Noting your depth with FSW or FFW

[emcbride81]

Just curious, I read here as well as some magazines where people attach feet salt water or feet fresh water along with their depth. Other than just letting you know the environment, is their any other reason to do this? I wasn't sure if there was some factor that makes this distinction important that I did not know.

 

[do it easy]

Some of us are proud to dive freshwater! I think the distinction is just to properly describe the environment.

It might make a difference for deep, deep, stupid deep dives if your tables are based on freshwater and your depth gauge reads in saltwater. For most dives, it would be a very small difference.

 

[SteveC]

The only difference is that salt water weighs about 3% more than fresh water. So if your saltwater calibrated depth gage says you’re at 100 ft your are actually about 103 ft deep.

 

[nereas]

My SUUNTO and my UWATEC both are factory-set for FFW. Therefore it shows that I am slightly deeper than I actually am in seawater.

For my logbook, I record the site location, and that will tell me whether I was diving a fresh water lake or a seawater ocean, for future reference. The weight belt / backplate weighting data comprise the most relevant issue, not the actual precise depth.

Either way, I do not record FFW nor FSW, only absolute numerical data.

It is not a perfect world.

 

[nereas]

The only difference is that salt water weighs about 3% more than fresh water. So if your saltwater calibrated depth gage says you’re at 100 ft your are actually about 103 ft deep.

34/33 = 1.030

Right on!

 

[jscott099]

If you were doing air consumption rate calculations you may wish to know fsw vs ffw. In the end though it makes only a small difference.

 

[brutus_scuba]

The only difference is that salt water weighs about 3% more than fresh water.

As people have mentioned there is a pressure difference in salt water and fresh water, and that is why FSW/FFW is noteworthy. The Pressure Gauge does not measure how deep we are in the water it measures the pressure and coverts it to depth. You learn in your Open Water class that every 33 feet of water is an additional ATM, well that is only true in Salt Water. If we were in Freshwater that number becomes 34 Feet.

To really know how deep you are you need to know how your SPG is calibrated. My SPG for example is set to FSW, and even though I dive in freshwater I would have to report my depth as FSW and use 33 feet as 1 ATM for calculations. The Nitrogen loading in your skin depends only on the pressure you're under, and not the linear distance to the surface.

You could also dive in Salt Water with an SPG calibrated to FFW, and the only difference would be that your SPG would say 34 when you were 33 linear feet from the surface, and you would have to use a NDL or decompression schedule based on Freshwater depths.

That's the scientific answer. The honest answer for most dives on here is that it doesn't matter. Many people write FFW because they dove in Freshwater and may not even be aware that the SPG is calibrated for Salt Water. And for the sake of conversation, and really all practicality it's fine. If someone has really done a dive where the distinction was critical to the dive. I would love to hear about it.