mmHg).
Evangelista Torricelli: Evangelista Torricelli was an Italian physicist and mathematician who is best known for his pioneering work in the field of pressure measurement. His invention of the barometer and his groundbreaking experiments on atmospheric pressure laid the foundation for our understanding of gauge pressure, absolute pressure, and pressure measurement.
Fluid Statics: Fluid statics is the study of fluids at rest, focusing on the principles that govern the behavior of stationary liquids and gases. It explores the concepts of pressure, buoyancy, and the factors that influence the distribution of pressure within a fluid system.
Gauge pressure: Gauge pressure is the pressure relative to atmospheric pressure. It is the difference between absolute pressure and atmospheric pressure.
Gauge Pressure: Gauge pressure is the pressure measured relative to the surrounding atmospheric pressure. It represents the pressure above or below the ambient atmospheric pressure at a given location.
Hydrostatic Equilibrium: Hydrostatic equilibrium is a state of balance where the pressure exerted by a fluid on its surroundings is constant and does not change over time. This concept is fundamental to understanding gauge pressure, absolute pressure, and pressure measurement in the context of fluid mechanics.
Hydrostatic Pressure: Hydrostatic pressure is the pressure exerted by a fluid, such as a liquid or gas, at rest. It is the pressure that arises due to the weight of the fluid itself and is directly proportional to the depth of the fluid. Hydrostatic pressure is a fundamental concept that underpins the understanding of various topics in physics, including pressure, variation of pressure with depth, gauge pressure, Archimedes' principle, pressures in the body, and Bernoulli's equation.
KPa: kPa, or kilopascal, is a unit of pressure in the International System of Units (SI). It is used to measure and quantify the amount of force exerted per unit area, which is a fundamental concept in the study of physics and various scientific and engineering applications.
Manometer: A manometer is a device used to measure pressure, particularly the pressure of fluids or gases. It is a crucial instrument in the study of fluid mechanics and is closely related to the concepts of pressure, variation of pressure with depth in a fluid, and the measurement of both gauge and absolute pressure.
Mercury Barometer: A mercury barometer is a device used to measure atmospheric pressure. It consists of a glass tube filled with mercury that is inverted in a mercury-filled reservoir, with the height of the mercury column indicating the pressure of the surrounding air.
MmHg: mmHg, or millimeters of mercury, is a unit of pressure that measures the height of a column of mercury in millimeters under the influence of gravity. This measurement is commonly used in various fields, including meteorology and medicine, to express pressure values. Understanding mmHg is crucial for comparing different types of pressure, including gauge and absolute pressure, and is vital for accurate pressure measurement in scientific applications.
Open-Tube Manometer: An open-tube manometer is a simple device used to measure pressure by comparing the height of a liquid column in a U-shaped tube. It is a fundamental tool for measuring gauge pressure, absolute pressure, and other pressure-related quantities.
Pascal: Pascal is a unit of pressure, which is the force applied perpendicular to a surface per unit area. It is a fundamental concept in physics that is closely tied to the study of fluids, gases, and the behavior of materials under stress and strain.
Pascal's Principle: Pascal's principle states that in a fluid, pressure applied to any part of the fluid is transmitted equally to all parts of the fluid. This means that when a force is applied to a fluid, the pressure increases equally throughout the fluid, and this increased pressure is exerted on all surfaces in contact with the fluid.
Pressure Head: Pressure head, also known as static head, is a measurement of the pressure exerted by a column of fluid at a given depth. It is the pressure that would be experienced at a specific point in a fluid, such as a liquid or gas, due to the weight of the fluid above that point. This concept is fundamental to understanding gauge pressure, absolute pressure, and pressure measurement in physics.
Pressure Transducers: A pressure transducer is a device that converts pressure into an electrical signal. It is used to measure and monitor pressure in a variety of applications, including in the context of gauge pressure, absolute pressure, and general pressure measurement.
Systolic pressure: Systolic pressure is the maximum pressure exerted on the walls of arteries during the contraction phase of the heart's cycle. It is typically measured in millimeters of mercury (mmHg).
Torr: Torr is a unit of pressure measurement that is commonly used in the context of vacuum and gas pressure. It is named after the Italian scientist Evangelista Torricelli, who invented the barometer. The torr is defined as the pressure exerted by a column of mercury 1 millimeter high at the standard acceleration of gravity.
ΔP: ΔP, or the change in pressure, refers to the difference between two pressure values, typically experienced within a fluid system. This concept is crucial in understanding gauge pressure and absolute pressure measurements, as well as how pressure affects energy transfer in thermodynamic processes. By calculating ΔP, one can determine how systems react to pressure changes and how those changes influence work done on or by the system.
ρgh: ρgh, where ρ (rho) represents the density of a fluid, g is the acceleration due to gravity, and h is the height or depth of the fluid, is a fundamental expression that describes the pressure exerted by a fluid column. This term is crucial in understanding the concepts of gauge pressure, absolute pressure, and pressure measurement in physics.