What Is Centimeter of Water (cmH₂O)?
Centimeter of water is a unit used to measure pressure. It tells you how strong a force is when it pushes on a surface, compared to the weight of a 1 centimeter tall column of water. This unit is small and easy to understand, so it is often used in medicine, breathing machines, and some industrial tools.
Definition
Pressure is how much force acts on a certain area. One centimeter of water is defined as the pressure made by a column of pure water that is 1 centimeter high at standard gravity.
In modern science, pressure is measured in pascals, written as Pa. One centimeter of water is about:
- 1 cmH₂O ≈ 98.1 Pa
- 1 cmH₂O ≈ 0.0981 kPa
So, if a pressure is 10 cmH₂O, it is about 981 Pa. This is still a fairly low pressure compared with air pressure around us.
History / Origin
Long ago, people used columns of liquids to measure pressure because they could see how high the liquid moved. Water was easy to find and to use, so height of water became a natural way to show pressure.
The idea is simple. If you have a tube filled with water, the water at the bottom feels the weight of the water above it. The taller the water column, the higher the pressure at the bottom. Scientists started to use centimeters of water and millimeters of water for smaller pressures, especially when dealing with air, gas, and breathing systems.
As the metric system and SI units grew popular, the pascal became the main pressure unit. But centimeter of water stayed in many fields where people were used to it and where small, easy to read numbers are helpful.
Symbol & Abbreviation
The common ways to write centimeter of water are:
- cmH₂O
- cm H₂O
- cm H2O (without the small 2 when typing is limited)
All of these mean the same thing. The symbol is made of:
- cm for centimeter
- H₂O for water, the chemical formula for water
Current Use Around the World
Centimeter of water is not an official SI unit, but it is still used in many places because it is very handy for low pressures.
Common uses include:
- Medical and hospital use: To set and check pressures on ventilators, CPAP machines, and other breathing support devices. Doctors may talk about PEEP or airway pressure in cmH₂O.
- Sleep medicine: Sleep apnea machines often show pressure settings like 5 to 20 cmH₂O.
- Respiratory research: Lab tests that study lungs and breathing often record pressures in cmH₂O.
- Industrial systems: Low gas pressures, soft air flows, or small pressure differences in ducts and filters can be given in cmH₂O.
- Hydraulics and fluid labs: Some manometers and water columns still use centimeters of water for simple experiments.
In many technical papers and machine manuals, you will see both cmH₂O and kPa or Pa listed together, to help people move between old and new units.
Example Conversions
Below are some useful example conversions. Values are rounded for easy use.
To SI units (Pascals and kilopascals)
- 1 cmH₂O ≈ 98.1 Pa
- 1 cmH₂O ≈ 0.0981 kPa
- 10 cmH₂O ≈ 981 Pa ≈ 0.981 kPa
- 20 cmH₂O ≈ 1 962 Pa ≈ 1.96 kPa
To millimeters of mercury mmHg
- 1 cmH₂O ≈ 0.735 mmHg
- 10 cmH₂O ≈ 7.35 mmHg
- 13.6 cmH₂O ≈ 10 mmHg (about)
To pounds per square inch psi
- 1 cmH₂O ≈ 0.0142 psi
- 50 cmH₂O ≈ 0.71 psi
- 70 cmH₂O ≈ 1.0 psi (about)
To meters of water mH₂O
- 100 cmH₂O = 1 mH₂O
- 1 cmH₂O = 0.01 mH₂O
In general, to change from cmH₂O to pascals, you can multiply by about 98.1. To change from pascals to cmH₂O, you can divide by about 98.1.
Related Units
Many other units are used to measure pressure. Here are some that are closely related to centimeter of water:
- Millimeter of water mmH₂O: Same idea as cmH₂O but uses millimeters instead of centimeters. 1 cmH₂O equals 10 mmH₂O.
- Meter of water mH₂O: Pressure from a 1 meter high column of water. 1 mH₂O is 100 cmH₂O.
- Inch of water in H₂O: Similar unit that uses inches instead of centimeters. Used in some English speaking countries for low air and gas pressures.
- Millimeter of mercury mmHg: Pressure from a column of mercury. Common in blood pressure and older lab work. Mercury is heavier than water, so 1 mmHg is larger than 1 cmH₂O.
- Pascals Pa and kilopascals kPa: Official SI units for pressure. Used in science, weather reports, and engineering.
- Bar and millibar mbar: Larger pressure units. 1 bar is 100 000 Pa. Weather maps often use millibar or hectopascal hPa.
- Atmosphere atm: About equal to the average air pressure at sea level. 1 atm is about 101 325 Pa, which is about 1 033 cmH₂O.
FAQs
What does 1 cmH₂O mean in simple words?
It is the pressure made by a 1 centimeter tall column of water. Think of a tiny stack of water pushing down on a surface.
Why do doctors use cmH₂O for breathing machines?
Because pressures in the lungs are quite small. Centimeter of water gives numbers like 5, 10, or 15, which are easy to read and remember. Using pascals would make the numbers look bigger and less friendly.
Is cmH₂O an SI unit?
No. The official SI unit of pressure is the pascal Pa. However, cmH₂O is accepted for practical use in many fields, especially medicine, because it is familiar and simple.
How do I convert cmH₂O to mmHg?
Multiply the value in cmH₂O by about 0.735. For example, 10 cmH₂O × 0.735 ≈ 7.35 mmHg.
How do I convert cmH₂O to kPa?
Multiply the value in cmH₂O by about 0.0981. For example, 20 cmH₂O × 0.0981 ≈ 1.96 kPa.
Is centimeter of water the same as centimeter of water column?
Yes. Many people say centimeter of water column or write cm WC. They are talking about the same type of pressure unit.
Where might I see cmH₂O in daily life?
You might see it on the screen of a CPAP machine, a hospital ventilator, or in manuals for fans, filters, and low pressure gas systems.
Why use water and not another liquid?
Water is cheap, safe, easy to find, and its properties are well known. That is why it became a standard choice for these types of pressure units.