þrýstingr

þrýstingr is the physical quantity that describes the force exerted per unit area on a surface. It appears in many contexts, including atmospheric pressure, fluid pressures, and stresses within solids. The SI unit of pressure is the pascal (Pa), defined as one newton per square meter; larger values are commonly expressed in kilopascals (kPa) or megapascals (MPa).

Absolute pressure refers to pressure relative to a perfect vacuum. Gauge pressure is the pressure above ambient atmospheric pressure, so in open air it is near zero; differential pressure is the difference between two pressures. The relation P_abs = P_atm + P_gauge holds, and P_gauge = P_abs − P_atm.

Measurement methods include barometers for atmospheric pressure, manometers and bourdon tubes for other pressures, and pressure transducers that convert pressure into electrical signals. Modern sensors may be capacitive, piezoelectric, or resonant.

Equations and concepts: P = F/A. In fluids, hydrostatic pressure increases with depth: P = ρ g h, where ρ is density, g is gravity, and h is depth. Pascal's law states that pressure applied to an enclosed incompressible fluid is transmitted undiminished to all parts of the fluid. Pressure concepts underpin hydraulics, pneumatics, meteorology, medicine, and many engineering fields. Blood pressure is commonly reported in millimeters of mercury (mmHg) as systolic/diastolic values.

Etymology: þrýstingr is Icelandic for pressure, derived from the verb þrýsta "to press" with the nominal ending -ingr; the modern form is þrýstingur.