Is gauge pressure defined as the pressure below atmospheric pressure?

Gauge pressure is defined as the pressure relative to atmospheric pressure, indicating the difference between the pressure being measured and the atmospheric pressure surrounding the measurement point. Therefore, it represents the pressure above atmospheric pressure, not below it.

For example, if a gauge reads 5 psi (pounds per square inch) of gauge pressure, it means that the pressure is 5 psi above the current atmospheric pressure. This understanding is crucial in process control and instrumentation, where differentiating between gauge pressure and absolute pressure (which includes atmospheric pressure in its measurement) is fundamental for accurate readings and effective system operation.

In contrast, absolute pressure is the measurement that considers atmospheric pressure as part of the total pressure. Understanding the distinction helps ensure that pressure readings are interpreted correctly in various applications, such as in closed systems or fluid dynamics, where pressure can have different implications based on its context.