Final answer:
A valve's pressure classification is always given in terms of gauge pressure, which is relative to atmospheric pressure and measured with devices like manometers. The SI unit of pressure is the pascal.
Step-by-step explanation:
A valve's pressure classification is always given in terms of gauge pressure, which is the pressure relative to atmospheric pressure. Gauge pressure can be positive for pressures above atmospheric pressure, and negative for pressures below it. The U-shaped device known as a manometer is used to measure gauge pressure using the height of a fluid column. Additionally, in engineering applications, the absolute pressure is also important, which is the sum of the gauge pressure plus atmospheric pressure. The SI unit for pressure is the pascal (Pa), but other units like atmospheres (atm) and pounds per square inch (psi) are also commonly used.