Here TDS without salinity will be a problem, because although salinity may vary gradually, the compounds apart from sea salt contributing to TDS fluctuate quite randomly and so it is always safer to rely on both readings rather than just TDS for calculations

Here TDS without salinity will be a problem, because although salinity may vary gradually, the compounds apart from sea salt contributing to TDS fluctuate quite randomly and so it is always safer to rely on both readings rather than just TDS for calculations

Sorry for the simple answer. If you care about precision salinity readings, I suggest to buy a new device.

Can you take samples, store in glass bottles to avoid evaporation, and determine salinity at a later point? that might be preferable.

For conversion from solute mass ("Absolute Salinity") to Practical Salinity of seawater, see the formula given at http://www.teos-10.org/publications.htm

TDS could not be used as a proxy of ocean salinity. TDS may be used in fresh water but never in sea water because the error is too high.

Well, TDS and salinity are not the same thing. You should check both terms definitions, an empirical linear relationship is not good. Are you sure that TDS and salinity behave one another linearly? Better buy a new instrument or keep the samples and in your lab use a conductivity meter to have better results

Felix, as others have said, you need to replace/fix your field salinometer. Alternatively, you can collect samples in airtight containers and measure salinity at a later point. Collect a minimum of 100ml and duplicate a few samples. To ensure no evaporation of sample, wrap the container closure with electrical tape. Check nearby universities or laboratory for a Guildline salinometer, which will give you excellent results.

OK. I'm convinced that there's no way to convert TDS ppms to Salinity PSUs. I'll buy a new one right away and thanks for all inputs :-)

I would also store samples, and analysis in the lab after the trip. Sampling and storage is not the problem. If you want to measure salinity with high precision, TDS is not an option.

There is no emprical relationship between salinity and TDS. If you plot TDS vs Salinity for a large range of differeing salinities i.e low salinity/TDS (fresh water) to high salinity/TDS (sea water) the line of best fit follows a curve. Salinity is essentially a measure of the sodium chloride content of water. But other ratios of cations and anions give different salinities for the same TDS. For sea water it is mostly sodium chloride. For other waters, particularly groundwater it varies. Salinity and TDS does not tell you what the consituents of the water sample is. It is just a vauge value for comparing waters from different areas. But it does not tell you any more about the content of the water. For detailed comparisum of waters you need a full siute of cations and anions.

I can get 4 beakers of water fill then with distilled deionised water put in a EC meter and get zero vaule. I can then get some common chemicals available from any supermarket such as table salt (mostly sodium chloride), baking soda (sodium bi-carbonate), epsom salts (magnesium sulphate) and make up a mixture of all three. I can add each one of each of the four groups of chemicals to a beaker to and if I doctor it enough I can get the exact same salinity reading from each beaker. The salinity tells me nothing about chemical composition of the water.

If your salinometer is broken buy a new one. Not sure what you hope to achieve by recording different salinities of sea water but If you are hoping to determine different sea water types by salinity without detailed chemical analysis I'd say your project is lacking something.

I suppose your salinometer is a refractometer with salinity (in ppm) and density scales. Firstly you can take the values ​​of density (with a densimeter, of course) and then compare them with a new salinometer, when you purchase it. The unit of practical salinity (PSU) has now been superseded by the scale of practical salinity (PSS), for which the units are dimensionless. Nevertheless, the values ​​remain the same, whether they are expressed in g / l, ppm, ppt, PSU or PSS. All these values ​​express the total soluble salts (TSS), which are certainly a different value respect to the total dissolved solids (TDS). Although, in my opinion, definitely in off shore, but also for certain near-shore areas, difference would be negligible. Clearly can not exist a conversion scale between TDS and TSS, because an area may have a different solute content from another for various reasons, entirely different from those of salinity. More anthropized an area is, more difficult it is to use TDS as TSS. I do not know where you go to do measurements, but I think you could take the value mg / L TDS as PSS with an accuracy around 0.5 (which can be acceptable or not because of the use you make of the data), for an off-shore marine area

Mauro's comment is somewhat outdated, sorry to say that here.

The new international seawater standard in force since 2010 is TEOS-10, see www.teos-10.org for the details. It is based on the definition of the 2008 Reference Composition Salinity Scale of IAPSO Standard Seawater (SSW) which gives the best estimate currently available for the mass fraction of dissolved sea salt, say, in g/kg.

The conductivity-based 1978 Practical Salinity Scale (PSS-78) is still in practical use; its readings return dimensionless numbers (inofficially sometimes denoted by "psu") which deviate from the mass fraction in g/kg by about 0.5% because of experimental errors made by Soerensen in 1900. This is known since the 1940s. There are no units "PSS" or "PSU" ever officially defined in any of the seawater standards since 1902, nor where those permitted to be used in scientific journals.

Rainer is right. He makes a rigorous reasoning, on which it does not rain.

However, I'm going back to say that a lot depends on what we want to do with the data. For example, if we want to use the salinity in a work of artificial breeding and larval rearing of marine fish species, the approximation I mentioned is more than acceptable and the data can also be used in the publication of scientific work. Otherwise, if the salinity values ​​should be placed in relation to dissolved oxygen, for a work regarding this aspect.

One should take samples of seawater, store in glass bottles in order to avoid evaporation, then determine the salinity which is a prefered procedure.

I am being asked if it is possible to take TDS as a proxy for ocean salinity.

As I said, it depends on the use that we make of data.

TDS = DIS + DOM

Dissolved inorganic solutes are a quantity substantially similar to TSS, while dissolved organic matter is relatively negligible, by weight, in off-shore areas (in accordance with certain uses of the data)

On the other hand, the estimated values with the salinometer (refractometer) and densimeter (the scale of density is considered in the salinometer) are, respectively, the result of the refraction of light by all contained solutes, and of the water density at a given temperature and at a certain solute content … not only dissolved salts. These are rough estimates, by their nature. Therefore, as rightly points Rainer, when necessary to maintain strict conditions, we refer to a sample of standard seawater!

TDS has a linear relationship with EC and it's formula is TDS=0.65*EC

then you can calculate salt concentration, C(p.p.m) with C=640*EC

If the TDS is measured by density then it can be converted to salinity very accurately(a few ppm) based on the equation of state. If the TDS is measured by conductivity, then it gives the old, conductivity based salinity.

Actually the TDS relates to the new salinity(TEOS-10) better than it does with the PSS-78 salinity.

Saline water is mainly dominant in Na and Cl. Thus TDS most depend upon these two constituents. Measure the electrical conductivity is a quick process. Ion meters for the measurement of TDS is equally good. Collection of water and analysis in a lab and INSTU analysis may give different results. However spot analysis is best.