Salinity convertion - confused on numbers.

[arking_mark]

This is not true for my meter. At 25 degree C my conductivity probe show 51.5 mS

If I change this to PSU - it show 33.8

And with density offset set to 0.0013 it shows 1.0225 kg/L as density

If I go to this calculator and put in 51.5 as mS I get salinity as 33.8 psu (the same as for my GHL result) and density to 1022.47 g/l thats is 1.02247 kg/l - rather close to what my GHL probe says 1.0225 kg/L - its a difference in the 4:th decimal - fair enough for me

I do not understand where the confusing is. Use the function density offset and you will have the density right. GHL use the standard for salinity in german speaking countries kg/L - not the specific gravity that most american use.

IMO - there is no confusion with the calculations

However - there can be problem with showing the "right" mS if you use automatic temp compensation. My temperature is rather stable around 25 degree C and therefor I use manual temperature compensation. I calibrate my refractometer with fresh standard of 35 psu (I have the standard in a syringe - therefore no evepropation - the standard is stable. I analyse my seawater in PSU and after that I adjust the manual temperature compensation in that way that it show my actual PSU from the refractometer. I have the probe in a stade, fast stream from my return pipe (without bubbles) - this give me a steady reading

Sincerely Lasse

Your measurements and conversions are spot on...I'm more confused now.

 

[Lasse]

Thank you @Randy Holmes-Farley

For further clarification using your table:

1.0229kg/L should equal about 34,5-ish PSU.

However, showing PSU gives me a different number:

What is the logic behind these differences?

Use the density offset in order to get the density to match the table - after that your PSU will be right to the shown density.

Sincerely Lasse

 

[Lasse]

Exactly - Randys table, Your table, my table and my GHL shows exactly the same.

I use the density offset to get the right value for the density. The mS and PSU in GHL follow the tables but not always the density because there could be different weight in the ingoing ions of the saltwater. Therefore it is possible to set an offset according to shown mS or PSU in order to get the density readings right.

Sincerely Lasse

 

[Lasse]

It would matter if the device thought the temp was a different temp than it actually was.

I´ll think that this is the real problem when calibrating (and compensating for temperature during continuous measurements) We do not have temperature readings that is exact enough. Its better now with the digital temp probe of P4 compared with the old PT1000 of P3. That´s the reason why I use my refractometer and manipulate the temperature my conductivity probe use in order to compensate with in order to get a good PSU reading. I do not use relative gravity or density - I use the more modern PSU readings

Sincerely Lasse

 

[kilnakorr]

Exactly - Randys table, Your table, my table and my GHL shows exactly the same.

I use the density offset to get the right value for the density. The mS and PSU in GHL follow the tables but not always the density because there could be different weight in the ingoing ions of the saltwater. Therefore it is possible to set an offset according to shown mS or PSU in order to get the density readings right.

Sincerely Lasse

But if the probe can only measure the conductivity and not the ions, why deliberatly use an obscure conversion and have users do the math and manuel correct it?

 

[Lasse]

Your measurements and conversions are spot on...I'm more confused now.

I use this

The difference between us is that you see the density as the measured value and mS and PSU as the calculated values - but the truth is the opposite - the mS is the real, hard value - PSU are calculated but it is an easier calculation but the density is very complicated to calculate - you need the possibility to set an offset if you want to use that parameter.

But the problem with an exact temperature compensation make it difficult to always trust the mS reading - therefore is good to use a refractometer (or TM german version of their hydrometer) in order to make the conductivity to show the right value

But if the probe can only measure the conductivity and not the ions

The conductivity is the amount of ions - not their total weight/L as the density is. If you invent a equipment that could in an electrical way measure the weight of all ions in a solution - you would get the finest seat in Stockholm City Hall some november evening in the future.

Sincerely Lasse

 

[Randy Holmes-Farley]

But if the probe can only measure the conductivity and not the ions, why deliberatly use an obscure conversion and have users do the math and manuel correct it?

What units of measure can you choose it to report?

 

[kilnakorr]

What units of measure can you choose it to report?

ms/cm
PSU
kg/L

 

[kilnakorr]

but the truth is the opposite - the mS is the real, hard value - PSU are calculated but it is an easier calculation but the density is very complicated to calculate - you need the possibility to set an offset if you want to use that parameter.

I understand completely.
I just don't understand how several online calculators, Randys table etc. can all agree on the same density value, but GHL can not.
It seems GHL uses a different formula to convert ms/cm to kg/L than everyone else.

 

[Randy Holmes-Farley]

I'd just use mS/cm since that requires no conversions other than temperature compensation, so is not dependent on trusting their conversions.

 

[kilnakorr]

I'd just use mS/cm since that requires no conversions other than temperature compensation, so is not dependent on trusting their conversions.

I'm not German so the kg/L value is not useful to me anyway.
I was simply wondering why these three values available, does not show the same salinity.
I wonder how many Germans have setup their profilux and started increasing the salinity based and a much lower and false density reading.

 

[Lasse]

I'm not German so the kg/L value is not useful to me anyway.

No but it seems that you are danish and this concept was developed in Denmark around 1900 and as GHL say in their article - my bold

GHL analyzed several calculators and tables which convert conductivity into salinity and density and created a formula which lies somewhere in the average of all them and is very close to the conversion table of “Copenhagen Standard Sea Water”.

Or use PSU - it is the normal way to express salinity nowadays and it is also a internal standard (SI unit) . Specific gravity is not an international standard (SI unit) as I know it.

Sincerely Lasse

 

[Randy Holmes-Farley]

Or use PSU - it is the normal way to express salinity nowadays and it is also a internal standard (SI unit) . Specific gravity is not an international standard (SI unit) as I know it.

Specific gravity is unitless, but there are potentially different definitions. In particular, at what temperature is the fluid in question, and what temperature of pure fresh water is it being compared to.

 

[Lasse]

Specific gravity is unitless, but there are potentially different definitions. In particular, at what temperature is the fluid in question, and what temperature of pure fresh water is it being compared to.

Yes I know and in our case the pure freshwater it compare to should be around +4 degree C. If so - specific gravity is equal to density. 1023 g/L = 1.023 kg/L

In our case - if we measure in 25 degree C we get a difference

Note PSU and mS does not vary but in this case when converting density - specific gravity change. If we had converted specific gravity - density would change


Sincerely Lasse

 

[kilnakorr]

GHL analyzed several calculators and tables which convert conductivity into salinity and density and created a formula which lies somewhere in the average of all them and is very close to the conversion table of “Copenhagen Standard Sea Water”.

So instead of using the same formular and tables that already exist, GHL decided to come up with their own formula, so all users needs to offset their readings by 0.013 to show the correct value?

 

[Randy Holmes-Farley]

So instead of using the same formular and tables that already exist, GHL decided to come up with their own formula, so all users needs to offset their readings by 0.013 to show the correct value?

I do not know what they did or why, but companies make chemistry mistakes all the time.

Quite a few companies don't even understand their own products, and make ridiculous claims about them.

 

[kilnakorr]

I do not know what they did or why, but companies make chemistry mistakes all the time.

Quite a few companies don't even understand their own products, and make ridiculous claims about them.

True. My mistake here might have been wondering why such a difference in values was shown, as I should have simply ignored it and just used the value that looked to be right.

 

[arking_mark]

True. My mistake here might have been wondering why such a difference in values was shown, as I should have simply ignored it and just used the value that looked to be right.

Love it! They're all approximates anyways...and stability is what counts.

For what's worth, I've been using my TM High-precision Hydrometer to identify the ms reading I target instead of recalibration of the probe. My probe is about 1.5 years old and drifts...this is quick and easy. Then every 3 months or so I recalibrate.

I'll probably replace the probe in another 6 months.

 

[kilnakorr]

Love it! They're all approximates anyways...and stability is what counts.

For what's worth, I've been using my TM High-precision Hydrometer to identify the ms reading I target instead of recalibration of the probe. My probe is about 1.5 years old and drifts...this is quick and easy. Then every 3 months or so I recalibrate.

I'll probably replace the probe in another 6 months.

I only use it to watch for swings.
Somehow I got a lower than usual reading, witch told me something was wrong - my numbers in this thread shows pretty low salinity.
Still not sure what happened, but did serve its purpose and showed my something was not right.

 

[Lasse]

so all users needs to offset their readings by 0.013 to show the correct value?

Help - have you read the links that explain the science behind salinity measurements at all? The correction factor is not always 0.0013 - its differ between different water mixes - in my case 0,0013 did the job. I had to calculated it from my real measurements and the tables. There is many different ways to calculate the density from the conductivity measurements - as I understand GHL use the most accepted reference salt water mix.- the Copenhagen Standard Sea Water.

For what's worth, I've been using my TM High-precision Hydrometer to identify the ms reading I target instead of recalibration of the probe. My probe is about 1.5 years old and drifts...this is quick and easy. Then every 3 months or so I recalibrate.

I do the same but use a refractometer that I have as correction tool. But the normal calibration is important because it gives you the slope coefficient for the curve (or line) from 0 to 50 mS. However - iME - the probes is rather good after the first calibration and if you now and when gently use a tooth brush on the plates in the gap in the electrode - it will work for very long time. My last worked for 5 years before I dropped it into the floor

Here is my salinity in PSU for the last week (sample period 5 min) The rise around 16:00 today is caused of an adaption to my refractometers readings. After this I had to take out a little amount of aquarium water and replace with RO water (of other reasons than salinity). The probe readings answered directly

Sincerely Lasse