f -> fsw -> msw -> m

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They used fresh water to create the "feet of salt water" unit? That makes no sense.
Nevertheless, the fact remains that the basic pressure unit was developed using a ratio of a depth (1 meter of fresh water) to an arbitrary unit (1). It needed to be adapted to work for salt water.

Following your conclusion, if I'm under 100 feet of salt water, I feel a pressure of 100fsw. Let's see, you say that 100 feet are 30,5 meters and 100fsw are 30,7msw, right?
You feel the same pressure at 30.5 meters depth in salt water that you would feel at 30.7 meters of depth in fresh water.

So I'm diving at a depth of 30,5 meters and I feel a pressure of 30,7msw?
Almost right. You feel the pressure of 30.7 mfw.

I know saltwater is denser than freshwater and that our depth gauges measure pressure, which are displayed as depth.
fsw is an abbreviation for feet sea water, msw is an abbreviaton for meters sea water.

Youre right, I do have a serious mental block with regards to why fsw->msw takes into account a liquid that is not present.
If I dive a freshwater calibrated gauge in seawater, I can see why I need to add ~2,5% depth to whats displayed, but that would be applicable both to feet and meters?
Yes. It is necessary to adjust for the density regardless of whether you are measuring your depth in feet or meters.
 
Almost right. You feel the pressure of 30.7 mfw.

mfw?

Ok, I'll just repeat the question, in case you decide answering it in msw:
Following your conclusion, if I'm under 100 feet of salt water, I feel a pressure of 100fsw. Let's see, you say that 100 feet are 30,5 meters and 100fsw are 30,7msw, right?

And, while answering, remember that the conversions 100ft:30,5m and 100fsw:30,7msw come from your previous post.
 
mfw?

Ok, I'll just repeat the question, in case you decide answering it in msw:
Following your conclusion, if I'm under 100 feet of salt water, I feel a pressure of 100fsw. Let's see, you say that 100 feet are 30,5 meters and 100fsw are 30,7msw, right?

And, while answering, remember that the conversions 100ft:30,5m and 100fsw:30,7msw come from your previous post.
No, that's not right. When you are at 30.5 meters depth in the ocean you feel the pressure equivalent to 30.7 mfw (not msw). No post of mine gave a value of 30.7 msw; I very specifically said 30.7 mfw.
 
Ok. you win.

The NOAA manual has an errata and where it says msw it should say mfw.

Or any other similar change that justifies the incoherence between the conversion factors ft:m and fsw:msw.
 
"win"? I'm not competitive, so "winning" is meaningless to me. All I hope is that some light has been shed on an often confusing topic. I know that the sentence in question comes from Chapter 16 "Oxygen and Mixed Gases" of the NOAA Dive Manual. If you're reading that chapter, you're a fairly dedicated student of diving science and are to be commended. Anybody who delves into that just for fun deserves the kind of respectful patience in a discussion of this sort that I think we have engaged in.
 
1. Go to post #1
2. Read the quote
3. Tell me if it says 1 fsw = 0.307 mfw there? (Witch would make perfect sense).

Hint: watch the middle letter in the TLAs very carefully..
 
I can obviously agree with that (as C and D are mine and A and B from the manual, but what you're saying there is that the NOAA diving manual is just wrong.…

I believe you are referring to the footnote at the bottom of Page 16-1 of the NOAA Diving Manual, Fourth Edition:

1 Metric pressure and depth conversions by agreement are shown to the nearest 0.1 meter or msw. In some cases, the references to pressure or length are to ranges and properly should not appear to be so exact, so any excess precision should be ignored. 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.

You are correct and this footnote is incorrect. The linear distance below the surface of sea water and pressure equivalent depth expressed in FSW or MSW are equal. Therefore the linear conversion factor between feet and meters apply. Conversion factors only apply when the water has a different density than sea water.

In reality, it has little practical effect unless you are planning diving operations in fresh water, or a high salinity body of water, based on linear measurements that appear in engineering drawings or on charts. Commercial divers often deal with this conversion when working on dams. Vertical location of the diver’s work site indicated by drawings must be converted because the pneumo-fathometers sensing the diver’s depth are high-precision instruments calibrated in units of Sea Water. Decompression is always based on pressure, not linear distance from the surface.

I apologize for not fully comprehending your initial question.
 
I completely agree with what Akimbo has said.

I mean, the word conversion implies that one unit will be expressed in terms of another unit (in our case another medium).
So, the discrepancy with the OP's question really lies in that the wrong units were used to begin with

What is true in terms of equivalent pressure is that:
pressure of 1 fsw = pressure of 0.305 msw
pressure of 1 fsw = pressure of 0.307 mfw (not msw)

This discution was well worth the read since it made me realise that maybe not all we read in manuals and books is true!
 
So, does all that hold true when that fresh water, at @0° C, is ice? :snicker:

Well now this is my cup of tea and here comes a hydrology lesson in ice!

Water at of above 0°C is 1000kg/m^3 ice is 916.7kg/m^3 at 0°C but here comes the exciting part! Water at 0.01°C is is the triple point, that being it can become liquid, gas, or a solid. Ice will form above 0°C but its in the .01°C range and the ice lattice structure is forming, at 0°C the lattice is set and the density changes from 1000kg/m^3 to 916.7kg/m^3.

Note this is all freshwater density.
 
I'm sorry, but that doesn't solve my problem with the paragraph I quoted in the first post:
"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."

Because, no matter which path or precision you follow, these four premises can't be all true at the same time:
A - 1 fsw = 0.307 msw
B -
1 ft = 0.305 m
C - 1fsw = 1ft of sea water
D - 1msw = 1m of sea water

OP is right, and both C and D in your quote are false. Erik Baker, who contributes to many of the decompression theory and models in use today, has an explanation in his Introductory Deco Lessions, at ftp://ftp.decompression.org/pub/Baker/Introductory Deco Lessons.pdf. Here is the relevant excerpt:

Erik Baker:
The American usage unit of depth (pressure) is feet of seawater (fsw) and is DEFINED as,

1 fsw = 1/33 standard atmosphere = 3.0705 kPa = 3.0705 x 10^3 Pascals (N/m^2)

This unit conforms to a specific gravity for sea water of 1.020.

The European usage unit of depth (pressure) is the meter of seawater (msw) and is DEFINED as,

1 msw = 1/10 bar = 10 kPa = 10^4 Pascals (N/m^2)

This conforms to a specific gravity for sea water of 1.027.

So the pressure at 10 msw is defined to be 1 bar, and the pressure at 33 fsw is defined to be 1 atm = 1.01325 bar. It's easy to see from the quote above, (3.0705 x 10^3 Pa) / (10^4 Pa) = 0.30705 fsw / msw, which agrees with OP and your point A. Interestingly these definitions ignore both the defined relation between feet and meters (length) and the definition of a standard atmosphere (1.01325 bar). Chalk another one up to arbitrarily-defined units.

I should add though... as has been mentioned in the thread a few times, these small differences are well under the margin of error in typical depth gauges, and should only matter for calculating decompression tables and such when errors can accumulate.

HTH
 
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