Fresh and salty deco

[tbone1004]

the cenotes are too shallow for the computers to show any significance.

what was said above is correct, your computers and algorithms really don't care what unit is displayed because they measure pressure and the pressure is what determines your deco schedules. Now, if you are doing dive planning and you know if the software is programmed for feet fresh water, then you can convert, but again, this is only 1ft difference per atmosphere, about 3% difference in depth, which is negligible for dive planning considering your computer is not always sitting at the centerline of your body. Even at 10ata/100m/330fsw/340ffw, you can see the difference is only 10ft. That is significant at shallower depths, but the difference is still only 3% and really only matters if you are doing survey work if you need to be spot on with depth *which in the ocean you would also have to factor in the tides which will change your depth more significantly than having the wrong salinity setting. If you are diving tables to 100m, then yeah you may want to know the exact salinity, so if your planner is in FSW but you're diving in FFW you'll have some more deco, and if you're planning is in FFW but you're diving in the salty stuff you'll have a bit more but even then it's just not that significant to really change anything in your deco obligations.

 

[decompression]

the cenotes are too shallow for the computers to show any significance.

what was said above is correct, your computers and algorithms really don't care what unit is displayed because they measure pressure and the pressure is what determines your deco schedules. Now, if you are doing dive planning and you know if the software is programmed for feet fresh water, then you can convert, but again, this is only 1ft difference per atmosphere, about 3% difference in depth, which is negligible for dive planning considering your computer is not always sitting at the centerline of your body. Even at 10ata/100m/330fsw/340ffw, you can see the difference is only 10ft. That is significant at shallower depths, but the difference is still only 3% and really only matters if you are doing survey work if you need to be spot on with depth *which in the ocean you would also have to factor in the tides which will change your depth more significantly than having the wrong salinity setting. If you are diving tables to 100m, then yeah you may want to know the exact salinity, so if your planner is in FSW but you're diving in FFW you'll have some more deco, and if you're planning is in FFW but you're diving in the salty stuff you'll have a bit more but even then it's just not that significant to really change anything in your deco obligations.

There are a couple deep ones....... El Zacatón sinkhole.....


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[tbone1004]

obviously, but the halocline is still around 30m IIRC and because it's a halocline you aren't actually going to see your depth gauge magically switch over by 3ft when you cross it unless you're diving one of the Cochrans where it detects salinity through some strange voodoo. It is still just a measurement of pressure and that doesn't change.

 

[Steve_C]

A perfect illustration of the fresh water / salt water density is found in the Cenotes of the Yucatan. About forty feet down the water changes from fresh water to salt water. Swimming through this 'halocline' is really a cool thing to do because you can actually see (and taste) the change from fresh to salt water. I did not notice any difference on my depth gauge nor can I find any graphical evidence of the change from fresh to salt when I downloaded my dive profiles.

The pressure will vary continuously through the thermocline. If you had a measure of true depth, and if you had a thermocline of zero thickness, and if your computer could compute the rate of change of pressure with respect to true depth (which is the density), you would notice a small jump discontinuity in the rate as you passed through.

 

[Centrals]

But how do you measure the salinity accurately before planning the dive?

 

[tbone1004]

you don't, because it doesn't matter. Plan to conservatism. If you are doing a 100m dive in some degree of salty stuff and you know that your depth gauge reads in either fully fresh or "normal" salt water, then leave it in fresh water mode because your computer will read deeper than you are by somewhere up to 3% deviation, and is only relevant when you are diving based off of tables instead of a computer. If you run tables to ensure that you have enough gas and a rough idea of your deco obligation, then use the computer for accuracy, it doesn't actually matter.

If you run tables completely, again it doesn't really matter because it is based off of pressure so as long as your computer and your dive plan are based on the same salinity, it doesn't matter what the actual salinity of the water is because you can never plan your depth that accurately, that is +_ 3m at 100m which is fine, but for normal recreational dives to 30m, you're only +_1m which is not significant.

The beauty of the depth sensors only measuring pressure and using the computers to calculate your actual deco is that they are constantly adapting to any changes in the water pressure whether caused by temperature or salinity differences. If you're in a blue hole and drop through the halocline, your depth won't be 100% accurate, but all of your decompression will be.

 

[Centrals]

I read certain soft ware have the choice of salinity to compute before the dive. So I asked the question (post #5) on the measurement of the salinity before the dive. All I received was a PM accusing me for being sided with another person but not with any answer on the question!!!

 

[tursiops]

I read certain soft ware have the choice of salinity to compute before the dive. So I asked the question on the measurement of the salinity before the dive. All I received was a PM from certain person accusing me for being sided with another person but not with any answer on the question!!!

I have no idea what you mean by this.
Salinity is rarely measured. Instead, conductivity and temperature are measured, and an equation is used to get salinity.
Use your software and put in values of 33-37 grams per liter (the normal range of variation in the ocean), and see if it makes any difference.
Call it 35 and you are very close everywhere. The Shearwater allows you to choose the EN13319 standard for European dive computers...which is 1020 grams/liter.
It make no difference except minor changes in your displayed depth. So, choose 0, 10, 20, 30, 40, 50 gms/liter; don't bother measuring it or calculating it.

 

[Centrals]

I have no idea what you mean by this.
Salinity is rarely measured. Instead, conductivity and temperature are measured, and an equation is used to get salinity.
Use your software and put in values of 33-37 grams per liter (the normal range of variation in the ocean), and see if it makes any difference.
Call it 35 and you are very close everywhere. The Shearwater allows you to choose the EN13319 standard for European dive computers...which is 1020 grams/liter.
It make no difference except minor changes in your displayed depth. So, choose 0, 10, 20, 30, 40, 50 gms/liter; don't bother measuring it or calculating it.

What is the point of having a choice if the end result is not significantly difference?
No wonder my Deco Planner does not offer such a choice!

 

[Jax]

What is the point of having a choice if the end result is not significantly difference?
No wonder my Deco Planner does not offer such a choice!

I'm pretty sure that at one time, the difference was considered important. But as we learn more about planning, DCI, and external effects, the differences became inconsequential.