View Full Version : Technical Question...help please
JT2
February 2nd, 2003, 02:09 AM
When I was at the local dive shop today I brought up something that has always had me curious. I have seen several dive videos where divers have encountered haloclines and I have always wondered what effect they would have on depth gauges, and what I mean by that is lets say I am at 100ffw when I encounter a halocline and I then drop down five feet into the halocline, what will my depth gauge read?? Since salt water is more dense than fresh water, wouldn't my gauge read deeper than 105ft?
While this debate was going on someone else asked, if you took two divers and had one of them descend in freshwater until he reached 100ft by his depth gauge and had the other diver descend to 100ft in seawater, and then you took out a tape measure to physically measure the distance from the surface down to the divers depth gauges what measurements would you come up with? I know that there are a lot of variables that would have to be taken into consideration, but can any of you roughly explain this to me? Thanks, and I'm sorry if this is a goofy question, I'm just curious to learn how to figure this out and have found nothing in my PADI Dive Encyclopedia or my Divemaster training material that I am currently learning.
The Pirate
February 2nd, 2003, 04:21 AM
It all depends on how your guage is calibrated. If your guage is calibrated to ffw then it will read deeper then you actually are in sea water. If your guage is calibrated to fsw then it will read shallower than you actually are in fresh water.
For example:
99ffw = 3.9118 ata
99fsw = 4.0 ata
So, if your guage is calibrated to ffw and it's reading 99' in sea water, you're actually 96 linear ft from the surface. If your guage is calibrated to fsw and it's reading 99' in fresh water, you're actually 102 linear ft from the surface. This is assuming the guage is accurate.
And BTW, this information is contained in your materials, and it really doesn't matter in the real world. Its just a few feet eiher way and most guages aren't that accurate anyway.
MikeFerrara
February 2nd, 2003, 07:41 AM
A depth gauge doesn't really measure depth it measures presure. A depth gauge that is calibrated at sea level will read different at other altitudes. A depth gauge that is calibrated for salt water will read different in fresh water. Cold water is slightly more dense so it will effect the gauge reading to some degree.
ABQdiver
February 2nd, 2003, 10:45 AM
How does this affect electronic gauges? I assume they are basically the same and will measure the pressure as well.
MikeFerrara
February 2nd, 2003, 11:18 AM
Many electronic gauges comensate for altitude. Some even have user selectable settings for salinity. Altitude compensation can be done by first sampling the atmospheric presure at the surface. With some computers this automatic and other have user selectable altitude ranges. I am of course speaking in general terms because I'm not familiar with every model.
Rush
February 2nd, 2003, 02:14 PM
JT2 once bubbled...
When I was at the local dive shop today I brought up something that has always had me curious. I have seen several dive videos where divers have encountered thermoclines and I have always wondered what effect they would have on depth gauges, and what I mean by that is lets say I am at 100ffw when I encounter a thermocline and I then drop down five feet into the thermocline, what will my depth gauge read?? Since salt water is more dense than fresh water, would'nt my gauge read deeper than 105ft?
While this debate was going on someone else asked, if you took two divers and had one of them descend in freshwater until he reached 100ft by his depth gauge and had the other diver descend to 100ft in seawater, and then you took out a tape measure to physically measure the distance from the surface down to the divers depth gauges what measurements would you come up with? I know that there are a lot of variables that would have to be taken into consideration, but can any of you roughly explain this to me? Thanks, and I'm sorry if this is a goofy question, I'm just curious to learn how to figure this out and have found nothing in my PADI Dive Encyclopedia or my Divemaster training material that I am currently learning.
You ask about THERMOCLINES, but did you really mean to ask about HALOCLINES ? From your first paragraph, I get the feeling that you are speaking of a layer of saltwater under a layer of fresh water; a condition that exists in places such as some cenotes in Mexico.
JamieZ
February 2nd, 2003, 04:57 PM
ther·mo·cline ( P ) Pronunciation Key (thûrm-kln)
n.
A layer in a large body of water, such as a lake, that sharply separates regions differing in temperature, so that the temperature gradient across the layer is abrupt.
This is from the dictionary. You will not see a depth change on your computer.
hal·o·cline ( P ) Pronunciation Key (hl-kln)
n.
A vertical gradient in ocean salinity.
I believe you were referring to Halocline.
JT2
February 2nd, 2003, 05:54 PM
Rush once bubbled...You ask about THERMOCLINES, but did you really mean to ask about HALOCLINES ? Yes, I meant to type halocline and not thermocline, it was very late and my mind was slippin', I will go back and edit.....I apologize:bonk:
omar
February 2nd, 2003, 05:55 PM
Depth, actually pressure gauges will measure the weight of the column of water above the gauge. If the water column consists of 25 feet of fresh water and 40 feet of sea water you will see a “depth” that corresponds to the sum of the depth of fresh water time the density of the fresh water and the depth of salt water times the density of the salt water.
For the example above you will see on the gauge the equivalent pressure of:
(25 * 62.4 lb/ft^3 FW) + (40 * 64.08 lb/ft^3 SW ) = 4,123.2 lb/ft^2 or 28.63 psi or 1.95 atm [OF WATER].
The depth on the gauge will depend on whether it is calibrated to sea water or fresh water. Most gauges are calibrated to fresh water. This means that the gauge will read deeper than the actual depth when in sea water.
Moving from fresh to salt water will not make the depth gauge increase dramatically, claiming otherwise is BS.
MikeFerrara
February 2nd, 2003, 08:20 PM
[i]
Moving from fresh to salt water will not make the depth gauge increase dramatically, claiming otherwise is BS. [/B]
I've never heard anyone claim otherwise but it would be BS.
omar
February 3rd, 2003, 01:17 AM
Rush did in his prior post... Before he edited it ;-)
DaveO
February 4th, 2003, 12:54 PM
scubanarc once bubbled...
hal·o·cline ( P ) Pronunciation Key (hl-kln)
n.
A vertical gradient in ocean salinity.
Does this condition exist as both a layer (horizontal gradient) and a "wall" (the vertical gradient)?
The Pirate
February 4th, 2003, 01:20 PM
By vertical gradient, I think its referring to the difference in salinity from top layer to bottom layer.
DaveO
February 4th, 2003, 01:31 PM
Got it... thanks.
gaschef
February 4th, 2003, 03:50 PM
DEar All
Following on from what has already been said the difference between depths of salt and fresh water is insignificant.
the specific gravity of salt water is 1030 kgs / cubic metre
the specific gravity of fresh water is 1000 kgs / cubic metre
the difference is not worth worrying about, besides which a normall 63mm depth gauge is at best plus or minus 5% accurate
thats its build specification
just make sure you add on an appropriate jesus factor and you should be alright
Ontario Diver
February 4th, 2003, 09:12 PM
It doesn't really matter to the diver because we use out depth to figure out our NDLs (or deco sched :D)- but nitrogen uptake is based on pressure not depth.
So your bottom profile for purposes of determining your nitrogen load at 99 ffw and 96swf (both reading on your depth guage as 96 f) - you are under the same pressure and the rate of N2 absorbtion will be the same.
