Metre sea water

Source: Wikipedia, the free encyclopedia.
"msw" redirects here. For other uses, see MSW (disambiguation).
metre sea water
Hyperbaric chamber pressure gauge calibrated in msw and bar
General information
Unit ofpressure
Symbolmsw
Conversions
1 msw in ...... is equal to ...
   
psi
foot sea water
Pressure gauge on Siebe Gorman manual diver's pump calibrated in fsw and psi
General information
Unit ofpressure
Symbolfsw
Conversions
1 fsw in ...... is equal to ...
   
psi

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

metric unit of pressure used in underwater diving. It is defined as one tenth of a bar.[1][2]

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

psi,[1][2] though elsewhere it states that 33 fsw is 14.7 psi (one atmosphere), which gives one fsw equal to about 0.445 psi.[3]

The msw and fsw are the conventional units for measurement of

pressure gauges.[4]

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.

gauge pressure
, 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 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.[6] Therefore at the surface the gauge pressure of 0 fsw is equivalent to an absolute pressure of 1 standard atmosphere (14.7 psi), 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) [7]

Usage

Surface supply air panel for 4 divers with pneumofathometer gauges calibrated in metres and feet seawater

In diving the

decompression algorithms and tables can be used, which eliminates the need to use calibration factors when diving in these environments.[8]

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.[1]

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.[1]

One standard metre sea water equals:[1]

  • 3.26336 fsw
  • 102.018 
    cmH2O
     at 15 °C
  • 0.1 bar by definition
  • 10.0 
    SI units
  • 100000 
    cgs units

One standard metre sea water is also approximately equal to:[1]

  • 0.0986923 
    atm
  • 1.45038 
    psi
  • 75.0062 mmHg
  • 75.0062 Torr
  • 2.95299 inHg

One standard foot sea water is approximately equal to:[1]

  • 0.30643 msw
  • 3.0643 
    SI units
  • 30643 
    cgs units
  • 0.030242 
    atm
  • 0.44444 
    psi
  • 22.984 mmHg
  • 22.984 Torr
  • 0.904884 inHg
  • 31.24616 
    cmH2O

Similar units

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

References

  1. ^ a b c d e f g h US Navy Diving Manual 2016, Table 2‑10. Pressure Equivalents..
  2. ^ a b Staff (2016). "2 - Diving physics". Guidance for Diving Supervisors (IMCA D 022 August 2016, Rev. 1 ed.). London, UK: International Marine Contractors' Association. p. 3.
  3. ^ Page 2-12.
  4. ^ US Navy Diving Manual 2016, Section 18‑2.8.3.
  5. ^ US Navy Diving Manual 2016, Section 2-9.1.
  6. ^ US Navy Diving Manual 2016, Section 2-9.3.
  7. ^ US Navy Diving Manual 2016, Table 2‑1. Pressure Chart, and Figure 2.7.
  8. ^ a b NOAA Diving Program (U.S.) (December 1979). Miller, James W. (ed.). NOAA Diving Manual, Diving for Science and Technology (2nd ed.). Silver Spring, Maryland: US Department of Commerce: National Oceanic and Atmospheric Administration, Office of Ocean Engineering.

Sources

Retrieved from "https://en.wikipedia.org/w/index.php?title=Metre_sea_water&oldid=1200457421"