Summary
Highlights
Pressure is defined as the force exerted by a fluid per unit area. Fluids include liquids, vapors, and gases. Pressure in a fluid at rest is uniform in all directions at a given point but increases with depth due to the weight of the fluid above. This effect is more pronounced in liquids due to their higher density compared to gases, where pressure can often be assumed uniform across different depths.
The standard unit for pressure is the Pascal (Pa), which is Newton per square meter. Due to its small magnitude, kilopascals (kPa) and megapascals (MPa) are commonly used. Other common pressure units include bar, standard atmosphere (atm), and kilogram-force per square centimeter (kgf/cm²). The video provides conversion factors between these units.
Atmospheric pressure is the pressure caused by the weight of the air column above the Earth's surface, varying with height and weather. Absolute pressure is measured relative to absolute zero pressure (a perfect vacuum). Gauge pressure is the difference between the measured pressure and local atmospheric pressure. Vacuum pressure is the amount by which a measured pressure is less than the local atmospheric pressure.
A barometer is used to measure atmospheric pressure. Invented by Torricelli, it consists of a mercury-filled tube inverted into a mercury container open to the atmosphere. The height of the mercury column is directly proportional to the atmospheric pressure, calculated by the formula P_atmospheric = rho * g * h, where rho is mercury density, g is gravity, and h is the column height.
A manometer is a U-shaped glass tube containing a liquid (e.g., water, alcohol, mercury), used to measure small pressure differences. One end is connected to the region of interest, and the other is open to the atmosphere. The difference in liquid levels indicates the pressure difference. The pressure in the measured region can be determined using the formula P_2 = P_atmospheric + (rho * g * h).
Bourdon tube gauges are common for measuring medium to high pressures. They consist of a curved, circular, spiral, or coiled tube that straightens or uncoils when internal pressure increases and contracts when it decreases. This motion is proportional to the pressure and is linked to a pointer mechanism to display the reading.
An example demonstrates how to calculate absolute pressure given vacuum gauge pressure and atmospheric pressure. If a vacuum gauge reads 5.8 PSI and atmospheric pressure is 14.5 PSI, the absolute pressure in the chamber is calculated as P_absolute = P_atmospheric - P_vacuum, resulting in 8.7 PSI. The local atmospheric pressure is crucial for determining absolute pressure.