f -> fsw -> msw -> m
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That's sort of it, yeah. The pressure term meters-of-seawater has as much to do with a meter of depth as kilometers-per- hour of, say, wind speed has to do with a kilometer of the earth the wind crosses.
Tigerman
Contributor
Youre making less sense than ever, as does using the term fsw or msw in the first place if its got "nothing to do" with linear depth whatsoever..
IS;
The pressure excerted at a depth 10 ft seawater == 10 fsw?
The pressure excerted at 10 ft seawater == the pressure excerted at 3,05 meters of seawater?
IS;
The pressure excerted at a depth 10 ft seawater == 10 fsw?
The pressure excerted at 10 ft seawater == the pressure excerted at 3,05 meters of seawater?
Okay, let's back up. When we are diving, we have instruments to measure the depth, correct? Well, these instruments actually don't measure depth--they measure pressure, and because pressure and depth are proportionately related, we can express this pressure as depth.
Now suppose that the depth gauge is calibrated to the technically correct 1:1 ratio of depth
ressure which is 1 m depth in fresh water = 1 unit of pressure. If you dive in salt water using that gauge calibrated to fresh water, when your "depth" reading is 10 on the gauge, you will actually not be 10 units of distance (feet or meters) from the surface--you'll be shallower. If you actually go to 10 (feet or meters), the pressure will make the gauge read at a greater "depth" than you are physically at.Of course it's possible to calibrate the depth gauge to accurately display depth in sea water by taking into account the greater pressure exerted by sea water at a given depth, and in fact, most depth gauges are calibrated to sea water. But that doesn't change the fact that the pressure units do not have an exact 1:1 correspondence to the linear units when it comes to sea water. It is because of this discrepancy that people refer to msw/fsw and mfw/ffw.
Tigerman
Contributor
I guess just answering my question was too hard..
is 10 feet of sewater == 10 fsw or not?
just FORGET about freshwater and saltwater differences for a second and answer that question if you want to "explain" why its not a typo in the op
is 10 feet of sewater == 10 fsw or not?
just FORGET about freshwater and saltwater differences for a second and answer that question if you want to "explain" why its not a typo in the op
You cannot just forget about the difference between fresh and salt water since that is the source of the .002 difference that is bugging you so much.
If you calibrate a depth gauge to salt water, when it reads 10 ft, you will indeed be at 10 ft. However, if your depth gauge is not calibrated to salt water, when it reads 10 ft, you will actually be closer to 9.9 feet.
Tigerman
Contributor
Thats not the topic of discussion whatsoever.. Good going on adding to the confusion..
The OP quoted a statement that states a difference between metric and imperial measures in saltwater, not between saltwater and freshwater..
Or are you actually suggesting that when its METERS/sw instead of FEET/sw that the water change salinity and that freshwater by default is metric and saltwater imperial?
The OP quoted a statement that states a difference between metric and imperial measures in saltwater, not between saltwater and freshwater..
Or are you actually suggesting that when its METERS/sw instead of FEET/sw that the water change salinity and that freshwater by default is metric and saltwater imperial?
Now you are just being silly. You clearly have a mental block for this, and it appears to be pointless to try to help you understand it.
Nevertheless, one last attempt:
The NOAA simply says that when making conversions for pressures in sw, the ratio to use to get from meters to feet is 1:.307 rather than the 1:.305 that is used to convert lengths expressed in meters and feet. The core reason for this apparent discrepancy is the greater density of sw as compared to the standard on which the pressure units were created, which was fresh water.
They used fresh water to create the "feet of salt water" unit? That makes no sense.
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?
So I'm diving at a depth of 30,5 meters and I feel a pressure of 30,7msw?
Tigerman
Contributor
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?
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?
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