f -> fsw -> msw -> m

[Thanshin]

would someone please explain this paragraph to me? As I understand it, it makes no sense, so...


"The conversion between feet and meters of sea water is 1 fsw = 0.307 msw. This conversion is for the pressure units (msw and fsw), not the units of length,meter and foot which is : 1 ft = 0.305 m."

 

[DevonDiver]

Have a look at these threads:

http://www.scubaboard.com/forums/new-divers-those-considering-diving/2398-whats-fsw.html

[URL]http://www.scubaboard.com/forums/basic-scuba-discussions/395918-fsw-what-telling-me.html[/URL]

 

[Thanshin]

Have a look at these threads:

I must be dense, as sea water, because I still don't see one detail:

Let's say we're 100ft deep on sea water.
So we're at: 100ft and also 100fsw, right?

Now, let's convert, following the indications of that paragraph:
100ft -> 30,5m
100fsw -> 30,7msw

So we're 30,5 meters deep and yet we feel the pressure of 30,7 meters of sea water.

 

[Akimbo]

Don’t confuse the linear distance (depth in feet or meters) with the equivalent pressure at that depth. Because decompression is based on the pressure exerted on the human body, we often indicate depth as FFW/MFW or FSW/MSW for fresh and sea or salt water. The short-hand of simply using Feet or Meters for a sea water depth is common, but what really matters is the pressure, not the linear distance from the surface.

Even the instruments we use to measure depth are actually pressure sensing instruments calibrated in FSW or MSW.

 

[Thanshin]

Don’t confuse the linear distance (depth in feet or meters) with the equivalent pressure at that depth.

Let me ask you in a different way. Diving in salt water:

A - What pressure (in fsw) would you expect to feel at a depth of 100ft?
B - What pressure (in msw) would you expect to feel at a depth of 30,5m?

 

[Tigerman]

The depth gagues should just read mmhg instead of meters/feet and give us more accurate readings, given that it really is pressure and not distance that is the important factor

 

[Akimbo]

I will start with the basics and high decimal precision to illustrate the point. Sea water weighs more than fresh water for the same volume. You will notice some variation in conversion values of water because of differences in temperature, rounding, salinity, and other “contaminants”, but these are close averages.

Weight of Water
[TABLE="class: grid, width: 811"]
[TR]
[TD]64.1[/TD]
[TD]Lbs/Ft³ of Sea Water[/TD]
[/TR]
[TR]
[TD]1028.940971[/TD]
[TD]Kg/M³ of Sea Water[/TD]
[/TR]
[TR]
[TD]62.2970625[/TD]
[TD]Lbs/Ft³ Fresh Water @0° C[/TD]
[/TR]
[TR]
[TD]1000[/TD]
[TD]Kg/M³ Fresh Water @0° C[/TD]
[/TR]
[/TABLE]


Note: Sea water is about 2.89% heavier than fresh water.

The weight of the water determines the pressure. Here are pressure equivalents to one Atmosphere since that is common in diving both in Imperial or Metric units.

One Standard Atmosphere Equals:
SI Units
[TABLE="class: grid, width: 810"]
[TR]
[TD]101,325[/TD]
[TD]Pascals[/TD]
[/TR]
[TR]
[TD]101.325[/TD]
[TD]KPa, Kilo Pascals, or 1000x[/TD]
[/TR]
[TR]
[TD]0.101325[/TD]
[TD]MPa or Mega Pascals or 1 Million x[/TD]
[/TR]
[TR]
[TD]1.01325[/TD]
[TD]Bar[/TD]
[/TR]
[TR]
[TD]10.06275861[/TD]
[TD]Meters of Sea Water[/TD]
[/TR]
[/TABLE]


Note: 0.100693064 Bar per MSW (Meter of Sea Water)

Imperial Units
[TABLE="class: grid, width: 810"]
[TR]
[TD]14.69594878[/TD]
[TD]PSI[/TD]
[/TR]
[TR]
[TD]33.89952425[/TD]
[TD]Feet of Fresh Water at 4° C[/TD]
[/TR]
[TR]
[TD]33.0142999[/TD]
[TD]Feet of Seawater based on a density of 64.1 Lbs/Ft³[/TD]
[/TR]
[/TABLE]


Note: One cubit foot divided by 144 (the number of square inches in a square foot) equals the pressure exerted in PSI. Therefore there is 0.445138889 PSI (usually rounded to .445) for every FSW (Foot of Sea Water) — 62.1 Lbs divided by 144 In²

From the chart above, here are your answers:

Let me ask you in a different way. Diving in salt water:

A - What pressure (in fsw) would you expect to feel at a depth of 100ft?...

100 x 0.445138889 PSI/FSW = 44.5138889 PSI

Another common method is:

100 FSW divided by 33.0142999 (usually rounded to 33 FSW) = 3.028990477 ATM (not ATA)

…B - What pressure (in msw) would you expect to feel at a depth of 30,5m?

30½ MSW x 0.100693064 Bar/MSW = 3.071138462 Bar

As you can see, the metric measurements conveniently round to 10 MSW = 1 ATA = 1 Bar — certainly within the accuracy of the computers and depth gauges we use. Since there is only about 2.89% in the difference in weight between fresh and sea water, there is about 2.89% difference in linear depth measurement.

 

[Akimbo]

The depth gagues should just read mmhg instead of meters/feet and give us more accurate readings, given that it really is pressure and not distance that is the important factor

You still have confusion because charts and fathometers are in linear units of measure. Decompression tables and computers need to match the depth sensing device. If we switch to a pressure unit, please let it be atmospheres!

 

[Jax]

I will start with the basics and high decimal precision to illustrate the point. Sea water weighs more than fresh water for the same volume. You will notice some variation in conversion values of water because of differences in temperature, rounding, salinity, and other “contaminants”, but these are close averages. Weight of Water [TABLE="class: grid, width: 811"] [TR] [TD]64.1[/TD] [TD]Lbs/Ft³ of Sea Water[/TD] [/TR] [TR] [TD]1028.940971[/TD] [TD]Kg/M³ of Sea Water[/TD] [/TR] [TR] [TD]62.2970625[/TD] [TD]Lbs/Ft³ Fresh Water @0° C[/TD] [/TR] [TR] [TD]1000[/TD] [TD]Kg/M³ Fresh Water @0° C[/TD] [/TR] [/TABLE] Note: Sea water is about 2.89% heavier than fresh water.

So, does all that hold true when that fresh water, at @0° C, is ice? :snicker:

 

[Akimbo]

So, does all that hold true when that fresh water, at @0° C, is ice? :snicker:

Sure, since it is still water until it is below 0° C and becomes ice. The way I understand it, they chose that temperature because it was easy to achieve by adding lots of ice to a beaker full the water and letting the temperature stabilize.