Pressure & Vacuum Conversions — PSI, Bar, kPa, MPa

Gauge pressure (psig, barg) reads zero at local atmospheric pressure. Absolute pressure (psia, bara) includes atmospheric pressure. At sea level: psia = psig + 14.7. At 5,000 ft altitude: psia = psig + 12.2. This correction is critical for compression ratio calculations and gas density corrections.

Inches of water (in H₂O) for low-pressure duct systems (1-10 in H₂O), inches of mercury (in Hg) for vacuum measurement, psi for refrigeration pressures, and bar for European HVAC equipment. Typical duct static pressure is 0.5-1.5 in H₂O.

The bar is a metric unit (1 bar = 100,000 Pa) widely adopted in European industry for convenience. Most industrial pressures fall in the 1-350 bar range, making bar an intuitive scale. PSI remains dominant in US industry due to historical adoption and legacy equipment specifications.

30 psi = 2.068 bar. This is a common tire pressure and low-pressure pneumatic system setting. Many European tools rated at 2 bar operate correctly on US 30 psi compressed air.

The conversion factor between units does not change with altitude or temperature — 1 psi always equals 0.0689476 bar. However, the atmospheric pressure reference for gauge readings changes with altitude: ~14.7 psia at sea level drops to ~12.2 psia at 5,000 ft. For density-dependent calculations (SCFM, gas mass flow), always use actual local atmospheric pressure.