i think it deeply depending on kind of salt and their purity, for example when i added 1 gr to 1 L water i had EC= 10 t0 13dS/m according to different kind of edible salt in our country. when i had %3.5 solution ECmeter showed 40dS/m.

Thank you, Saeid.  In Oceanography we operate  with the practical salinity, which has accuracy  equivalent to 0.003 g/kg  and aware about errors, produced by actual ionic composition  of seawater .  Here is my reference website about oceanographic salinity laboratory measurements and history of oceanographic salinity determination:

http://salinometry.com/

Salinity is dimensionless and is usually shown as 35 (symbol o/oo) or parts per thousand (rather than % parts per hundred) by weight. Instantaneously reading probes (rather than actual chemical analysis) are usually just a conductivity measurement that has been calibrated to read in salinity. The relative concentrations of the constituents in sea water in the open ocean is approximately constant (Dittmar's Principle).

Thank you, Dudley, for the salinity primer, but I have my own website with that information: http://salinometry.com/early-determination-of-salinity-from-ancient-concepts-to-challenger-results/

I asked a particular question to Oceanographers, who are working with the acquisition, analysis and modelling of the salinity data. 

Thank you, Guillermo. I hope that have enough understanding of salinity definitions and units. My question is in the area of the choice of particular kind of salinity (practical salinity, reference salinity, absolute salinity, density salinity, added-mass salinity, preformed salinity etc). For example, would the absolute salinity work better for the thermodynamic modelling compare to the practical salinity? Would replacement of the  Practical salinity (unit-less) with the Reference salinity (expressed in g/kg),  be good enough for compromise of having salinity units and simple form of deriving this parameter (just by multiplying the practical salinity on coefficient 1.004715)?

Hello Igor,

If I understand right the meaning of your question, I may say that it is a difficult one.

Oceanographers during the past 30 yrs are working with PSS (Sp). All publications use the Sp values, so changing to Sa is quite difficult because one cannot directly compare Sp and Sa values. So how can you read a paper using Sp and compare with a newer paper using Sa?

In my opinion we are not yet ready to say goodbye to Sp. On the other hand we should gradually get used to Sa. In conclusion, we need to know both, and if I had a choice I would stick to PSS (Sp)........

By the way do you still produce the RBR portable salinometer?

Thank you, Dimitris! You are right, we can't directly compare Sp and Sa, but we can do it for Sp and Sr (just multiplying Sp on coefficient 1.004715). In a first order of assimilation of the TEOS-10, why do not adopt Sr and correct Sp on 0.47% to be close to Sa? Rest of corrections (Absolute Salinity Anomaly factor) from Sr to Sa  gives about 0.1%, which could be a second order of assimilation. But do we need it at all for the practical use?

Unfortunately, last year RBR removed the MS-315 from the production line due to lack of orders, but, probably, we can make a few for a special order (please contact RBR, if you need one: http://www.rbr-global.com/about-rbr/contact-us) . 

Happy New Year!

Thanks for your answer Igor!

A happy new year to you too!!

Let's make an example:

Suppose PSS salinity Sp=39. This will make Sr=Sp*1.0047515=39.185. In this case and IF we speak about surface, then Sa=39.184 (for Eastern Mediterranean) and Sa=39.188 (for Western Mediterranean). If we speak about larger depths Sa tends to be the same for both basins.

Now let's try to walk in the shoes of an oceanographer. Can he understand  that the Sr salinity of 39.185 at for example 2000m depth that he reads in a paper is the same as Sp=39.000 he used to read in older publications for the same water mass at the same (2000m) depth? Can he understand in such a paper that it is the same water mass at the same depth?

Believe me he can't and believe me that salinity 39 and salinity 39.185 make a HUGE difference especially at non-surface depths. The only possible think to do is to report both salinities in a paper so that the readers will get used to it gradually. This is why I am telling you that many years will pass in order to adopt Sr, if it will ever be adopted.

Now as far as Sa is concerned I think this is even harder to adopt. And that is because it is location and depth depended, and in my opinion this adds more ambiguity to it than that it tries to resolve.

By the way in most cases an accuracy from 0.005 up to 0.01in salinity is quite OK.

However this is a very interesting topic and a great discussion, and I thank you for that!

P.S.: It is a petty you stopped producing MS-315. It must have been an excellent and innovative instrument. Perhaps, better marketing is what it needed! But it is pleasant to hear that you can still produce a few.

Thank you, Dimitris, it is a great topic for discussion and I am glad that we start it.  I always puzzled Why  the international oceanographic community do not  challenging the cost of redefinition of salinity. You are right, we can not replace Sp even with Sr without  breaking historical data record and confusing researchers. Actually, this is why Sp is numerically different from Sr. In 1970th founders of the PSS-78 were wise enough do not open "Pandora box"  and adopted Sp=35 as Chlorinity method defined S=35%o salinity. This was done only for the sake of  keeping historical record of salinity unbroken and thanks them for that. Rilley and Chester in 1971 mentioned that "The total salt content of seawater is about 0.45% greater than its salinity, defined in this( chlorinity) way." (p.13 "Introduction in Marine Chemistry").  So, what is novelty in introduction of Sr, despite of interest to have salinity expressed in SI units? And who is really care? All thermodynamic oceanography based on differentiations, where scaling constant will become zero. Only the salt balance models and TS-analysis of the water masses would be the winners. 

Now a few words about salinometry of the measuring Sp. In the blue water oceanography salinity accuracy QC  based on salinity samples, measured with Autosal (I am talking about WOCE/CLIVAR etc requirements). I found that there is a lot to improve in this technique, and try to introduce the alternative method, which I called the thermodynamic salinometry. In fact, PSS-78 based on conductivity ratio of sample, measured not at the Autosal set temperature, but rather at in situ temperature. If you will try to measure deep water Mediterranean samples just after arriving CTD on deck with my MS-315, set in the thermo-controlled bath at in situ temperature, you will be surprised how different practical salinity of sample would be, compare to the same sample, measured at room temperature. I've done these measurements for the North Atlantic and difference was significant (0.004-0.01) (see my paper "The New Method of Salinometry").  This is the main advantage of the Micro-Salinometer (in additional to small size and no needs for the air conditioning room), but, on the other hand,  this is where is MS-315 marketing problem - it is measure conductivity ratio (thus derive the practical salinity) in more correct way, compare to Autosal technique, but nobody really care, because Autosal technique adopted as industrial standard in oceanography. I've done salts in many cruses when hundreds of samples in a day were taken just to show statistical distribution of uncertainty of Autosal technique compare to the precise enough CTD (I don't say about cases with a bad CTD or Rosette, we've already  passed that bad times).  But request for QC of CTD data with following exactly Autosal technique makes the cruise chiefs very nervous with implementation of other technique and getting controversial results (my personal impression, sorry if I offended somebody).  By the way, in 1960-70th,  during the first comparisons of chlorinity method with conductometry, exactly the same method with in situ set temperature  Bradshaw/ Schleicher conductivity ratio measured salinometers (salinity bridges) were used. Maybe we should just step back and revise methods of salinometry before introduction of all others salinities, which are, finally, still all based on the PSS-78.  

Dear Igor,

Thanks for your answer.

I must admit that I have not read your paper "The New Method of Salinometry" and I also must admit that I am not familiar with the different methods between MS-315 and Autosal.

However I would like to know more on your method, so I 'll read your paper (hopefully soon) and get back to you with questions.

Regards

For the majority of applications, Sp is fine and it should continue to be reported in papers. Conversion to Sr is very straight forward, simple and easy to adopt for those that are ready to adopt it. For the few applications that need Sa conversion is not as straight forward but fairly simple with a little extra knowledge of the sample.

Whether an instrument reports Sp or Sr probably isn't important at this point as long as the user knows which it is. From there it's easy to convert as needed for comparison with historical data. Adopting Sr will not cause a break with historical records, but people have to be aware of the difference. Implimentation of Sr is important and should be done even though it will take a long time to fully transistion. PSS-78 kicked the can down the road and now it's time to address the issue. I think it's great that instrument companies are intrested in doing so. For the forseeable future Sa will have to be calculated and implemented by the user.

As with any paper it's very imporant to define all your methods and units, so it's extremely important for all published papers to define which salinity they are using and how it was determined, doing so will make the transistion to Sr much easier and prevent breaks or confusion with historical records.

Thank you, Ryan!

Yes, we can add a new Reference Salinity (Sr) deriving parameter in our CTD's firmware/software, which will be expressed in SI units (g/kg).  That step will satisfy request for having SI units in salinity expression (with all uncertainties which included in definition of Sr). Absolute Salinity deriving is a process of more complex methodology of counting all factors in the Absolute Salinity Anomaly deriving for the specific waters (or simply knowledge of this Absolute Salinity Anomaly as function of longitude, latitude and pressure:)

Therefore, adoption of the Reference Salinity will give oceanographers the first approach in having SI units for salinity expression without compromising historical records. All future steps to the Absolute Salinity deriving will only improve an uncertainty of determination of these units (g/kg) for natural waters, but must be well documented in presented papers and research results. 

Thank you again for your valuable input!