Atmospheric pressure is the force exerted by the weight of the air above a given point, while differential pressure measures the pressure difference between two points in a system.
Understanding the Pressures
Let's break down these two concepts:
Atmospheric Pressure
Atmospheric pressure, often called barometric pressure, is the force exerted by the weight of the air molecules above us. It varies with altitude and weather conditions. At sea level, standard atmospheric pressure is approximately 101.325 kilopascals (kPa), 14.7 pounds per square inch (psi), or 1 atmosphere (atm). Think of it as the constant pressure exerted on everything due to the air surrounding us.
Differential Pressure
Differential pressure, on the other hand, is concerned with the difference in pressure between two specific locations. According to the reference provided, it's "the contrast in pressure between two points in a system". It doesn't necessarily rely on atmospheric pressure directly. Instead, it focuses on a pressure difference within a contained environment or process.
Key Differences Summarized
Here's a table to clearly outline the main distinctions:
| Feature | Atmospheric Pressure | Differential Pressure |
|---|---|---|
| Definition | Force exerted by the weight of the air. | Pressure difference between two points. |
| Measurement | Measured relative to a vacuum or indirectly with gauge pressure. | Measured as the difference between two specific points. |
| Reference Point | The Earth's atmosphere. | Two defined points within a system. |
| Purpose | Indicates general air pressure. | Indicates pressure variations within a confined space. |
Practical Examples
-
Atmospheric Pressure:
- A barometer measures atmospheric pressure to predict weather changes.
- The pressure in a car tire is measured relative to atmospheric pressure; it's the gauge pressure.
- High altitude areas have lower atmospheric pressure compared to low-lying areas.
-
Differential Pressure:
- A flow meter uses differential pressure to measure fluid flow in pipes.
- A filter in a HVAC system will show a differential pressure that rises as the filter becomes clogged.
- A medical ventilator monitors the differential pressure between a patient's airway and the machine.
How They Relate
While distinct, these pressures are related. Gauge pressure is an example of how we often refer to pressure in the context of atmospheric pressure. Gauge pressure is an absolute pressure measured relative to atmospheric pressure, and we can find the absolute pressure by adding the gauge pressure reading to the atmospheric pressure. While differential pressure measures a difference in pressure, it doesn't directly measure the absolute or gauge pressure and neither are influenced by atmospheric pressure.
Conclusion
In essence, atmospheric pressure is the pressure due to the air surrounding us, a general environment measure. Differential pressure focuses on the pressure contrast between two specific points within a system and doesn't rely on atmospheric pressure to be measured. It is the difference in pressure between these two points, and this is a very useful measure when tracking flow, filter condition, and other process-based readings.
