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

If a dive computer does not have a fresh-water setting, does using such a dive computer in fresh water result in a dive that is slightly more aggressive than it would be in seawater?

Seawater = 1 ATM @ 33 feet.
Fresh water = 1 ATM @ 34 feet.

If you are in fresh water 68 feet, the computer will think you are in seawater at 66 feet, and you would be at a 3.03% greater depth than the computer is using for its calculations, correct?

My dive computer has a resolution error of 1 foot of depth. I suspect that is common. So if the reading is 66 feet, you might be one foot deeper or three feet deeper.

Most dive computers have a personal adjustment setting to make the algorithm more conservative, but the amout of conservatism that is introduced by the setting is unknown to the diver and may be far more than what is needed to compensate for the difference between fresh water and seawater.

Does using a dive computer with only a seawater algorithm require any special adjustments on the part of the diver when diving on a single tank in fresh water to 100 feet or shallower?

Could be wrong, but it seems to me that the issue to be concerned about is the pressure, not the exact depth. As the computer is reading pressure and not depth and as our bodies are taking in / off gassing excess nitrogen as a function of pressure and not depth, the computer's depth readout may be off, but its primary function of computing pressure loading would be accurate.

Not to mention which, the algorithms are based on inexact statistical models. They can easily accommodate that margin of error. Different computers give different results too.

If you have an issue, that ain't why.

Scratch what I said, I hadn't had my morning coffee yet.

fisheater:

Could be wrong, but it seems to me that the issue to be concerned about is the pressure, not the exact depth. As the computer is reading pressure and not depth and as our bodies are taking in / off gassing excess nitrogen as a function of pressure and not depth, the computer's depth readout may be off, but its primary function of computing pressure loading would be accurate.

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Winner winner chicken dinner.

---------- Post added February 9th, 2014 at 11:10 AM ----------

chrispete:

In FW you'll actually be at a slightly lower ambient pressure for a given depth. As such, the computer would actually be slightly more conservative in FW vs. SW using SW setting.

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Incorrect unless your depth gauge is a measuring tape.

It's a non-enent IMO.

The computer reads the pressure but shows you that reading in feet. Diving in fresh water your depth reading will be off but the pressure readings and their effects on your body will be correct. The only exception to that would be diving at a high altitude where the surface pressure that you are returning to at the end of the dive is well below 14.7psi. For that type of diving you would need a different type of computer or use high altitude tables, watch and depth gauge.

I've never heard of a dive computer these days that is seawater only. What kind is it?

Suunto Zoop does not have a fresh-water setting.

I expect that the difference between fresh water and seawater is insignificant for practical purposes, but (1) I wanted to make sure of that instead of assuming it and (2) I am trying to understand the principles involved.

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.