Bit of a classic case of the man with a watch always knows what time it is.
Man with 2 watches never sure.

You will always get small differences between computers, and testing them in
only 2m of water is hardly fair on them.  They will barely be turned on.

Simple answer is pick the one that reads deepest and gives the most deco
then you are following the safest profile.  Using the deepest reading one
will also mean you are less likely to drop deeper than your nitrox MOD.

Don't worry about it too much.  I have 2 of the same make and model and they
read 1.1m different.

Regards M.

Might computer reads 0 at 1.9m! - it doesn't turn on til 2m

My idea is - so what?

To give my views in turn - the discrepancy of depth read. Irrelevant. Put
these various depthdifferences into tables for dives of 40mins, and see what
difference it makes when after a 2 hr surface interval you are about to
embark on a subsequent dive.

The depth at which the comparison was made is hardly a suitable comparison
over which to make a decision. What about at 40m? If the difference is still
only 0.5m, so what? That's the sort of distance that is better measured with
a ruler or tape measure when barely extending the arms sideways!

As to salt or fresh water, it's irrelevant. Unless you're into setting
records, it doesn't matter what depth you're at. As far as the computers are
concerned, they read the pressure around you. OK, so in salt water at a
given depth they read deeper than in fresh water at the same depth. The
difference here is around 2.5% (1m difference in 40m depth). Given that the
prime reason for using a computer is to track your nitrogen exposure (and so
keep within no-deco limits or manage a decompression dive) and nitrogen
absorbtion / elimination is dependent on PRESSURE not depth, whether this
pressure is achieven in fresh or salt water is irrelevant.

My thoughts, FWIW.

Ken