NTR: psu - practical salinity units

[cmungall]

As part of https://github.com/INCATools/biosample-analysis we are normalizing units for mixs properties like salinity

We are seeing 100s of samples with values like 26.49 PSU

https://en.wikipedia.org/wiki/Salinity#Seawater

The use of electrical conductivity measurements to estimate the ionic content of seawater led to the development of the scale called the practical salinity scale 1978 (PSS-78).[9][10] Salinities measured using PSS-78 do not have units. The suffix psu or PSU (denoting practical salinity unit) is sometimes added to PSS-78 measurement values.[11] The addition of PSU as a unit after the value is "formally incorrect and strongly discouraged

This indicates that PSU should not be added to UO. However, where should the concept live?

@pbuttigieg @wdduncan should be add docs to mixs6 that say not to use PSU?

I think you can assign @kaiiam to this

[dr-shorthair]

Is it a 'Quantity Kind' in the sense defined in QUDT?

[kaiiam]

According to http://www.salinityremotesensing.ifremer.fr/sea-surface-salinity/definition-and-units practical salinity unit (PSU): is equivalent to per thousand or (o/00) or to g/kg.

The Minimum Information about any X Sequence standard (MIxS) v5 lists the preferred units to express salinity as milligram per liter, practical salinity unit, and percentage, and has the following definition of salinity:

Salinity is the total concentration of all dissolved salts in a liquid or solid (in the form of an extract obtained by centrifugation) sample. While salinity can be measured by a complete chemical analysis, this method is difficult and time consuming. More often, it is instead derived from the conductivity  measurement. This is known as practical salinity. These derivations compare  the specific conductance of the sample to a salinity standard such as seawater

As conductivity based salinity measurements are easier and more commonly done, scientists standardized Hence the need for the PSS scale, which according to this researchgate thread

The 'Pactical Salinity Scale' (PSS) was defined in 1978 and later promulgated by the UNESCO/ICES/SCOR/IAPSO Joint Panel on Oceanographic Tables and Standards in Sidney, BC, Canada, 1-5 September 1980. Because it makes no sense to say the salinity is, for example, 35 PSS, the term PSU was introduced, as the scale is composed of units. However, the use of PSU is discouraged and, because salinity is by definition a dimensionless parameter, it should simply be represented by a number. As practical salinity has been given the symbol S (cf. Unesco Technical Papers in Marine Science 45, 1985, or IAPSO Publication Scientifique No. 32, 1985), one should speak of a salinity of, for example, S = 35.

The thread also says:

The term ppt was therefore initially replaced by the term psu, which simply means 'practical salinity units'. Later even this term was dropped, although it is still occasionally used. In this context one needs to know that the salinity or the salt content of seawater is officially defined as the mass of salt in 1 kg of seawater.

I think the essential thing here is that the PSS scale is composed of units, hence why it's redundant to add a unit, but we want to represent the fact that these salinity measurements are using the PSS scale, @dr-shorthair this might be similar to the QUDT quantity kind idea. Although it seems to me that those are often in reference to an entity e.g. quantitykind:ElectricConductivity for the unit unit:MicroS (usiemens).

Although I don't see PSU in QUDT, a similar idea, the Beaufort scale (BFT), which consists of a scale with it's own units is in QUDT with it's own unit class unit:BFT that links to quantitykind:Speed. So I guess in the QUDT usage, quantitykind here would just link back to the concept of salinity?

Finally, I previously thought that PSU was equivalent to parts per thousand because it's (a proxy for) the mass of salts in g per kg of seawater, but perhaps I'm mistaken. This book supports this:

Some sources now use PSU to express salinity values, where 1 PSU = 1 ppt

This post, however, says PSU and ppt are distinct from one another, and that PSU has it's own standard based on a mix of ions rather than the actual mix of ions (which differ from ocean to ocean). I can consult some oceanographers if needed.

[reality]

Does this need action from UO?

[kaiiam]

upto @cmungall if you still need this for MIxS. My understanding is that PSU should be equivalent to parts per thousand, but some people might want the unit still.