Torr to Centimeter Water (4 °C)

No — a true 0 torr vacuum is physically impossible. Even the best laboratory cryo-pumps bottom out around 10⁻¹³ torr, where stray molecules still occasionally wander through. Interstellar space is roughly 10⁻¹⁷ torr but still contains a few hydrogen atoms per cubic centimeter. Quantum field theory predicts that even "empty" space seethes with virtual particle pairs, so absolute nothingness does not exist. In practice, engineers define "good enough" vacuum levels for each application — 10⁻³ torr for freeze-drying, 10⁻⁶ for electron microscopes, 10⁻⁹ for particle accelerators.

Because the torr maps neatly to the range of vacuum pressures: rough vacuum is 1–760 torr, medium vacuum 10⁻³–1 torr, high vacuum 10⁻⁶–10⁻³ torr, and ultra-high vacuum below 10⁻⁹ torr. Each regime is a clean power of ten. Expressing the same range in pascals (133,000 down to 0.00000013 Pa) is clumsy. The torr gives vacuum engineers a log-friendly scale that spans thirteen orders of magnitude in tidy notation.

Routine lab turbo-pump systems reach 10⁻⁸ torr. Particle accelerators like CERN's LHC operate at about 10⁻¹⁰ torr — comparable to the vacuum of outer space near the Moon. The lowest laboratory pressure ever achieved is around 10⁻¹³ torr, using cryogenic pumps at liquid-helium temperatures. At that level, a molecule might travel thousands of kilometers before hitting another molecule.

Freeze-drying food and pharmaceuticals operates at 0.1–4 torr. Vacuum-sealed food storage bags pull to about 5–10 torr. Incandescent light bulbs were historically evacuated to ~0.01 torr. Vacuum-assisted braking in cars uses roughly 400–500 torr of manifold vacuum. Even your thermos flask has a vacuum of perhaps 10⁻³ torr between its double walls to block heat conduction.

Boiling point plummets. Water boils at 100 °C at 760 torr (sea level), but at only 25 °C at about 24 torr and at 0 °C at just 4.6 torr. This is how freeze-drying works: reduce pressure to 0.1–1 torr and ice sublimates directly to vapor without ever becoming liquid. Vacuum distillation in chemistry exploits the same principle — heat-sensitive compounds that would decompose at their normal boiling point can be distilled gently at a fraction of the temperature under reduced torr.

Respiratory medicine adopted cmH₂O because the original ventilators literally used water columns to regulate pressure — a jar of water with a submerged tube set the pressure at whatever depth the tube was immersed. A CPAP setting of 10 cmH₂O meant the air bubbled out at 10 cm depth. The unit stuck even after electronics replaced water seals, because clinicians, patients, and device manuals all speak the same scale.

Most adults are prescribed between 6 and 14 cmH₂O, with 10 cmH₂O being a common starting point. Severe obstructive sleep apnoea may require 15–20 cmH₂O. Auto-titrating (APAP) machines vary pressure within a set range — typically 4–20 cmH₂O — adjusting breath by breath. Higher pressures are more effective at splinting the airway open but can cause discomfort and air swallowing.

ICU ventilators also use cmH₂O. Positive end-expiratory pressure (PEEP) is usually set at 5–15 cmH₂O to keep alveoli open. Peak inspiratory pressure above 30–35 cmH₂O raises the risk of lung injury. Plateau pressures are monitored to stay below 30 cmH₂O. The entire field of mechanical ventilation runs on this single unit because it directly corresponds to the pressures inside the lung.

Measured via lumbar puncture with the patient lying on their side, normal CSF pressure is 10–18 cmH₂O in adults. Above 25 cmH₂O suggests raised intracranial pressure — potentially from a tumor, meningitis, or hydrocephalus. Below 6 cmH₂O indicates low pressure, often from a CSF leak. Neurologists use cmH₂O rather than mmHg because spinal fluid is essentially water, making the unit a direct physical analogue.

1 cmH₂O ≈ 0.981 mbar ≈ 0.0981 kPa. For bedside estimates, 1 cmH₂O ≈ 1 mbar is close enough (error under 2%). A CPAP setting of 12 cmH₂O is about 11.8 mbar or 1.18 kPa. Since respiratory equipment universally reads cmH₂O, conversion is mainly needed when interfacing with industrial instruments or when charting pressures alongside blood gas data reported in mmHg.