What is Absolute Pressure?
Absolute pressure is a measurement of pressure relative to a perfect vacuum, meaning zero pressure. It includes the pressure exerted by the atmosphere in addition to any other applied pressure. In scientific and engineering contexts, absolute pressure is the fundamental measure as it accounts for all pressure acting on a system, making it suitable for calculations involving gas laws or phase changes.
What is Gauge Pressure?
Gauge pressure is a measurement of pressure relative to the surrounding atmospheric pressure. It indicates how much the measured pressure is above or below the current local atmospheric pressure. Gauge pressure is often used in practical applications where the difference from ambient conditions is more relevant than the total absolute pressure, such as in tire inflation or blood pressure readings.
The Key Difference: Reference Points
The primary distinction lies in their reference points. Absolute pressure uses a perfect vacuum (0 psi or 0 Pascal) as its zero reference, while gauge pressure uses the local atmospheric pressure as its zero reference. This means that at sea level, where atmospheric pressure is approximately 14.7 psi or 101.3 kPa, a gauge pressure reading of 0 psi actually corresponds to an absolute pressure of 14.7 psi.
Practical Applications and Conversion
To convert between the two, you add or subtract atmospheric pressure. Absolute Pressure = Gauge Pressure + Atmospheric Pressure. Conversely, Gauge Pressure = Absolute Pressure - Atmospheric Pressure. For example, a car tire inflated to 32 psi gauge pressure in an environment with 14.7 psi atmospheric pressure actually has an internal absolute pressure of 46.7 psi. This distinction is vital for processes where atmospheric changes could affect results, like vacuum systems or high-altitude operations.