Natural Science Forum / Earth Science / Oceanography / January 2004

There is no equivalence.

Salinity S is *defined* - according to IAPSO - by a fifth order polominal:

imagine k = sqrt(K15)

S := a0 + a1*k^1 + a2*k^2 + a3*k^3 + a4*k^4 + a5*k^5

Where K15 is the ratio of the *CONDUCTIVITY* of seawater and a very
precisely defined KCL solution at 15 degrees Temperature und a Normal
pressure of 1013 HPa.

The ai (0 < i < 5)have been found by very precise analyses of the
conductivity sea water.

This IAPSO definietion is only valid for the range 2 < S < 42.  

The International Units for oceanography are defined and fixed in
UNESCO Technical Paper of Marine Sciences No 45 whereas a description of the
underlying backgrouns can be found in UNESCO Technical Paper of Marine
Sciences No 37.

A value of "ppt" or "promille" thus makes absolutely no sense at all.

Bernd

 Turns out that I got that wrong.  An emailed correction, with the
important citation, is below.

 In a sense, salinity, now, has no correspondance to the old
parts per thousand or anything else.  The salinity is defined
as function of conductivity.  This is somewhat like the speed
of light, which is now defined to be exactly 2.99whatever m/s.

 Also analogous is the fact that the new values (though
not definition) correspond pretty well to the old values.  
The new one is built on a foundation of conductivity, which
is easy to measure, and can be measured accurately easily.
The old approach was based on a chemical approach (mass of
non-water).  The differences in answer that you would get
between the two methods mostly depend on variation in
chemical composition of sea water, which is known to be
small.

 But for the real details, see the UNESCO report.