The metre (or meter) sea water (msw) is a

metric unit The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the Internati ...

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

used in

underwater diving Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also often referred to as diving, an ambiguous term with several possible meanings, depending on contex ...

. It is defined as one tenth of a

bar Bar or BAR may refer to: Food and drink * Bar (establishment), selling alcoholic beverages * Candy bar * Chocolate bar Science and technology * Bar (river morphology), a deposit of sediment * Bar (tropical cyclone), a layer of cloud * Bar (u ...

. The unit used in the US is the foot sea water (fsw), based on

standard gravity The standard acceleration due to gravity (or standard acceleration of free fall), sometimes abbreviated as standard gravity, usually denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. ...

and a sea-water density of 64 lb/ft³. According to the US Navy Diving Manual, one fsw equals 0.30643 msw, , or , though elsewhere it states that 33 fsw is (one atmosphere), which gives one fsw equal to about 0.445 psi.Page 2-12. The msw and fsw are the conventional units for measurement of diver pressure exposure used in

decompression tables There are several categories of decompression equipment used to help divers decompress, which is the process required to allow divers to return to the surface safely after spending time underwater at higher ambient pressures. Decompression o ...

and the unit of calibration for

pneumofathometer Surface-supplied diving is diving using equipment supplied with breathing gas using a diver's umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell. This is different from sc ...

s and

hyperbaric chamber A diving chamber is a vessel for human occupation, which may have an entrance that can be sealed to hold an internal pressure significantly higher than ambient pressure, a pressurised gas system to control the internal pressure, and a supply of ...

pressure gauge Pressure measurement is the measurement of an applied force by a fluid ( liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pres ...

Feet of sea water

One atmosphere is approximately equal to 33 feet of sea water or 14.7 psi, which gives 4.9/11 or about 0.445 psi per foot. Atmospheric pressure may be considered constant at sea level, and minor fluctuations caused by the weather are usually ignored. Pressures measured in fsw and msw are

gauge pressure Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressur ...

, relative to the surface pressure of 1 atm absolute, except when a pressure difference is measured between the locks of a hyperbaric chamber, which is also generally measured in fsw and msw. The pressure of seawater at a depth of 33 feet equals one atmosphere. The

absolute pressure Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressu ...

at 33 feet depth in sea water is the sum of atmospheric and hydrostatic pressure for that depth, and is 66 fsw, or two atmospheres absolute. For every additional 33 feet of depth, another atmosphere of pressure accumulates. Therefore at the surface the gauge pressure of 0 fsw is equivalent to an absolute pressure of , and the gauge pressure in fsw at any depth is incremented by 1 ata to provide absolute pressure. (Pressure in ata = Depth in feet/33 + 1)

Usage

Surface supply air panel for 4 divers P3053737

In diving the

is used in most computations, particularly for decompression and

breathing gas A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed h ...

consumption but depth is measured by way of

hydrostatic pressure Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imme ...

. In metric units the

ambient pressure Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording), also known as atmospheres or backgrounds * Ambient music, a genre of music that puts an emphasis on tone and atmosphere * ''Ambient'' (album), by Moby * ...

is usually measured in metres sea water (msw), and converted to bar for calculations. In US customary units ambient pressure is normally measured in feet of sea water (fsw), and converted to atmospheres absolute or pounds per square inch absolute (psia) for decompression computation. Feet and metres sea water are convenient measures which approximate closely to depth and are intuitively simple to grasp for the diver, compared to the options of more conventional units of pressure which give no direct indication of depth. The distinction between gauge and absolute pressure is important for calculation of gas properties and pressure must be identified as either gauge or absolute. Gauge pressure in msw or fsw is converted to absolute pressure in bar or atm for decompression and gas consumption calculation, but

are usually provided ready for use directly with the gauge pressure in msw and fsw.

Depth gauge A depth gauge is an instrument for measuring depth below a reference surface. They include depth gauges for underwater diving and similar applications, and engineering instruments used to measure the depth of holes and indentations from a refer ...

s and

dive computers A dive computer, personal decompression computer or decompression meter is a device used by an underwater diver to measure the elapsed time and depth during a dive and use this data to calculate and display an ascent profile which according to t ...

with readouts calibrated in feet and metres are actually displaying a pressure measurement, usually in feet or metres sea water, as most diving is done in the sea. If ambient pressure in fresh water and hyperbaric chambers is measured in feet and metres sea water, the same

decompression algorithm There are several categories of decompression equipment used to help divers decompress, which is the process required to allow divers to return to the surface safely after spending time underwater at higher ambient pressures. Decompression o ...

s and tables can be used, which eliminates the need to use calibration factors when diving in these environments.

Conversions

In the metric system, a pressure of 10 msw is defined as 1 bar. Pressure conversion between msw and fsw is slightly different from length conversion between metres and feet; 10 msw = 32.6336 fsw and 10 m = 32.8083 ft. The US Navy Diving Manual gives conversion factors for "fw" (feet water) based on a fresh water density of 62.4 lb/ft³ and for fsw based on a sea water density of 64.0 lb/ft³. One standard metre sea water equals: * * at * by definition * , in

SI units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...

* , in cgs units One standard metre sea water is also approximately equal to: * * * * * One standard foot sea water is approximately equal to: * * , in

* , in cgs units * * * * * *

Similar units

*Feet fresh water (ffw) or Feet water (fw), equivalent to 1/34 atm.

References

Sources

* {{authority control Units of pressure Underwater diving physics