Atmospheric pressure is the force exerted by the weight of the air above a given point, while absolute pressure is the total pressure, including atmospheric pressure, measured relative to a perfect vacuum (absolute zero).
Understanding the Key Differences
To clarify the difference, consider these key aspects:
- Atmospheric Pressure: This is the pressure exerted by the Earth's atmosphere. It varies with altitude and weather conditions. At sea level, standard atmospheric pressure is approximately 101.325 kPa (14.7 psi).
- Absolute Pressure: This is the pressure measured relative to a perfect vacuum. A perfect vacuum has zero pressure. Therefore, absolute pressure is always a positive value.
- Gauge Pressure: Gauge pressure is a third related pressure measurement. It is the difference between the absolute pressure and the atmospheric pressure. Many pressure gauges read gauge pressure.
Pressure Relationships Explained
The relationship between these pressure types can be summarized as follows:
- Absolute Pressure = Atmospheric Pressure + Gauge Pressure
- Gauge Pressure = Absolute Pressure - Atmospheric Pressure
Illustrative Table
| Pressure Type | Definition | Measurement Reference | Example |
|---|---|---|---|
| Atmospheric Pressure | Pressure exerted by the Earth's atmosphere. | Sea Level (variable) | Approximately 14.7 psi at sea level |
| Absolute Pressure | Pressure measured relative to a perfect vacuum (absolute zero pressure). | Perfect Vacuum | Pressure in a tire plus atmospheric pressure |
| Gauge Pressure | Pressure measured relative to atmospheric pressure. | Atmospheric Pressure | Pressure read on a typical tire gauge (pressure above atmospheric) |
Real-world Relevance
- Tire Pressure: When checking tire pressure, you're typically reading gauge pressure. The gauge indicates the pressure above the atmospheric pressure. The absolute pressure in the tire is the gauge pressure plus the atmospheric pressure.
- Altitude: As altitude increases, atmospheric pressure decreases. This is why cabins in airplanes are pressurized to maintain a more comfortable pressure for passengers. Absolute pressure decreases with altitude as well, however, any pressure reading will still be absolute.
- Vacuum Systems: In vacuum systems, absolute pressure is critical. A "perfect" vacuum has an absolute pressure of zero. Actual vacuum systems have very low, but non-zero, absolute pressures.
Conclusion
In summary, atmospheric pressure is the pressure exerted by the surrounding air, absolute pressure is the total pressure referenced to a vacuum, and gauge pressure is the pressure relative to atmospheric pressure. Understanding the relationships between these pressures is crucial in many engineering and scientific applications.
