Fluid columns create pressure proportional to depth and density.

• p = ρ g h
• Water: ~9.81 kPa per meter
• 1 bar ≈ 10 m of water head

Weather uses hPa (same as mbar). Sea-level standard is 1013.25 hPa.

• 1 atm = 101.325 kPa
• Low pressure → storms
• High pressure → fair weather

Gauge pressure (suffix 'g') measures relative to ambient. Absolute pressure (suffix 'a') measures relative to vacuum.

• Absolute = Gauge + Atmospheric
• At sea level: add ~101.325 kPa (14.7 psi)
• Altitude changes atmospheric baseline

Absolute = Gauge + Atmospheric

At sea level: add 101.325 kPa or 14.696 psi

• Adjust baseline for altitude
• Always document which scale

Water head to pressure

• 1 mH₂O ≈ 9.80665 kPa
• 10 mH₂O ≈ ~1 bar

Altimeter settings

• 1013 hPa = 29.92 inHg
• 1 inHg ≈ 33.8639 hPa

Know your reference

• QNH: Sea‑level pressure (sets altimeter to field elevation)
• QFE: Field pressure (altimeter reads 0 at the field)
• QNE: Standard 1013.25 hPa / 29.92 inHg (flight levels)

Rules of thumb

• ±1 inHg ≈ ∓1,000 ft indicated
• ±1 hPa ≈ ∓27 ft indicated
• Cold/Hot air: density errors affect true altitude

Measures thermal conductivity of gas

• Range: ~10⁵ → 10⁻¹ Pa (approx)
• Gas‑dependent; calibrate for gas type
• Great for rough to low vacuum

Ionization current vs pressure

• Range: ~10⁻² → 10⁻⁹ Pa
• Sensitive to contamination and gas species
• Use with isolation to protect at high pressure

Absolute diaphragm deflection

• High accuracy; gas‑independent
• Ranges span ~10⁻¹ → 10⁵ Pa
• Ideal for process control

Altimeters use inHg or hPa set to local QNH; pressure affects indicated altitude.

• 29.92 inHg = 1013 hPa standard
• High/low pressure shifts indicated altitude

Blood pressure uses mmHg; respiratory and CPAP use cmH₂O.

• Typical BP 120/80 mmHg
• 5–20 cmH₂O for CPAP

Process equipment and hydraulics often use bar, MPa, or psi.

• Hydraulic lines: tens to hundreds of bar
• Pressure vessels rated in bar/psi

Weather maps show sea-level pressure in hPa or mbar.

• Strong lows < 990 hPa
• Strong highs > 1030 hPa

Vacuum tech uses torr or Pa across rough, high, and ultra-high vacuum.

• Rough vacuum: ~10³–10⁵ Pa
• UHV: < 10⁻⁶ Pa

1 hPa = 1 mbar exactly — they're the same! Meteorology switched from mbar to hPa for SI consistency, but they're numerically identical.

Mercury manometers were the gold standard for over 300 years. Despite being phased out due to toxicity, blood pressure is still measured in mmHg worldwide!

Atmospheric pressure halves roughly every 5.5 km (18,000 ft) elevation. At Mount Everest's summit (8.8 km), pressure is only 1/3 of sea level!

At the Mariana Trench (11 km deep), pressure reaches 1,100 bar — enough to crush a human instantly. That's like having 1,100 kg sitting on every square centimeter!

Outer space has a pressure of ~10⁻¹⁷ Pa — that's 100 million trillion times less than Earth's atmosphere. Your blood would literally boil (at body temperature)!

A car tire at 32 psi is actually experiencing 46.7 psi absolute (32 + 14.7 atmospheric). We measure gauge pressure because it's the 'extra' pressure doing the work!

The pascal (Pa) is named after Blaise Pascal, who proved atmospheric pressure exists by carrying a barometer up a mountain in 1648. He was only 25 years old!

At 1 bar (15 psi) above atmospheric, water boils at 121°C instead of 100°C. This cuts cooking time by 70% — pressure literally speeds up chemistry!

Birth of the Barometer

Evangelista Torricelli invents the mercury barometer while studying why water pumps couldn't lift water beyond 10 meters. Creates the first artificial vacuum and establishes mmHg as the first pressure unit.

Proved that air has weight and pressure, revolutionizing our understanding of the atmosphere. The torr unit (1/760 atm) is named in his honor.

Pascal's Mountain Experiment

Blaise Pascal (age 25) has his brother-in-law carry a barometer up Puy de Dôme mountain, proving atmospheric pressure decreases with altitude. Mercury dropped from 760mm to 660mm at the summit.

Established the relationship between altitude and pressure, fundamental to aviation and meteorology. The pascal (Pa) unit honors his work.

Boyle's Law Discovery

Robert Boyle discovers the inverse relationship between pressure and volume (PV = constant) using improved vacuum pumps and J-tube apparatus.

Foundation of gas laws and thermodynamics. Enabled scientific study of pressure-volume relationships in confined gases.

Bourdon Tube Invention

Eugène Bourdon patents the Bourdon tube gauge—a curved metal tube that straightens under pressure. Simple, robust, and accurate.

Replaced fragile mercury manometers in industrial applications. Still the most common mechanical pressure gauge design 175 years later.

Bar Standardization

The bar is officially defined as 10⁶ dyne/cm² (exactly 100 kPa), chosen to be close to atmospheric pressure for convenience.

Became the standard engineering unit across Europe. 1 bar ≈ 1 atmosphere made mental math easy for engineers.

Pascal as SI Unit

The pascal (Pa = N/m²) is adopted as the official SI unit for pressure, replacing bar in scientific contexts.

Unified pressure measurement with Newton's force unit. However, bar remains dominant in engineering due to convenient scale.

Meteorology's SI Transition

Weather services worldwide switch from millibar (mbar) to hectopascal (hPa). Since 1 mbar = 1 hPa exactly, all historical data remained valid.

Painless transition to SI units. Most weather maps now show hPa, though some aviation still uses mbar or inHg.

MEMS Pressure Revolution

Micro-electromechanical systems (MEMS) enable tiny, cheap, accurate pressure sensors. Found in smartphones (barometer), cars (tire pressure), and wearables.

Democratized pressure measurement. Your smartphone can measure altitude changes of just 1 meter using atmospheric pressure.

Use absolute for thermodynamics/vacuum; gauge for practical equipment ratings. Always label units with 'a' or 'g' suffix (e.g., bara vs barg, kPaa vs kPag).

Legacy altimetry scales in inches of mercury; many countries use hPa (QNH).

1 torr is exactly 1/760 of a standard atmosphere (≈133.322 Pa). Common in vacuum technology.