Using a dive computer with a seawater-only algorithm in fresh water

[Hatul]

As others have pointed out the computer works on pressures and displays the depth for your pleasure. At 34 feet fresh water it will show 33 feet. Think of the depth display as showing the pressure in feet salt water.

 

[dmoore19]

I don't know about the Suunto, but the Oceanic OC1 doesn't have a setting but does in fact do it automatically, but only after an altitude of 3001' has been reached.

ALTITUDE SENSING AND ADJUSTMENT
Prior to the first dive of a series of repetitive dives, Altitude (i.e., ambient pressure) is measured upon activation of Dive
Surface Mode and every 15 minutes
until a dive is made or operation reverts to Watch Mode.
> While it is operating in Watch modes after a dive, measurements are taken every 15 minutes during the 24 hour period after surfacing.
> Measurements are only taken when the unit is dry.
> Two readings are taken, the second reading 5 seconds after the first. The readings must be within 1 foot (30 cm) of each other to record that ambient
pressure as the current Altitude.
> No adjustments are made during any time that the Wet Contacts are bridged.
When diving in high altitude waters from 3,001 to 14,000 feet (916 to 4,270 meters), the OC1 automatically adjusts to these conditions providing cor
-
rected Depth, and reduced No Deco and O2 Times at intervals of 1,000 feet (305 meters).
At an elevation of 3,001 feet (916 meters), Depth calibration automatically changes from feet of seawater to feet of fresh water
. This is the first adjustment
to the algorithm.
When the Conservative Factor feature is set On, NDLs are calculated based upon the next higher 3,000 foot (915 meter) Altitude. All adjustments for
Altitudes greater than 11,000 feet (3,355 meters) are then made to allowable dive times for 14,000 feet (4,270 meters). At Sea Level, calculations are
based upon an Altitude of 6,000 feet.
The OC1 will not function as a Dive Computer above 14,000 feet (4,270 meters).

 

[Agility]

To make things worse, the sensor might be off by +/-1% :sigh:

 

[DivemasterDennis]

It should be noted somewhere in this thread to the biggest issue for fresh water diving vs salt water diving and computer use relates to altitude, and every computer I have owned ( 6, of 4 different manufaturers) has had a built in "altitude adjustment." or notation. I still use PADI's altitude tables when planning dives here in Colorado from 5000 feet to 10000 above sea level. I never owned, saw, or heard of a computer that has a separate setting for salt or fresh water. Has anyone?
DivemasterDennis

 

[Ricky B]

From the Mares Puck manual:

SET DATA – FRESH / SALT
​

To ensure maximum accuracy, you need to set PUCK for either fresh water (“Fresh”) or seawater (“Salt”) as appropriate. Check this setting often, especially if you use the instrument in a variety of environments (lake, sea, swimming pool).
​

I have never used this computer so I am taking the manual's word for it. I assumed that this was a common feature if an entry-level computer like the Puck has it.

​

 

[Easdem]

If you are diving so close to the NDL's that the difference between SW and FW makes a difference it's time to back off or start learning about deco. General comment, not aimed at the OP specifically.

NO!! NO!! NO!! This statement is wrong on many levels.

It doesn't sound like you have a clear understanding of how the body on-gases. If you remove physiological differences between people, there are two main factors (there could be others) that come into play in this case.

1.) Breathing gas mix.
2.) Ambient pressure.

The gas mix can be set by the diver regardless of salinity, so that doesn't really apply to the original question.

Ambient pressure on the other hand has to do with the density of the water you are in and how it exerts pressure on your body. Fresh water is less dense, so the ambient pressure at a set linear distance from the surface will be lower than the same linear distance from the surface in salt water.

Simply put, the more pressure around you and the richer the mix, the more quickly you absorb gases. The inert gases, in this case Nitrogen, are the problematic ones.

The computer is simply detecting the ambient pressure and making calculations by using that reading to give you your estimated absorbed Nitrogen (or Nitrogen Load). The actual linear distance to the surface does not figure into the equation.

The same is also true of dive tables. Those are calculated for salt water and your analog depth gauges are as well.

Bottom line, the same computer can be used for fresh and salt water.

 

[DaleC]

If you are diving so close to the NDL's that the difference between SW and FW makes a difference it's time to back off or start learning about deco. General comment, not aimed at the OP specifically.

What can I say. You can't remove the physiological difference between people or the guesstimate nature of any deco model so my statement stands. It's the difference between understanding the practical nature of on/off gassing and becoming mired down in the minutiae of theoretical semantics.

 

[rjack321]

. If you remove physiological differences between people

Let us know when you got this one figured out

 

[NAM001]

I dont recall any of my computers having a menu selectoin for fresh or salt water. could be mistaken. Salt water would be the conservitive setting if any. I agree that it is a moot thing as the computer runs on presure (salt or fresh ignorant) it only does the comversion for depth readouts. Also if you were doing a deco the depth stops called for would have so little error it would be moot. And on top of that I have never seen my puter say stop at 27 ft. it says 30 ft and varies the time. I guess that answer is probably it doesnt matter.