When discussing gas or liquid pressure, three common terms are often used: atmospheric pressure, gauge pressure, and absolute pressure. Understanding these concepts is essential in fields like engineering, meteorology, and fluid dynamics. Let’s break down each term and explore how they are related.
1. Atmospheric Pressure
Atmospheric pressure is the pressure exerted by the weight of the air in Earth’s atmosphere on any surface. At sea level, atmospheric pressure is approximately 101.3 kPa (kilopascals) or 1 standard atmosphere (atm). It varies slightly with altitude and weather conditions, but 101.3 kPa is generally used as a reference for calculations.
- 1 atm = 101.3 kPa
- Atmospheric pressure is the pressure exerted by the air around us.
2. Gauge Pressure
Gauge pressure is the pressure measured relative to atmospheric pressure. It essentially ignores the weight of the air and measures only the pressure above or below atmospheric pressure. For example, the pressure in a car tire is typically measured as gauge pressure. When the gauge pressure reads zero, it means the pressure inside the system equals atmospheric pressure.
- Gauge pressure = pressure measured relative to atmospheric pressure.
- At atmospheric pressure, gauge pressure = 0 kPa.
Example: The pressure gauge on a car tire might read 200 kPa. This is the pressure over and above the atmospheric pressure.
3. Absolute Pressure
Absolute pressure is the total pressure exerted on a system, measured relative to a complete vacuum (where no pressure exists). It is the sum of the atmospheric pressure and the gauge pressure. Absolute pressure gives a true measure of the force being applied, since it includes both the atmospheric pressure and any additional pressure inside the system.
- Absolute pressure = gauge pressure + atmospheric pressure.
- It reflects the total pressure on an object, including the atmosphere.
Relationship Between These Pressures:
The relationship between absolute pressure, gauge pressure, and atmospheric pressure can be expressed using the following equation:
Absolute Pressure=Gauge Pressure+Atmospheric Pressuretext{Absolute Pressure} = text{Gauge Pressure} + text{Atmospheric Pressure}Absolute Pressure=Gauge Pressure+Atmospheric Pressure
- When gauge pressure is zero (such as when the pressure in the system is equal to atmospheric pressure), absolute pressure equals atmospheric pressure.
- If the pressure inside a system is lower than atmospheric pressure (a vacuum), gauge pressure becomes negative, but absolute pressure remains positive as long as it is greater than zero.
Example:
Let’s consider a container at sea level, where atmospheric pressure is 101.3 kPa:
- The gauge pressure inside the container is measured as 50 kPa.
Using the formula:
Absolute Pressure=50 kPa+101.3 kPa=151.3 kPa
In this case, the absolute pressure in the container is 151.3 kPa.
Practical Insights
- Gauge pressure is commonly used in everyday applications like tire pressure or water pumps because it reflects how much pressure is being exerted beyond the surrounding atmosphere.
- Absolute pressure is important in scientific calculations and for understanding the total force exerted on a system, especially in environments like space or under high-pressure conditions.
Understanding the distinction between these types of pressures is crucial for correctly interpreting pressure readings and applying them to real-world situations.
Pressure Unit Conversion Table and Detailed Explanation
Pressure is a critical concept in science and engineering, and it’s measured in various units depending on the context. Below is a detailed guide to common pressure units, their symbols, and how they convert to one another.
Common Pressure Units and Conversion Table
| Unit | Symbol | Conversion Relationship |
|---|---|---|
| Pascal | Pa | 1 Pa = 1 N/m² |
| Kilopascal | kPa | 1 kPa = 1,000 Pa |
| Megapascal | MPa | 1 MPa = 1,000 kPa = 1,000,000 Pa |
| Standard Atmosphere | atm | 1 atm = 101,325 Pa |
| Torr (Millimeters of Mercury) | Torr (mmHg) | 1 Torr = 133.322 Pa |
| Bar | bar | 1 bar = 100,000 Pa = 100 kPa |
| Pounds per Square Inch | psi | 1 psi = 6,894.76 Pa |
| Kilogram-force per Square Centimeter | kgf/cm² | 1 kgf/cm² = 98,066.5 Pa |
| Meter of Water Column | mH₂O | 1 mH₂O = 9,806.65 Pa |
| Inch of Mercury | inHg | 1 inHg = 3,386.39 Pa |
Explanation of Pressure Units
- Pascal (Pa)
The Pascal is the SI (International System of Units) unit of pressure and is defined as one newton per square meter. It is a relatively small unit, so kilopascals (kPa) and megapascals (MPa) are more commonly used in practical applications.- 1 Pa = 1 N/m²
- Example: Atmospheric pressure at sea level is approximately 101,325 Pa.
- Kilopascal (kPa)
The kilopascal is 1,000 Pascals, and it’s commonly used for expressing everyday pressures like tire pressures and weather reports.- 1 kPa = 1,000 Pa
- Example: Typical car tire pressure is around 220-240 kPa.
- Megapascal (MPa)
The megapascal is 1,000,000 Pascals. It’s often used in engineering to describe high pressures, such as in hydraulic systems or material stress tests.- 1 MPa = 1,000,000 Pa
- Example: High-pressure hydraulic systems might operate at 5-10 MPa.
- Standard Atmosphere (atm)
The atmosphere is a unit based on the average pressure at sea level on Earth. It is a convenient reference point for many scientific and engineering applications.- 1 atm = 101,325 Pa
- Example: Atmospheric pressure at sea level is 1 atm.
- Torr (mmHg)
The Torr is a unit that is often used in medicine and vacuum science. It was named after Evangelista Torricelli, the inventor of the barometer. One Torr is equivalent to the pressure exerted by a 1-millimeter column of mercury (mmHg).- 1 Torr = 133.322 Pa
- Example: Blood pressure is often measured in mmHg, where normal systolic pressure might be around 120 mmHg.
- Bar
The bar is a unit of pressure that is slightly less than the atmosphere. It is commonly used in meteorology and engineering, especially in Europe.- 1 bar = 100,000 Pa = 0.98692 atm
- Example: The pressure at the bottom of a swimming pool might be 0.1-0.2 bar higher than at the surface.
- Pounds per Square Inch (psi)
PSI is a widely used unit in the United States and some industries, particularly for things like tire pressure and hydraulic systems.- 1 psi = 6,894.76 Pa
- Example: The recommended pressure for most car tires is around 30-35 psi.
- Kilogram-force per Square Centimeter (kgf/cm²)
This unit is commonly used in mechanical engineering and material science, particularly in older or non-SI systems.- 1 kgf/cm² = 98,066.5 Pa
- Example: The pressure inside a high-performance engine could exceed 10 kgf/cm².
- Meter of Water Column (mH₂O)
This unit is used to measure the pressure exerted by a column of water of a certain height, typically in plumbing or hydrology.- 1 mH₂O = 9,806.65 Pa
- Example: The pressure at the bottom of a 10-meter-deep water tank would be approximately 98,066.5 Pa or about 10 mH₂O.
- Inch of Mercury (inHg)
This unit is used in the United States for atmospheric pressure measurements, especially in weather reports and aviation.
- 1 inHg = 3,386.39 Pa
- Example: Standard atmospheric pressure is about 29.92 inHg at sea level.
Common Unit Conversions
- From Atmospheres (atm):
- 1 atm = 101,325 Pa
- 1 atm = 1.01325 bar
- 1 atm ≈ 14.696 psi
- 1 atm = 760 Torr
- From Pascals (Pa):
- 1 Pa = 0.00001 bar
- 1 Pa = 0.000145 psi
- 1 Pa = 0.0075 Torr
- From Bar:
- 1 bar = 100,000 Pa
- 1 bar ≈ 0.9869 atm
- 1 bar ≈ 14.5038 psi
Example Calculations
- Converting from atm to psi: If the pressure of a gas container is measured at 2 atm, it can be converted to psi using the following conversion factor:2 atm×14.696 psi/atm=29.392 psi,
- Converting from bar to Pa: If a hydraulic system operates at 5 bar, to convert it to Pascals:5 bar×100,000 Pa/bar=500,000 Pa,
This guide provides a comprehensive overview of pressure units and how to convert between them. Understanding these conversions is crucial in fields like engineering, physics, and meteorology.