Question 1

What pressure should car tyres be?

Accepted Answer

Usually 1.8–2.5 bar (26–36 psi), and front and rear may differ. Saloons run 2.0–2.3 bar, SUVs 2.0–2.5 bar, motorbikes 2.0–2.5 bar front and 2.5–3.0 rear. Always check when the tyres are cold.

Question 2

Does a blood-pressure monitor read pascals or mmHg?

Accepted Answer

Millimetres of mercury. A normal reading is 120/80 mmHg (about 16,000/10,666 Pa), and hypertension is above 140/90 mmHg. The instrument reads in mmHg because pressure was historically measured by a column of mercury.

Question 3

Why are there so many pressure units?

Accepted Answer

Different fields settled on different ones: medicine uses mmHg, meteorology hPa/mbar, chemistry atm/kPa, engineering MPa, and tyres bar/psi. Standard atmospheric pressure ties them together: 101,325 Pa = 1,013.25 hPa = 1 atm = 760 mmHg.

Question 4

How does pressure affect water's boiling point?

Accepted Answer

At sea level water boils at 100°C under 1 atm. At 3,000 m altitude (~0.7 atm) it boils at about 90°C, so eggs take longer. A pressure cooker at 1.5–2 atm boils at about 120°C and cooks faster; an autoclave at 2 atm reaches 134°C.

Question 5

What is "gauge pressure"?

Accepted Answer

Engineers often measure pressure relative to the atmosphere (gauge pressure). A tyre gauge showing 2 bar means 2 bar above atmospheric, i.e. 3 bar absolute. A vacuum gauge shows negative values. Absolute pressure = gauge pressure + atmospheric.

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