Low altitude, Fresh water diving

[diverrick]

I recently upgraded to a nitrox compatable Computer, and it automatically switches to FW dive tables, base on current altitude after 2000 Ft. ASL.
The problem is, I want to dive on FW at just a bit above SW altitude, say 600 Ft. and it is not able to be set to FW dive tables manually from what I can tell. This got me thinking about the following questions.
1) does FW/ SW effect your Nitrogen intake differently, if the altitude were the same?
2) (depending on the answer to #1) Can I simply use SW calculations for FW dives below 2000 Ft? or 1000 Ft? or any other elevation above SW?

 

[devilfish]

There is a tendency to think of nitrogen absorbtion in terms of measured depth. However absorbtion is in relation to pressure. Does not matter if for example 29.4 psig is in sw or fw, the absorbtion will be the same, although the depth display in ft or m will be different.

 

[Dr Deco]

Hello diverrick:

Inert Gas Uptake

The uptake and elimination of inert gas is not affect by whether you dive in fresh or salt water per se . In fact, the uptake and elimination is not changed directly by anything. The body does not regulate the concentration of any inert gas. While it can regulate the metabolic gases such as oxygen or carbon dioxide, inert gases are simply carried along by the blood and that blood flow is controlled by events that do not take into account the fact that inert gas is present. However, since the absolute pressure in fresh water is less for a given depth than in salt water, there is a lower pressure in the alveoli and hence in the arterial blood.

The same is true of inert gas elimination with the added complication of the growth of a free gas phase.

Gas Phase Growth

The presence of micronuclei produces a change in the off gassing of inert gas. The micronuclei present will grow if they are larger than a certain radius (the Laplace limit) that is controlled for every pressure change by the surface tension of the bubble interface (“skin”). The pressure change for any given change in depth will be different whether it is freshwater or salt water since there is a difference in density from the added salts.

Decompression tables are derived to reflect the reduction in absolute pressure. For this reason, the tables are different for fresh and salt water and for different altitudes. The NDLs in fresh water are different from those in salt water – a little different anyway.

Your Computer

I do not know how to adjust your computer. If you indicate the brand, I trust that one of the readers will be able to give you the necessary tips.

Good Luck! :scuba:

Dr Deco :doctor:

 

[diverrick]

I just got this one. It's a VEO 200. I called the Manufacturer after reading the Manual 3 times through. They said it cannot be manually adjusted, They said it will automatically adjust to FW at 2000 Ft. That is what got me on this subject in the first place. I guess to clarify.. Would the difference in 800 Ft. v.s. sea level be enough to worry about, or can I simply dive it using the sea water settings. I am not doing decompession dives anyway, just was wondering If I had to make special considerations for that bit of altitude. I understand that if you go into high altitude it would have a effect, but it was the slight level above sea level I was thinking about.
Inert Gas Uptake

The uptake and elimination of inert gas is not affect by whether you dive in fresh or salt water per se . In fact, the uptake and elimination is not changed directly by anything. The body does not regulate the concentration of any inert gas. While it can regulate the metabolic gases such as oxygen or carbon dioxide, inert gases are simply carried along by the blood and that blood flow is controlled by events that do not take into account the fact that inert gas is present. However, since the absolute pressure in fresh water is less for a given depth than in salt water, there is a lower pressure in the alveoli and hence in the arterial blood.

The same is true of inert gas elimination with the added complication of the growth of a free gas phase.

Gas Phase Growth

The presence of micronuclei produces a change in the off gassing of inert gas. The micronuclei present will grow if they are larger than a certain radius (the Laplace limit) that is controlled for every pressure change by the surface tension of the bubble interface (“skin”). The pressure change for any given change in depth will be different whether it is freshwater or salt water since there is a difference in density from the added salts.

Decompression tables are derived to reflect the reduction in absolute pressure. For this reason, the tables are different for fresh and salt water and for different altitudes. The NDLs in fresh water are different from those in salt water – a little different anyway.

Your Computer

I do not know how to adjust your computer. If you indicate the brand, I trust that one of the readers will be able to give you the necessary tips.

Good Luck! :scuba:

[/font] Dr Deco :doctor:[/QUOTE]

 

[pipedope]

BE happy.

Remember that the DIR folks use the Bottom Timer and it always measures in Ft fresh water.

Your computer depth won't match exactly a measurement by lead line but that is not a problem for sport divers.

 

[DepartureDiver]

There is a tendency to think of nitrogen absorbtion in terms of measured depth. However absorbtion is in relation to pressure. Does not matter if for example 29.4 psig is in sw or fw, the absorbtion will be the same, although the depth display in ft or m will be different.

DevilFish is right. It doesn't matter. Ignoring the altitude (which is basically inconsequential at 600’ and can be ignored), pressure is pressure and your depth reading can be directly used with a table and the computer function of your gauge will do just fine. It is not the depth that is important, but the pressure of the dive. For example, if you are at 34’ of fresh water, your sea water gauge will read 33’. But the pressure is identical since 34’ of fresh water pressure is the same as 33’ of sea water pressure. When a table (or computer) gives a no-stop limit at a certain depth, it is really giving a no-stop time for a certain pressure, which is then displayed in terms of a depth. So the answer to question 1 is just use the gauge reading in your sea level dive table. No adjustments are necessary. If a sea water gauge says you are at 33’ even though you are at 34’, the 33’ depth is used in the dive table and the computer knows the actual pressure you are at.

In answering question 2, again the reading doesn’t matter. But make sure you do proper altitude corrections. Some altitude conversion tables read in terms of fresh water and others don’t. Due to rounding in the altitude correction tables (unless you calculate them yourself), the tiny differences in depth won’t really matter. But, if your gauge reads fresh water and your altitude table does not take the fresh water into account, again you are conservative since the fresh water gauge will put you slightly deeper than necessary. However, if your altitude table reads in terms of fresh water and your gauge hasn’t kicked over yet and is still reading in terms of sea water, then you can just add 2.5% to your depth if you would like. But again, the differences are small especially when the altitude corrections are small at the low altitudes.

Also as Pipedope said, many gauges read everything in terms of fresh water, so if anything, it gives a slight conservatism factor.

Hope that helps.

 

[diverrick]

Thanks for all the info. and I learned alot. Who knew about "laplace limit", micronuclei, and bubble skins! I don't think that was covered in OW class.
Can't wait to go out and get wet.