Millimeter Mercury to Bar

Clinical medicine is deeply conservative about units because misreadings kill people. Doctors, nurses, and patients worldwide have memorized "120/80 is normal" in mmHg. Converting to kPa (16.0/10.7) would require retraining millions of clinicians and rewriting every guideline. The WHO considered the switch and decided the risk of transcription errors during transition outweighed the elegance of SI compliance. So mmHg stays — likely for decades more.

The top number (systolic) is the peak pressure when the heart contracts and pushes blood into the arteries — typically 90–120 mmHg. The bottom number (diastolic) is the lowest pressure between beats when the heart relaxes — typically 60–80 mmHg. A reading of 140/90 mmHg or above is classified as hypertension. The gap between the two (pulse pressure) also matters: a wide gap above 60 mmHg may signal stiff arteries.

In 1643, Evangelista Torricelli filled a glass tube with mercury, inverted it into a dish of mercury, and watched the column drop to about 760 mm. The empty space above was the first laboratory vacuum. The height of the mercury column became the measurement of atmospheric pressure — 760 mmHg at sea level. Nearly 400 years later, we still use his column height as a pressure unit in medicine and vacuum science.

For all practical purposes, they are identical — 1 torr = 1/760 atm ≈ 133.322 Pa, and 1 mmHg ≈ 133.322 Pa. The difference is about 0.00015% and arises from the torr being defined from the atmosphere while mmHg is defined from mercury density. Medicine uses mmHg; vacuum physics uses torr. They are interchangeable in any real-world measurement.

Intraocular pressure (glaucoma screening): normal is 10–21 mmHg, above 21 is suspicious. Partial pressure of oxygen in arterial blood (PaO₂): normal is 80–100 mmHg. Central venous pressure: 2–6 mmHg. Intracranial pressure: normal below 15 mmHg, dangerous above 20 mmHg. Carbon dioxide in blood (PaCO₂): 35–45 mmHg. The unit pervades clinical monitoring far beyond the blood pressure cuff.

Europe adopted metric units broadly in the 19th and 20th centuries, and the bar (100,000 Pa) became the natural metric pressure unit for everyday engineering. The US never metricated, so pounds per square inch persisted. A car tire at 2.2 bar is the same as 32 psi — most modern tire placards list both. If you rent a car abroad and the pump reads bar, just divide your usual psi number by 14.5.

Standard aluminum scuba cylinders are rated to 200 bar (2,900 psi); steel tanks often go to 232 or 300 bar. At 200 bar, the air inside is compressed to 1/200th of its surface volume — a 12-liter tank holds 2,400 liters of breathing gas. Deep technical divers using trimix may use 300-bar steel tanks to maximize bottom time at extreme depths.

Close, but not quite. One bar is exactly 100,000 Pa; one standard atmosphere is 101,325 Pa — about 1.3% higher. The bar was designed as a round-number metric unit, not an exact atmospheric equivalent. For most practical purposes (cooking, tire inflation, diving rules of thumb) the difference is negligible, but in chemistry and calibration work the distinction matters.

Espresso machines run at 9 bar, car tires at 2–2.5 bar, a fire extinguisher at 12–15 bar, a garden pressure washer at 100–150 bar, and a diesel fuel injection rail at up to 2,500 bar. The range from gentle (carbonated water at 2–4 bar) to extreme (waterjet cutting at 4,000+ bar) makes the bar a versatile everyday engineering unit.

The SI only recognizes base and coherently derived units — pressure in SI is strictly the pascal (kg·m⁻¹·s⁻²). The bar is accepted "for use with SI" but is technically an outside unit, like the liter or the hour. The reason it thrives anyway is convenience: 2.2 bar is far friendlier than 220,000 Pa for a tire label, and industry adoption is too deep to reverse.