View source for Physics:Metre sea water

{{short description|Unit of pressure equal to one tenth of a bar}}
{{Infobox unit
| name     = metre sea water
| symbol   = msw
| image    = File:Manómetro de la cámara.jpg
| caption  = Hyperbaric chamber pressure gauge calibrated in msw and bar
| quantity = [[Physics:Pressure|pressure]]
| units1   = SI units
| inunits1 = ={{thinsp}}{{val|10000.0|ul=Pa}}
| units2   = CGS units
| inunits2 = ={{thinsp}}{{val|100000|ul=Ba}}
| units3   = U.S. customary
| inunits3 = ≈{{thinsp}}{{val|3.263}}&nbsp;fsw<br/>&emsp;≈{{thinsp}}{{val|1.45038|ul=psi}}
}}
{{Infobox unit
| name     = foot sea water
| symbol   = fsw
| image    = File:Pressure gauge on Siebe Gorman manual diver's pump P3220126.jpg
| caption  = Pressure gauge on Siebe Gorman manual diver's pump calibrated in fsw and psi
| quantity = [[Physics:Pressure|pressure]]
| units1   = SI units
| inunits1 = ≈{{thinsp}}{{val|3064.3|ul=Pa}}
| units2   = CGS units
| inunits2 = ≈{{thinsp}}{{val|30643|ul=Ba}}
| units3   = U.S. customary
| inunits3 = ≈{{thinsp}}{{val|0.44444|ul=psi}}
}}

The '''metre''' (or '''meter''') '''sea water''' ('''msw''') is a [[Metric units|metric unit]] of [[Physics:Pressure|pressure]] used in underwater diving. It is defined as one tenth of a [[Bar (unit)|bar]].{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}<ref name="IMCA D022" />

The unit used in the US is the '''foot sea water''' ('''fsw'''), based on [[Physics:Standard gravity|standard gravity]] and a sea-water density of 64&nbsp;lb/ft<sup>3</sup>. According to the US Navy Diving Manual, one fsw equals 0.30643&nbsp;msw, {{val|.030643|ul=bar}}, or {{val|0.44444|ul=psi}},{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}<ref name="IMCA D022" /> though elsewhere it states that 33&nbsp;fsw is {{val|14.7|u=psi}} (one atmosphere), which gives one fsw equal to about 0.445&nbsp;psi.<ref>Page 2-12.</ref>

The msw and fsw are the conventional units for measurement of diver pressure exposure used in decompression tables and the unit of calibration for pneumofathometers and hyperbaric chamber pressure gauges.{{sfn|''US Navy Diving Manual''|2016|loc=Section 18‑2.8.3}}

==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.{{sfn|''US Navy Diving Manual''|2016|loc=Section 2-9.1}} Pressures measured in fsw and msw are 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.{{sfn|''US Navy Diving Manual''|2016|loc=Section 2-9.3}} Therefore at the surface the gauge pressure of 0 fsw is equivalent to an absolute pressure of {{Convert|1|atm|psi|1}}, 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) {{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑1. Pressure Chart, and Figure 2.7}}

==Usage==
[[File:Surface supply air panel for 4 divers P3053737.jpg|thumb|Surface supply air panel for 4 divers with pneumofathometer gauges calibrated in metres and feet seawater]]
In diving the absolute pressure is used in most computations, particularly for decompression and [[Chemistry:Breathing gas|breathing gas]] consumption but depth is measured by way of hydrostatic pressure.<ref name="NOAA Diving Manual 1979" /> In metric units the [[Physics:Ambient pressure|ambient pressure]] 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 decompression tables are usually provided ready for use directly with the gauge pressure in msw and fsw. [[Depth gauge]]s and dive computers 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 algorithms and tables can be used, which eliminates the need to use calibration factors when diving in these environments.<ref name="NOAA Diving Manual 1979" />

== Conversions ==

In the metric system, a pressure of 10&nbsp;msw is defined as 1&nbsp;bar. Pressure conversion between msw and fsw is slightly different from length conversion between metres and feet; 10&nbsp;msw&nbsp;=&nbsp;32.6336&nbsp;fsw and 10&nbsp;m&nbsp;=&nbsp;32.8083&nbsp;ft.{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}

The US Navy Diving Manual gives conversion factors for "fw" (feet water) based on a fresh water density of 62.4&nbsp;lb/ft<sup>3</sup> and for fsw based on a sea water density of 64.0&nbsp;lb/ft<sup>3</sup>.{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}

One standard metre sea water equals:{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}
* {{val|3.26336|u=fsw}}
* {{val|102.018|u=[[Physics:Centimetre of water|cmH<sub>2</sub>O]]}} at {{val|15|u=Celsius}}
* {{val|0.1|u=bar}} by definition
* {{val|10.0|ul=kPa}}, in SI units
* {{val|100000|ul=Ba}}, in cgs units

One standard metre sea water is also approximately equal to:{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}
* {{val|0.0986923|ul=atm}}
* {{val|1.45038|ul=psi}}
* {{val|75.0062|ul=mmHg}}
* {{val|75.0062|ul=torr}}
* {{val|2.95299|ul=inHg}}

One standard foot sea water is approximately equal to:{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}
* {{val|0.30643|u=msw}}
* {{val|3.0643|ul=kPa}}, in SI units
* {{val|30643|ul=Ba}}, in cgs units
* {{val|0.030242|ul=atm}}
* {{val|0.44444|ul=psi}}
* {{val|22.984|ul=mmHg}}
* {{val|22.984|ul=torr}}
* {{val|0.904884|ul=inHg}}
* {{val|31.24616|u=[[Physics:Centimetre of water|cmH<sub>2</sub>O]]}}

== Similar units ==
*Feet fresh water (ffw) or Feet water (fw), equivalent to 1/34 atm.{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}

== References ==
{{Reflist|refs=

<ref name="IMCA D022">{{cite book|last=Staff |title=Guidance for Diving Supervisors |url=http://www.imca-int.com/diving |edition=IMCA D 022 August 2016, Rev. 1|year=2016 |publisher=International Marine Contractors' Association |location=London, UK |chapter=2 - Diving physics|page=3}}</ref>

<ref name="NOAA Diving Manual 1979" > {{cite book |title=NOAA Diving Manual, Diving for Science and Technology |author=NOAA Diving Program (U.S.) |edition=2nd |editor-first=James W. |editor-last=Miller |date=December 1979 |publisher=US Department of Commerce: National Oceanic and Atmospheric Administration, Office of Ocean Engineering |location=Silver Spring, Maryland |isbn=|ref={{sfnRef|NOAA Diving Manual|1979}} }}</ref><!--{{sfn|''NOAA Diving Manual''|1979|loc=}}--> 

}}

===Sources===
*{{cite book|last=US Navy|title=U.S. Navy Diving Manual Revision 7 SS521-AG-PRO-010 0910-LP-115-1921|url=http://www.navsea.navy.mil/Portals/103/Documents/SUPSALV/Diving/US%20DIVING%20MANUAL_REV7.pdf?ver=2016-12-14-135043-757|archive-url=https://web.archive.org/web/20161228033101/http://www.navsea.navy.mil/Portals/103/Documents/SUPSALV/Diving/US%20DIVING%20MANUAL_REV7.pdf?ver=2016-12-14-135043-757|archive-date=December 28, 2016|url-status=live|date=1 December 2016|publisher=US Naval Sea Systems Command|location=Washington, DC.|ref={{harvid|US Navy Diving Manual|2016}}}}


[[Category:Units of pressure]]
[[Category:Underwater diving physics]]

{{Sourceattribution|Metre sea water}}