Understanding Centimeters of Water to Technical Atmospheres Conversion
The centimeter of water (cmH2O) is the pressure of a 1 cm water column at 4°C. The technical atmosphere (at) is defined as one kilogram-force per square centimeter, equal to 98066.5 pascals, and appears in older European engineering and hydraulics literature. Because a technical atmosphere equals exactly the pressure of a 1000 cm water column, this conversion is an elegant factor of one-thousandth.
Conversion Formula
To convert centimeters of water to technical atmospheres, multiply by this factor:
Step-by-Step Example
Convert 25 centimeters of water to technical atmospheres.
How to Convert Centimeters of Water to Technical Atmospheres
Use the clean thousandth-scale relationship between the two units.
- Note the cmH2O value: For example, 25 cmH2O.
- Multiply by 0.001: One centimeter of water is a thousandth of a technical atmosphere.
- Compute: .
- Report the result: 25 cmH2O equals 0.025 at.
centimeters of water to technical atmospheres conversion table
| centimeters of water (cmH2O) | technical atmospheres (at) |
|---|---|
| 0 | 0 |
| 1 | 0.001 |
| 2 | 0.002 |
| 3 | 0.003 |
| 4 | 0.004 |
| 5 | 0.005 |
| 6 | 0.006 |
| 7 | 0.007 |
| 8 | 0.008 |
| 9 | 0.009 |
| 10 | 0.01 |
| 15 | 0.015 |
| 20 | 0.02 |
| 25 | 0.025 |
| 30 | 0.03 |
| 40 | 0.04 |
| 50 | 0.05 |
| 60 | 0.06 |
| 70 | 0.07 |
| 80 | 0.08 |
| 90 | 0.09 |
| 100 | 0.1 |
| 150 | 0.15 |
| 200 | 0.2 |
| 250 | 0.25 |
| 300 | 0.3 |
| 400 | 0.4 |
| 500 | 0.5 |
| 600 | 0.6 |
| 700 | 0.7 |
| 800 | 0.8 |
| 900 | 0.9 |
| 1000 | 1 |
| 2000 | 2 |
| 3000 | 3 |
| 4000 | 4 |
| 5000 | 5 |
| 10000 | 10 |
| 25000 | 25 |
| 50000 | 50 |
| 100000 | 100 |
| 250000 | 250 |
| 500000 | 500 |
| 1000000 | 1000 |
What is the centimeter of water?
The centimeter of water (cmH2O) is a unit of pressure equal to the pressure exerted by a one-centimeter-high column of water under standard conditions. It is used heavily in medicine and respiratory therapy, where small pressures are common.
Definition
The conventional centimeter of water is defined as the pressure of a 1 cm column of water with a density of 1000 kg/m³ under standard gravity (9.80665 m/s²):
This is the conventional value. Because water density varies with temperature, more precise variants exist (for example, cmH2O at 4 °C uses water's maximum density of about 999.972 kg/m³, giving roughly 98.064 Pa).
Origin and History
The unit comes directly from the water manometer, an instrument in which pressure is read as the height difference of a water column. Water was chosen for low pressures because it produces a taller, more readable column than mercury (water is about 13.6 times less dense). The convention fixes water density and gravity so the unit has a single agreed value independent of local conditions.
Law and Notable Facts
The centimeter of water is not an SI unit but is permitted in medical contexts, where it remains standard for measuring airway, ventilator, and cerebrospinal-fluid pressures. Roughly 1 cmH2O equals 0.7356 mmHg, so about 1.36 cmH2O make up 1 mmHg.
Real-World Examples and Conversions
- Mechanical ventilators typically deliver positive end-expiratory pressure (PEEP) of 5 to 10 cmH2O.
- Normal cerebrospinal-fluid opening pressure in a lumbar puncture is roughly 10 to 18 cmH2O.
- 1 cmH2O ≈ 98.0665 Pa ≈ 0.098 kPa.
- A standard atmosphere equals about 1033.2 cmH2O.
What is the technical atmosphere?
The technical atmosphere (at) is a non-SI unit of pressure equal to one kilogram-force per square centimeter. It was widely used in engineering, especially in Europe, before the pascal became standard.
Definition
The technical atmosphere is defined as the pressure of one kilogram-force acting on one square centimeter:
This equals exactly 1 kgf/cm² (98,066.5 Pa), because standard gravity is 9.80665 m/s². It is close to, but distinct from, the standard atmosphere (atm = 101,325 Pa) and the bar (100,000 Pa).
Origin and History
The technical atmosphere arose from the gravitational metric system of units, which used the kilogram-force rather than the newton. Expressing pressure as kilogram-force per square centimeter was intuitive for engineers weighing loads, and the unit became common in mechanical and hydraulic engineering, boilers, and pneumatics through much of the 20th century.
Law and Notable Facts
The technical atmosphere is not part of the SI and its use is discouraged in favor of the pascal or bar. It is easily confused with the standard atmosphere; the two differ by about 3.3%. Gauge and absolute variants were often written "atü" (gauge) and "ata" (absolute) in German-language engineering.
Real-World Examples and Conversions
- 1 at = 1 kgf/cm² = 98.0665 kPa ≈ 0.9678 atm.
- 1 at ≈ 14.223 psi, close to but slightly below the 14.696 psi of one standard atmosphere.
- A pressure of 10 at (about 981 kPa) is a common rating benchmark for industrial hydraulic components.
- 1 at ≈ 0.980665 bar, so the bar and technical atmosphere differ by under 2%.
Frequently Asked Questions
Why is the factor exactly 0.001?
A technical atmosphere equals the pressure of a 1000 cm (10 m) water column, so each centimeter of water is precisely one-thousandth of a technical atmosphere.
How many technical atmospheres are 25 cmH2O?
Multiplying 25 by 0.001 gives 0.025 at.
How does the technical atmosphere differ from the standard atmosphere?
The technical atmosphere (at) is 98066.5 Pa, based on kgf/cm², whereas the standard atmosphere (atm) is 101325 Pa, so they are close but not equal.
Where is the technical atmosphere still used?
It survives in some legacy hydraulics, boiler, and compressor documentation, particularly older European and Russian engineering sources.
How do I convert technical atmospheres back to cmH2O?
Multiply the technical-atmosphere value by 1000; for example, 0.025 at returns to 25 cmH2O.
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Complete centimeters of water conversion table
| Unit | Result |
|---|---|
| pascals (Pa) | 98.0665 Pa |
| kilopascals (kPa) | 0.0980665 kPa |
| megapascals (MPa) | 0.0000980665 MPa |
| hectopascals (hPa) | 0.980665 hPa |
| millibar (mbar) | 0.980665 mbar |
| bar (bar) | 0.000980665 bar |
| torr (torr) | 0.7355592 torr |
| meters of water @ 4°C (mH2O) | 0.01 mH2O |
| millimeters of mercury (mmHg) | 0.7355591 mmHg |
| standard atmospheres (atm) | 0.0009678411 atm |
| technical atmospheres (at) | 0.001 at |
| centimeters of mercury (cmHg) | 0.07355591 cmHg |
| pounds per square inch (psi) | 0.01422334 psi |
| kilopound per square inch (ksi) | 0.00001422334 ksi |
| Inches of mercury (inHg) | 0.02895902 inHg |