The difference between total pressure (also known as absolute pressure) and atmospheric pressure is that total pressure includes atmospheric pressure in its measurement, whereas atmospheric pressure is measured independently. Think of total pressure as the sum of atmospheric pressure and any additional pressure present.
Here's a more detailed breakdown:
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Total Pressure (Absolute Pressure): This is the pressure measured relative to a perfect vacuum (zero pressure). It's the sum of atmospheric pressure and gauge pressure. It represents the actual pressure at a specific point.
- Formula:
Pabs = Pg + Patm - Where:
Pabs= Absolute Pressure (Total Pressure)Pg= Gauge PressurePatm= Atmospheric Pressure
- Formula:
-
Atmospheric Pressure: This is the pressure exerted by the weight of the air above a given point. It varies with altitude and weather conditions. At sea level, standard atmospheric pressure is approximately 101.325 kPa (14.7 psi or 1 atm).
To further illustrate:
Imagine a tire gauge reading 30 psi. This is the gauge pressure (Pg). The total pressure (Pabs) inside the tire is the gauge pressure plus the atmospheric pressure. If atmospheric pressure is 14.7 psi, then the total pressure inside the tire is 30 psi + 14.7 psi = 44.7 psi.
| Pressure Type | Definition | Measurement Relative To | Includes Atmospheric Pressure? | Example |
|---|---|---|---|---|
| Total Pressure (Absolute) | Pressure measured relative to a perfect vacuum. | Perfect Vacuum (Zero) | Yes | Actual pressure inside a sealed container. |
| Atmospheric Pressure | Pressure exerted by the weight of the air above a given point. | Dependent on Location | No (measured independently) | Air pressure on a weather report, typically around 14.7 psi at sea level. |
| Gauge Pressure | Pressure measured relative to atmospheric pressure. This is often what gauges read directly. | Atmospheric Pressure | No | Pressure reading on a tire gauge or a pressure cooker (above atmospheric) |
In summary, total pressure is the absolute measure of pressure, including atmospheric pressure, while atmospheric pressure is a separate measurement of the surrounding air pressure. Gauge pressure, often read directly by instruments, indicates the pressure above atmospheric pressure.
