Metre sea water

Key takeaways

Metre sea water belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Metre sea water to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Metre sea water from memory before moving on to harder problems.

Reference excerpt

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. or as 1 msw = 10.0381 kPa according to EN 13319. The unit used in the US is the foot sea water (fsw), based on standard gravity and a sea-water density of 64 lb/ft3. According to the US Navy Diving Manual, one fsw equals 0.30643 msw, 0.030643 bar, or 0.44444 psi, though elsewhere it states that 33 fsw is 14.7 psi (one atmosphere), which gives one fsw equal to about 0.445 psi. 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.

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, 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. 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)

Usage

In diving the absolute pressure is used in most computations, particularly for decompression and breathing gas consumption but depth is measured by way of hydrostatic pressure. In metric units the 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 gauges 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.

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:

3.26336 fsw 102.018 cmH2O at 15 °C 0.1 bar by definition 10.0 kPa, in SI units 100000 Ba, in cgs units One standard metre sea water is also approximately equal to:

0.0986923 atm 1.45038 psi 75.0062 mmHg 75.0062 Torr 2.95299 inHg One standard foot sea water is approximately equal to:

0.30643 msw 3.0643 kPa, in SI units 30643 Ba, in 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.

References

Sources US Navy (1 December 2016). U.S. Navy Diving Manual Revision 7 SS521-AG-PRO-010 0910-LP-115-1921 (PDF). Washington, DC.: US Naval Sea Systems Command. Archived (PDF) from the original on December 28, 2016.

Illustrations

Worked examples

Example 1 — a first encounter with Metre sea water

Start with the simplest possible case. Write down what Metre sea water claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Metre sea water before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Metre sea water ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Metre sea water

In research: Metre sea water appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Metre sea water in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Metre sea water is common in secondary-school and first-year university syllabi. It links to neighbouring topics Units of pressure, so understanding it makes those chapters shorter.
In everyday life: Look for Metre sea water outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Metre sea water in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Metre sea water means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Metre sea water out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Metre sea water in simple terms?

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. or as 1 msw = 10.0381 kPa according to EN 13319.

Why does Metre sea water matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Metre sea water?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Metre sea water.