FSW - What is this telling me?

[MXGratefulDiver]

FSW stands for "feet salt water". Why use it? Well, because when we calculate things like no-decompression limit, or average air consumption, we're not really doing so based on depth ... we're doing so based on the ambient pressure that surrounds us ... and that depends on the density of the water we're swimming in.

Salt water is heavier than fresh water. What this means to us as divers is that in order to reach the same pressure, we have to go deeper in fresh water than in salt water. So to make a more specific determination for our depth calculations, we specify which kind of water we're diving in ... FFW or FSW.

As an example ... four atmospheres of pressure in salt water is 99 FSW ... four atmospheres of pressure in fresh water is 103 FFW. Different depths, but both will give you the same no-decompression limit (assuming both are at sea level) and both will give you the same air consumption rate, even though one is deeper than the other ... because both are at the same ambient pressure.

In diving, pressure is what matters ... depth is an expression of pressure only as it relates to the density of the water you're diving in.

... Bob (Grateful Diver)

 

[txapacheguy]

The difference between the two pressures (FSW and FFW) vs. the actual depth began to take meaning for me when I took the Advanced Nitrox/Deco Procedures class awhile back. I dive at altitude primarily (4500' or so), but also make it down to sea level occasionally. When calculating oxygen partial pressure and air consumption based on depth(pressures in atmospheres absolute or ATAs) and surface air consumption (SAC rate), the difference between fresh and salt water density begins to make a bit of difference to me. FWIW, I calculate that every 33' of salt water adds one additional atmosphere of pressure and that every 34' of fresh water adds one atmosphere of pressure. That would make about six feet of difference at 200' deep 7.06ATA with salt water, and 6.882 ATA with fresh water. That would make my partial pressure of oxygen on EAN23 at 200FSW 1.62 and at 200FFW it would be 1.58. One of those partial pressures is generally considered inside the allowable, and the other is not.

Disclaimer: I threw these numbers up simply to demonstrate why one might need to know the difference between FFW and FSW for dive planning. I do not advocate conducting the working portion of a 200' dive at a PPO2 at or near 1.6 ATA nor would I dive a mixture of EAN23 at that depth.

Safe Diving!!!

Will

 

[boat sju]

That's OK. I really don't mind looking stupid as long as I am learning something.

:bonk:

I didn't mean to imply anything like that. I was just having fun with you. My sense of humor is kind of off after I've been tailgating all day.

 

[gcarter]

I didn't mean to imply anything like that. I was just having fun with you. My sense of humor is kind of off after I've been tailgating all day.

Hey, no worries, I took it as your sense of humor, and came back at you with mine. All good

Thanks all for the feedback, I am in fact learning here - and hopefully I have saved some other poor schmuck from having to look silly by asking the question!

 

[PhuketPaul]

This is all pretty irrevelevant really because to make life simple we say that sea water is 3% more dense than fresh water - but this is a huge approximation:

Density of water (and thus the pressure) depends upon:

• Salinity - This changes from place to place dependent upon temperature, geographic position (centre of ocean, near fresh water outlet etc)
• Temperature - As water reduces in temperature (down to 4Celsius) it gets denser
• Pressure - Water does get compressed and thus denser as we get deeper

This show that there are many variable and to say that the density of fresh water is 1 and sea-water is 1.03 is a huge approximation and usually not accurate at all.

However the dive tables (and computers)are based up on trials (and not strict physiology rules) and are not guaranteed for everybody and thus for all practical purposes we can ignore the differences between sea-water and fresh water.

So just enjoy your dive and ignore whether your computer is calibrated for fresh or salt water. just err on the safe side.

 

[dmoore19]

Specific Gravity of pure fresh water is expressed as 1. The Density of Fresh water is also expressed as 1 but it is 1 g/ml or 1 g/cu cm.[FONT=Arial, Helvetica, sans-serif][SIZE=-1]

1000kg of pure water @ 4°C = 1 cubic metre, those materials under 1000kg/cubic metre will float; more dense will sink ie. those materials with a specific gravity more than 1[/SIZE][/FONT]
[TABLE="align: center"]
[TR="bgcolor: #FFFFEE"]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Temp
( °C )[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Density
pure
water ( g/cm³ )[/SIZE][/FONT]​

[/TD]
[TD="width: 12%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Density
pure water
( kg/m^{3 )}[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Density
tap
water
( g/cm³ )[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Density
pure
water
lb/cu.ft[/SIZE][/FONT]​

[/TD]
[TD="width: 14%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Specific Gravity
4°C reference[/SIZE][/FONT]​

[/TD]
[TD="width: 22%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Specific
Gravity
60°F
reference[/SIZE][/FONT]​

[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0 (solid)[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.9150[/SIZE][/FONT]​

[/TD]
[TD="width: 12%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]915.0[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]-[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][FONT=Arial, Helvetica, sans-serif][SIZE=-1]-[/SIZE][/FONT][/FONT]​

[/TD]
[TD="width: 14%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.915[/SIZE][/FONT]​

[/TD]
[TD="width: 22%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]-[/SIZE][/FONT]​

[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0 (liquid)[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.9999[/SIZE][/FONT]​

[/TD]
[TD="width: 12%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]999.9[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.99987[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]62.42[/SIZE][/FONT]​

[/TD]
[TD="width: 14%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.999[/SIZE][/FONT]​

[/TD]
[TD="width: 22%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]1.002[/SIZE][/FONT]​

[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]4[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]1.0000[/SIZE][/FONT]​

[/TD]
[TD="width: 12%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]1000[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.99999[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]62.42[/SIZE][/FONT]​

[/TD]
[TD="width: 14%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]1.000[/SIZE][/FONT]​

[/TD]
[TD="width: 22%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]1.001[/SIZE][/FONT]​

[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]20[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.9982[/SIZE][/FONT]​

[/TD]
[TD="width: 12%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]998.2[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.99823[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]62.28[/SIZE][/FONT]​

[/TD]
[TD="width: 14%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.998[/SIZE][/FONT]​

[/TD]
[TD="width: 22%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.999[/SIZE][/FONT]​

[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]40[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.9922[/SIZE][/FONT]​

[/TD]
[TD="width: 12%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]992.2[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.99225[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]61.92[/SIZE][/FONT]​

[/TD]
[TD="width: 14%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.992[/SIZE][/FONT]​

[/TD]
[TD="width: 22%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.993[/SIZE][/FONT]​

[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]60[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.9832[/SIZE][/FONT]​

[/TD]
[TD="width: 12%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]983.2[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.98389[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]61.39[/SIZE][/FONT]​

[/TD]
[TD="width: 14%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.983[/SIZE][/FONT]​

[/TD]
[TD="width: 22%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.985[/SIZE][/FONT]​

[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]80[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.9718[/SIZE][/FONT]​

[/TD]
[TD="width: 12%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]971.8[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.97487[/SIZE][/FONT]​

[/TD]
[TD="width: 13%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]60.65[/SIZE][/FONT]​

[/TD]
[TD="width: 14%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.972[/SIZE][/FONT]​

[/TD]
[TD="width: 22%"]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1]0.973[/SIZE][/FONT]​

[/TD]
[/TR]
[/TABLE]


Taken from this site.

Mass, Weight, Density or Specific Gravity of Water at Various Temperatures

 

[Akimbo]

I see a lot of posts where rather than just say, for example 90 feet, someone will say 90 FSW. Other that telling me that it is in salt water, is there any other reason to use this convention?....

Technically yes. The reason goes back to a diver’s primary reason for monitoring their depth: decompression. I fear these replies might be a little confusing to others reading this that haven’t been through a diving class yet. I hope this will help.

I believe most of the confusion is because depth (linear vertical distance) and pressure are often intermixed in conversation and literature as convenient shorthand. First, let’s define the relationships. Fresh water is easy since there is an international standard. I will stick with imperial units for this discussion. Fluid pressure is directly derived from the weight.

• 1 Ft³ of fresh water weights 62.2970625024 Lbs at STP (Standard Temperature & Pressure, 0° C at sea level)
• For discussion, let’s round to 62.3 Lbs which is 0.43263888 PSI per foot (62.3Lbs/144 Inches²). For discussion I will round to 0.433 PSI/Ft of depth.
• The average weight of sea water I have seen most often is 64.1 Lbs/Ft³. I know 64 Lbs is used for most buoyancy calculations but that is rounded a little too much for the precision necessary for this discussion.
• 64.1 Lbs/144 Inches² (1 Ft²) = 0.445138888888889 PSI. During my Naval and commercial diving career, 0.445 PSI/FSW was the standard for gauge calibration and calculations.

So, here are the numbers for this discussion accepting the rounding error:

0.445 PSI/FSW (Foot of Sea Water)
0.433 PSI/FFW (Foot of Fresh Water)
​

For the sport diver, what matters most often is the pressure equivalent to FSW because that is what decompression tables and the vast majority of US decompression computers are calibrated in. The depth instrument is actually a pressure sensing device with a display that is calibrated in depth of sea water.

I will use 100 FSW as an example instead of your 90 FSW because I am a lazy. When your depth instrument reads 100', what the sensor is actually reading is 44.5 PSI (gauge, not absolute). As a result, you will actually be 102.8 feet below the surface in a fresh water body that is at or near sea level. That does not place you in violation of a decompression table because they are based on the pressure at 100 FSW.

The difference matters a lot more for most surface supplied diving operations because the pneumo-fathometer gauges that monitor the diver’s depth are much higher precision and resolution. That allows the gauge to be used as a vertical navigation tool for locating points on a platform and directing crane operators. Even then, the diving supervisor must make adjustments for things like tide and variation in instruments that other operators may be using.

In practice it is not critical to most recreational divers because their instruments aren’t that accurate, though many newer dive computers are, and we are only talking about ~3’/100. It is important if you want to really understand the physics and math because glossing over the differences won’t yield accurate answers leaving you to question if you are doing it right.