The only stupid question......

[Pauly854]

Damn...I'm almost sorry I asked! Bottom line is...Yes, the pressure at identical depths is different....No, it doesn't make a difference. Thanks for all of the replies.

 

[Walter]

Bottom line is...Yes, the pressure at identical depths is different....No, it doesn't make a difference.

Pretty much. You need to be concerned about pressure, not depth. That pressure is measured in terms of depth (sort of since your linear distance from the surface will not be the exact depth displayed). You can dive with a computer (or tables) calculated in feet of sea water (fsw) in either fresh or salt water or you can dive with a computer (or tables) calculated in feet of fresh water (ffw) in either fresh or salt water.

Good question, but one that should have been covered in your OW class.

 

[AfterDark]

Damn...I'm almost sorry I asked! ......

Welcome to ScubaBoard!

 

[Pauly854]

Pretty much. You need to be concerned about pressure, not depth. That pressure is measured in terms of depth (sort of since your linear distance from the surface will not be the exact depth displayed). You can dive with a computer (or tables) calculated in feet of sea water (fsw) in either fresh or salt water or you can dive with a computer (or tables) calculated in feet of fresh water (ffw) in either fresh or salt water.

Good question, but one that should have been covered in your OW class.

Walter, it may very well have been covered in my OW class...but that was a long time ago, and I can't remember what I had for breakfast. The question came to me in one of my insomniac episodes(where most of my strange thoughts come from), and I had another episode last night. Last night I had the "AhhHaa" moment where everything came clear regarding the water pressure. The gauges measure pressure, not actual distance. It just translates the pressure into a numerical value (depth), so we can apply it to a calculation.

I AM interested in the difference between salt/fresh water though. I think it would be curious to place a depth gauge on a 33ft section of line, drop it to an exact depth in both salt and fresh water...and see what difference there is. If I am thinking correctly, the salt water would show deeper than the fresh water...even though the ACTUAL depth is identical.

Again, not that it matters to the calcualtions.

I finally GET IT!

 

[Rich Keller]

This may qualify as a stupid question, so blast me if you feel the need. Salt water is heavier than fresh water. Fresh water is lighter than salt water. But the atmospheric pressure is the same? Unless I am just thinking too much, it seems that the depth of each ATM of pressure would be different. Example: if 33ft of salt water is 2ATM, then why isn't 33ft of fresh water 1.8ATM(est)?

The only stupid question is the one you do not ask.

Atmospheric pressure is the same at sea level, once you get too far above that the reduced pressure at altitude must be taken into consideration. As far as sea water vs fresh water you gain .445 PSI per foot of sea water you descend into and .432 PSI per foot of fresh water. These rates for water are the same regardless of altitude as water can not be compressed.

 

[Fish in a Barrel]

Huh. My dive computer does actually have a fresh/salt setting. I never even thought to check/change that before I went to Cozumel. Oh well, no harm done, except I suppose my saved dive profiles are a little deeper than reality.

 

[Walter]

Walter, it may very well have been covered in my OW class...but that was a long time ago, and I can't remember what I had for breakfast.

Remember a few key points (or have them someplace handy for easy reference) and you can always figure out other things.

Sea water weighs 64 lbs/cu ft. Fresh water weighs 62.4 lbs/cu ft.

Sea water:

Weight of 1 cu ft of sea water divided by 144 gives us the weight of 1 sq in of sea water 1 ft high which is the PSI for every foot we descend.

64/144 = .4444 PSI

Since 1 atm = 14.7 PSI, we can figure out how deep we have to descend to reach an addition atm of pressure.

14.7/.4444 = 33.075 ft

Every 33.075 ft of linear depth we add 1 atm of pressure in sea water. The difference of .075 ft is insignificant and can be ignored. Also, remember we do not actually use linear depth in diving, we use fsw, or ffw as units of pressure. They are defined as 33 fsw = 34 ffw = 1atm.

Weight of 1 cu ft of fresh water divided by 144 gives us the weight of 1 sq in of fresh water 1 ft high which is the PSI for every foot we descend.

62.4/144 = .4333 PSI

Since 1 atm = 14.7 PSI, we can figure out how deep we have to descend to reach an addition atm of pressure.

14.7/.4333 = 33.92 ft

Keep in mind that salinity varies from place to place and time to time. These are average weights and can change, but rarely by a significant amount unless you plan to dive in places like the Great Salt Lake of the Dead Sea.