f -> fsw -> msw -> m

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LouieLouie

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Aaargh. Use proper SI units for pressure, and everything else for that matter, and we no longer have to waste time on silly things like this.
 

Phil_C

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For most things I'm bilingual so to speak, having learnt feet, inches, chains, furlongs, miles and so on when I was at school, and still mainly use miles, gallons and so on when driving, but frequently now use Kilometres, metres, litres and so on.

However I 're-learned' my diving abroad when I returned to it last year and I must say calculating pressure in ATA/ATM and bars, depth in metres and consumed air in litres is so much easier than the calculations I see people trying to do the Imperial Way. It seems like decimalisation got at least one thing right. - P
 

Thanshin

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From the chart above, here are your answers:
100 x 0.445138889 PSI/FSW = 44.5138889 PSI

30½ MSW x 0.100693064 Bar/MSW = 3.071138462 Bar

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

A and B come from the quote. C and D are what I thought to be axioms and must not be (which I'm trying to understand).

Because, if A, B, C and D are all true, by reductio ad absurdum:

[C] Under 1ft of water one has 1fsw,
[A] which is 0.307 msw,
[D] which is 0,307 meters under water

which is >1ft

And 1ft cannot be >1ft.

Thus, taking into account the quote comes from a respected diving manual, either C or D must be false. Which is what I still don't understand.
 

gcarter

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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

A and B come from the quote. C and D are what I thought to be axioms and must not be (which I'm trying to understand).

Because, if A, B, C and D are all true, by reductio ad absurdum:

[C] Under 1ft of water one has 1fsw,
[A] which is 0.307 msw,
[D] which is 0,307 meters under water

which is >1ft

And 1ft cannot be >1ft.

Thus, taking into account the quote comes from a respected diving manual, either C or D must be false. Which is what I still don't understand.


Assuming Ihave understood the explanations given me properly, the confusion you are having is this:

feet -> meters is a linear conversion
fsw -> msw is a pressue conversion

fsw <> feet, and msw <> meters.

Does that help?
 

Akimbo

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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

A and B come from the quote. C and D are what I thought to be axioms and must not be (which I'm trying to understand)…

One linear foot or 12 inches = 0.3048 Meters exactly (25.4 mm/Inch, as of July 1, 1959)
Inch - Wikipedia, the free encyclopedia

Therefore:
A: 1 FSW = 0.3048 MSW, not 0.307 MSW. Note that 1 FFW also equals 0.3048 MFW
B: 1 Linear foot equals 0.3048 Meters (not rounded to 0.305)
C: OK
D: OK

…Because, if A, B, C and D are all true, by reductio ad absurdum:

[C] Under 1ft of water one has 1fsw,
[A] which is 0.307 msw,
[D] which is 0,307 meters under water
which is >1ft

And 1ft cannot be >1ft.

Thus, taking into account the quote comes from a respected diving manual, either C or D must be false. Which is what I still don't understand.


I am not sure I understand your exact question but it seems that you are mixing linear measurements with pressure equivalent measurements. Feet of Sea Water, Feet of Fresh Water, Meters of Sea Water, and Meters of Fresh Water are all pressure equivalent measurements. To prevent confusion, I recommend that you convert them to pressure in your choice of units — Atmospheres is a good choice.
  • 1 Linear Foot below the surface of sea water = 0.0302899047692747 Atmospheres
  • 1 Linear Meter below the surface of sea water = 0.0993761292305287 Atmospheres
  • 1 Linear Foot below the surface of fresh water = 0.0294989390578542 Atmospheres
  • 1 Linear Meter below the surface of fresh water = 0.0967811025589768 Atmospheres
Therefore, the pressure at one linear Meter below the surface of fresh water is less than the pressure at one linear Meter below the surface of sea water… I hope I didn’t screw up all this copying and pasting. :blush:
 

Quero

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Maybe visualize it?

Imagine you have a cube of foam 1 foot long on each side, and also a cube of wood one foot long on each side. Lie down on your back and place the cube of foam on your stomach and notice how much pressure it exerts. Then lie down with the cube of wood on your stomach. How much pressure does it exert in comparison with the cube of foam? Why?

Now apply this principle to water.

Take a cube-shaped container that is 1 foot long on each side. Weigh the container. Then fill it with fresh water. Weigh it again. Dump that water out and fill it with sea water. Weigh it again. You will find that the same linear distance of one foot will produce different weight when the container holds sea water than when it holds fresh water. The pressure that comes to bear on a body depends on the weight of the material exerting the pressure. The notations fsw and ffw refer to the relative pressures found at a particular distance below the surface of the water rather than to the depth itself.

You can repeat this experiment with a container measuring 1 meter on each side, and you will find that the weight of the container when it is filled with sea water is greater than its weight when filled with fresh water. The notation for these pressures is msw and &#8203;mfw.

Your one-foot and one-meter cubes have not changed size, nor have the proportions between them changed (the one foot sides remain equivalent to .305 meters regardless of whether the container holds fresh or sea water). However, the weight of the same volume of water contained in each cube is different depending on whether the volume is fresh or salt water. And when we calculate the proportions, we find that 1 fsw is equivalent to .307 msw in terms of its weight.
 

Tigerman

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Except in the quote of the OP the argument was that while 1ft = 0.305 m, 1 fsw = 0.307 msw, and never mentioned freshwater with a single letter...

I might be tired, but to me the quote of the OP seems to be just wrong tho.. If youre submerged 1 foot in seawater you are submerged 0.305 meters in seawater as well, and not magically 0.002 meters deeper because youre in the water instead of on land..
 

Quero

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Except in the quote of the OP the argument was that while 1ft = 0.305 m, 1 fsw = 0.307 msw, and never mentioned freshwater with a single letter...

I might be tired, but to me the quote of the OP seems to be just wrong tho.. If youre submerged 1 foot in seawater you are submerged 0.305 meters in seawater as well, and not magically 0.002 meters deeper because youre in the water instead of on land..
It's true, T, that he didn't mention fresh water, and it's possible that this additional information will confuse him even further, but to my mind, it shows that pressure is relative to the medium, regardless of the linear dimension of that medium.

This is where you are also confused. Yes, a diver who is one foot under water is also .305 meters under water. Because metric calculations equate volume and weight of fresh water (i.e., one liter of fresh water weighs exactly one kilogram), the pressure exerted at a depth of one foot in fresh water would be .305 ffw. Since sea water is denser due primarily to dissolved salt, a diver who is .305 meters below the surface of the sea will be subject to a slightly greater pressure. In fact, this pressure is .307 msw.

So you see, it's not that it's magically .002 meters deeper in the sea than on land--the distance on land has nothing to do with it. Instead that .002 represents a relatively greater pressure per foot of depth in the sea than in fresh water.
 

Akimbo

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Not sure if this will help or confuse, but consider the pressure equivalent in very dense bodies of water like the American Great Salt Lake in Utah or the Dead Sea. The average density of the lake is about 1.15Kg/Liter the Dead Sea is about 1.24 Kg/Liter. That is about 15% and 24% heavier than fresh water respectively.

The pressure at a linear distance below the surface will increase as will the amount of weight required to make a diver neutrally buoyant. Sort of makes the ~ 3% difference between fresh and salt water seem insignificant, but demonstrates the principal.

Anybody got decompression tables calibrated in MDSW (Meters of Dead Sea Water)? :wink:
 

gcarter

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Assuming Ihave understood the explanations given me properly, the confusion you are having is this:

feet -> meters is a linear conversion
fsw -> msw is a pressue conversion

fsw <> feet, and msw <> meters.

Does that help?

For perhaps a bit more clarity, miles & kilometers are a measure of distance while mph and kph are a measure of speed.

Similarly, feet and meters are a measure of distance, while fsw and msw are a measure of pressure.

The error in understanding is to try to equate a pressure measure with a linear measure. Some of the other explanations here explain why the conversion factor is different, but the key is that they are measuring different things.

If you understand that, the rest is just math that will not matter to most people. For my purposes, it is the pressure I am concerned about. I am not fussed if the pressure measure is out from the physical measure by 3%. My computer works on the pressure.
 
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