Pressure Converter (Physics)

Blood pressure measurement originated with mercury sphygmomanometers (the cuff device), where pressure was directly read from the height of mercury in a glass tube. Although electronic devices are now standard, the mmHg unit persists by convention. Normal blood pressure is 120/80 mmHg (systolic/diastolic) = 16.0/10.7 kPa. Hypertension is defined as consistently above 130/80 mmHg.

The Challenger Deep reaches about 10,935 m depth. Pressure ≈ ρgh = 1025 kg/m³ × 9.81 m/s² × 10,935 m ≈ 109.9 MPa ≈ 1085 atm ≈ 15,900 psi. At this pressure, water is compressed by about 5% compared to surface. The Trieste bathyscaphe (1960) and DSV Limiting Factor (2019) have reached this depth.

Pascal's law states that pressure applied to an enclosed fluid is transmitted equally in all directions (for a static fluid). This is the principle behind hydraulic systems: a small force applied to a small piston creates a pressure that acts on a large piston, amplifying the force proportional to the area ratio. A hydraulic jack with 1 cm² input and 100 cm² output amplifies force by 100×.

Diamond anvil cells (DAC) can compress tiny samples (micrometers in diameter) between two diamond faces to pressures up to 600 GPa (6 million atm) — exceeding the pressure at Earth's center (~360 GPa). At these pressures, hydrogen may become metallic (an outstanding experimental question), and new phases of all materials are discovered. X-ray synchrotron radiation is used to characterize samples in situ.

When a vacuum system is pumped down, adsorbed gases on the chamber walls slowly desorb (outgas), limiting the achievable vacuum. Baking the chamber (heating to 150-250 °C under vacuum) accelerates outgassing and allows ultra-high vacuum (UHV, <10⁻⁶ Pa) to be achieved. UHV is required for surface science experiments, particle accelerators, and some semiconductor processes.

The Sun's core pressure is approximately 2.5 × 10¹⁶ Pa = 2.5 × 10¹¹ atm = 250 billion bar. This is maintained by the weight of the overlying solar material and is balanced by the pressure gradient from nuclear fusion energy release. The temperature at the core is ~15 million K. Together, these conditions maintain the proton-proton fusion chain that powers the Sun.

Standard atmospheric pressure is 1013.25 mbar. Meteorological pressure maps use millibars because changes of a few millibars are physically significant for weather (1 mbar ≈ 100 Pa). High pressure systems (1020-1040 mbar) bring fair weather; low pressure systems (980-1000 mbar) bring storms. Tropical cyclones can have central pressures below 900 mbar (Typhoon Tip: 870 mbar in 1979, the record low).

Electromagnetic radiation exerts pressure: P_rad = I/c for perfectly absorbing surfaces, where I is intensity (W/m²). Sunlight at Earth's surface (≈1000 W/m²) exerts radiation pressure of about 3.3 × 10⁻⁶ Pa — negligible for everyday objects but significant for solar sails (proposed spacecraft propulsion). Inside stars, radiation pressure competes with gas pressure; for massive stars (M > 50 M_sun), radiation pressure dominates.

White dwarfs are supported against gravitational collapse by electron degeneracy pressure — a quantum mechanical effect arising from the Pauli exclusion principle. Electrons packed to densities of ~10⁶ g/cm³ generate pressures of ~10²² Pa, independent of temperature (cold pressure). Above the Chandrasekhar limit (1.4 solar masses), even this quantum pressure cannot prevent collapse, leading to a neutron star or supernova.