Bar to Millimeter Water (4 °C)
bar
mmH2O
Conversion History
| Conversion | Reuse | Delete |
|---|---|---|
1 bar (Bar) → 10,197.448 mmH2O (Millimeter Water (4 °C)) Just now · Rounding: Up to 8 significant figures | Recalculate this conversion using your current rounding settings. |
Quick Reference Table (Bar to Millimeter Water (4 °C))
| Bar (bar) | Millimeter Water (4 °C) (mmH2O) |
|---|---|
| 0.01 | 101.97448 |
| 0.1 | 1,019.7448 |
| 1 | 10,197.448 |
| 2 | 20,394.895 |
| 10 | 101,974.48 |
| 100 | 1,019,744.8 |
| 300 | 3,059,234.3 |
Results rounded to a maximum of 8 significant figures
About Bar (bar)
The bar equals exactly 100,000 pascals — approximately 1.3% less than standard atmospheric pressure. It is widely used in engineering, hydraulics, industrial gas systems, and compressed-air applications, particularly in Europe and internationally. Tire pressures, hydraulic system operating pressures, scuba cylinder pressures, and industrial gas supplies are commonly quoted in bar. The bar is not an SI unit but is formally accepted for use alongside SI. Its decimal prefixes — millibar for meteorology, kilobar for high-pressure research — extend its range across many disciplines.
Car tire inflation is typically 2.0–2.5 bar. Scuba diving cylinders are filled to 200–300 bar.
About Millimeter Water (4 °C) (mmH2O)
The millimeter of water at 4 °C (mmH₂O) is the pressure exerted by a 1 mm column of pure water at 4 °C under standard gravity. Using the fixed reference density of 999.972 kg/m³, it equals 9.8063754138 pascals. It is used for very low pressure measurements where even pascals give large numbers: HVAC duct static pressures, spirometry and respiratory mechanics, building ventilation system balancing, and manometer readings in laboratory work. The 4 °C reference specifies a temperature close to water's maximum density for reproducible reference conversions.
HVAC supply duct static pressures typically range from 25 to 250 mmH₂O. A forced exhalation against resistance generates roughly 10–50 mmH₂O.
Bar – Frequently Asked Questions
Why do European tire gauges show bar while American ones show psi?
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.
How many bar of pressure can a scuba tank hold?
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.
Is one bar the same as one atmosphere?
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.
What bar pressures are common in industrial and everyday settings?
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.
Why is the bar not an official SI unit even though it is metric?
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.
Millimeter Water (4 °C) – Frequently Asked Questions
Why is HVAC duct pressure measured in millimeters of water instead of pascals?
HVAC technicians originally measured duct pressure with a simple U-tube manometer filled with water — you literally read the height difference in millimeters. One mmH₂O ≈ 9.81 Pa, so a typical 25–250 mmH₂O duct pressure range corresponds to 245–2,450 Pa. The water column scale is still used because the instruments are cheap, intuitive, and field-rugged, even though digital gauges now display the same numbers electronically.
What does the "at 4 °C" part of the unit mean?
Water is densest near 4 °C, but its density is not exactly 1 gram per cubic centimeter. Column pressure follows p = ρgh, so both density and gravity matter. This converter combines a fixed reference density of 999.972 kg/m³ with standard gravity (9.80665 m/s²), giving 98.063754138 Pa per centimeter without rounding the reference calculation. NIST publishes the shorter approximate value 98.0638 Pa/cm. The result also differs from the conventional 98.0665 Pa/cm factor that assumes a density of 1,000 kg/m³. Real manometer readings may require temperature and local-gravity corrections.
How do you read a water manometer in mmH₂O?
Connect one side of a U-tube to the duct and leave the other open to atmosphere. The water level drops on the pressurized side and rises on the open side. The total height difference in millimeters is the gauge pressure in mmH₂O. Inclined (slant) manometers amplify small readings by tilting the tube — a 10:1 slope makes each millimeter of travel represent 0.1 mmH₂O, improving resolution for filter pressure-drop testing.
What mmH₂O range indicates a clogged HVAC filter?
A clean residential furnace filter creates 12–50 mmH₂O of pressure drop. When the drop exceeds 125–250 mmH₂O (varies by manufacturer), the filter is restricting airflow enough to hurt efficiency and strain the blower motor. Commercial systems set alarms at specific mmH₂O thresholds — when the differential pressure sensor hits the limit, a "replace filter" indicator lights up on the building management system.
How does mmH₂O relate to inches of water column (inH₂O)?
For water-column units with the same temperature and gravity reference, 1 inch of water = 25.4 mmH₂O, since 1 inch = 25.4 mm. Thus 0.5 inches of water at 4 °C equals 12.7 mmH₂O at 4 °C. US HVAC specs often use inches of water gauge ("in. w.g."); European and Asian specs use mmH₂O. Check the stated reference before comparing factors: a water-column unit at 4 °C is slightly different from one at another temperature or the conventional density-based unit.