Understanding Gauge Pressure
Gauge pressure is the pressure measured relative to the surrounding atmospheric pressure. It represents the difference between the absolute pressure within a system and the current local atmospheric pressure. This means that a gauge reading of zero corresponds to the ambient atmospheric pressure, not a perfect vacuum.
How It's Measured and Interpreted
Most common pressure-measuring devices, such as tire gauges or industrial pressure sensors, are designed to read gauge pressure. A positive gauge pressure indicates that the system's pressure is higher than the atmosphere, while a negative gauge pressure (often called vacuum pressure) means the system's pressure is lower than the atmosphere. It simplifies many calculations because it directly reflects the pressure that drives flow or exerts force in an open system.
A Practical Example
When you inflate a car tire to 32 psi (pounds per square inch), this value is a gauge pressure. It signifies that the pressure inside the tire is 32 psi greater than the air pressure outside the tire. To find the absolute pressure inside the tire, you would add the local atmospheric pressure (e.g., approximately 14.7 psi at sea level) to the gauge pressure, resulting in an absolute pressure of about 46.7 psi.
Importance and Applications
Gauge pressure is highly practical and widely used in fields like engineering, manufacturing, and medicine. It's particularly useful for applications where the surrounding atmospheric pressure is the relevant reference, such as designing piping systems, monitoring blood pressure, or ensuring proper inflation of mechanical components. Its direct relevance to the 'working pressure' of many systems makes it a fundamental concept.