Fresh - Salt water and computer calculations...

[Charlie99]

biscuit7 once bubbled...
Forgive me if this is the single stupidest question ever, but I always assumed that the depth on my gauge was the actual number of feet I was below the surface. Is this not correct?

Rachel

The only stupid question is the one that isn't asked, and therefore is never answered.

If your gauge is calibrated for salt water, with a density of about 1.027 g/cc, then it will read 100' when you are really 103' below the surface in fresh water (which is about 1g/cc). For deco calculations, it doesn't matter since it is the pressure that counts.

Charlie

 

[GeekDiver]

You are correct in understanding that a depth gauge may not read the actual measurement in ft.

It's kind of like this.

Let’s say you want to count how many pennies’ you have in a jar. You take a 10 penny’s then weigh it on a scale. You determine those 10 penny’s weighs one ounce. Then you weigh the total amount of penny’s you have and they weigh altogher 1 lbs or 16 ounces. You could then multiple 16 ounces by 10 and say you have 160 penny’s. Now if the pennies didn't have a consistent weight or some were made of heaver material your count would be off.

Since your depth gauge is set to a specific point it thinks that x amount of water (or say depth in Ft) is equal to X amount of weight (or pressure). Since not all water weighs the same it may think there is more or less water. The reason why water doesn't always way the same is because of what is in the water. Some water may have more salt or other particles so it weighs more.

Hope that helps your understanding

Geek

 

[Charlie99]

Big-t-2538 once bubbled...

You will be within a foot if you're diving shallower than 80ft....and within 1 meter if you are european.

I was under the impression that the fsw to ffw correction was a little bit more ---- around 2% if using the 33fsw = 1 ATM = 1013.25mb system, and about 2.7% if using the 10msw = 1 bar system.

Charlie

 

[leadweight]

Aladin/Uwatec computers are calibrated for fresh water. If your Alacin computer reads 100 feet in salt water, you are actually about 97 feet below the surface.

Try to explain that to a DM who said 100 feet max when your Aladin reads 101 feet.

 

[Walter]

Your depth gauge and/or computer does not measure depth. It measures pressure. It is calculated to show either 33 ft or 10 m at 1 ATM gauge (2 ATMA). On average (there are differences in salinity and disolved solids from one place to another and even day to day in the same place) we reach 1 ATM gauge at 33.075 linear feet (10.081 linear meters) in sea water and 33.92308 linear feet in fresh water. Both of those numbers are close enough to 33 ft and 34 ft, respectively, to not matter.

Depth is totally unimportant except how it relates to pressure and the effect that pressure has on our bodies. Ignore the differences between ffw and fsw. Measure all pressure in fsw (or msw) and use the tables as if you were in salt water. Your computer already does this for you.

Keep in mind that 33 linear feet = 10.058 linear meters, but when measuring pressure 33 fsw = 10 msw = 1 ATM (gauge).

This should either make it clear or really confuse the issue.