Salinity convertion - confused on numbers.

[Lasse]

The real measurements is the conductivity, the real figure you get is x mS (milli Siemens). This should be transformed to a form we can refer to. Since long time ago - there is a standard of seawater salt content in use. If you want to understand the differences please start reading here. Go further here. here, here, here, here and here

But - this is one problem and the one you ask for clarifications of. Your real measurement of the conductivity could be false if you have not done the calibration well enough. The conductivity measurement is very dependent of temperature - therefore it is important that you do the calibration right and in a temperature as close as possible to the standards demand of temperature. A constant measurement of conductivity also demands a rather high flow to the probe because if tiny air bubbles forms on the electrode - the reading will be erratic.

To measure salinity by conductivity its not an easy task. Its easy to do mistake. When i calibrate my probe - I first test with my refractometer - calibrated with a fresh calibration fluid (for refractometers) When I calibrate my conductivity probe - I manipulate the input sample temperature in a way that I get a rather alike PSU reading as i get from my refractometer.

Sincerely Lasse

 

[kilnakorr]

The real measurements is the conductivity, the real figure you get is x mS (milli Siemens). This should be transformed to a form we can refer to. Since long time ago - there is a standard of seawater salt content in use. If you want to understand the differences please start reading here. Go further here. here, here, here, here and here

But - this is one problem and the one you ask for clarifications of. Your real measurement of the conductivity could be false if you have not done the calibration well enough. The conductivity measurement is very dependent of temperature - therefore it is important that you do the calibration right and in a temperature as close as possible to the standards demand of temperature. A constant measurement of conductivity also demands a rather high flow to the probe because if tiny air bubbles forms on the electrode - the reading will be erratic.

To measure salinity by conductivity its not an easy task. Its easy to do mistake. When i calibrate my probe - I first test with my refractometer - calibrated with a fresh calibration fluid (for refractometers) When I calibrate my conductivity probe - I manipulate the input sample temperature in a way that I get a rather alike PSU reading as i get from my refractometer.

Sincerely Lasse

Sure the readings could be wrong, if calibration was not done right, and readings could be influenced by other factors.
It is not about the accuracy.
If we take a loik at all the online converting tools, it seems you can convert conductivity to density. I wonder why GHL is not doing the same conversion?
Is it doing to seperate measurements for conductivity and density? This would make more sense.

 

[Lasse]

It convert to density - it is kg/L but it does not convert to specific gravity Specific gravity is a one-dimensional number with no dimensions . The difference between those is very good explained here. Note that it has been a tradition - at least in english aquarium literature to use specific gravity and name it density. In german literature (and scientific) is density or PSU used mostly - at least in the good old days

Sincerely Lasse

 

[Lasse]

Is it doing to seperate measurements for conductivity and density? This would make more sense

A conductivity probe is only measure mS. You can´t do a direct measurement of PSU, density or specific gravity with a conductivity probe. It is the same way when a scale give you a weight of something and you want to know the volume of the same thing - you need to have formulas that calculate that from the weight, density and so on

Sincerely Lasse

 

[kilnakorr]

A conductivity probe is only measure mS. You can´t do a direct measurement of PSU, density or specific gravity with a conductivity probe. It is the same way when a scale give you a weight of something and you want to know the volume of the same thing - you need to have formulas that calculate that from the weight, density and so on

Sincerely Lasse

That is what I thought.
Does this simply mean that GHL is coverting to density kg/L using a different calculations than other calculators?

 

[KStatefan]

It convert to density - it is kg/L but it does not convert to specific gravity Specific gravity is a one-dimensional number with no dimensions . The difference between those is very good explained here. Note that it has been a tradition - at least in english aquarium literature to use specific gravity and name it density. In german literature (and scientific) is density or PSU used mostly - at least in the good old days

Sincerely Lasse

Seems to be a fairly common mistake for owners in the states that they are the same.

 

[Lasse]

Once again - the most used value of salinity in the aquarium business nowadays is specific gravity. Specific gravity is a one-dimensional number with no dimensions. You compare the density with waters density at 4 degree C. At that temperature - density and specific gravity is nearly the same in numbers but density have the dimension kg/L.

at 25 degree C - there is difference between the number. A specific gravity of 1.026 - 25 degree C as reference will give a density of 1.023 kg/L (1022.97 g/l is around 1.023 kg/L)

You use specific gravity and name it density and its here its going wrong.

Sincerely Lasse

 

[arking_mark]

I'm going to try and simplify the frustration of using GHL salinity measurements.

Most people would agree that the target salinity number that they would like to maintain is that of NSW.

The accepted Specific Gravity of NSW @77f is roughly 1.0263.

When measured with an EC probe, 53ms/cm.

When measured by density, 1.0233 kg/L.

When measured by PSU, 35 ppt.

This is what most salinity calculators show.

The issue with GHL salinity is that for it's EC probe measurements, it does a significantly different calculation for PSU and Density. In GHL land, 53ms/cm does NOT convert to accepted norms.

These different different GHL salinity conversions cause confusion.

If you trust the EC measurement, then great...don't worry about anything else.

I wanted to get to ground truth. I used a high precision TM hydrometer with a NIST certified thermometer to pinpoint what I believed was my actual salinity. With a freshly calibrated GHL EC probe, I found that the only measurement that was accurate for my tank was the GHL density calculation.

 

[Lasse]

I used a high precision TM hydrometer with a NIST certified thermometer to pinpoint what I believed was my actual salinity. With a freshly calibrated GHL EC probe, I found that the only measurement that was accurate for my tank was the GHL density calculation.

Interesting that you mention the TM hydrometer. It is from Germany and comes in two different models. One for german speaking countries where density in kg/l is given and one for the rest of the world that give the result as specific gravity. Which did you use?

Sincerely Lasse

 

[Randy Holmes-Farley Verified Community Expert]

Forgive me if you already understand this, but density and specific gravity values WILL be different. The density of fresh water is not exactly 1.000000. Most people do not report density on this forum.

Instead of a calculator, a table might be easier to understand. I show some relative values in this article:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

Table 3. Density and specific gravity as a function of salinity of seawater.3 The darker blue rows represent the range usually encountered in the open ocean.​
Salinity (PSU)​	Density (25° C)​	Specific Gravity (25° C)​
0​	997.05​	1.0000​
29​	1018.8​	1.0218​
30​	1019.6​	1.0226​
31​	1020.3​	1.0233​
32​	1021.1​	1.0241​
33​	1021.8​	1.0249​
34​	1022.6​	1.0256​
35​	1023.3​	1.0264​
36​	1024.1​	1.0271​
37​	1024.9​	1.0279​
38​	1025.6​	1.0286​
39​	1026.4​	1.0294​

 

[kilnakorr]

Once again - the most used value of salinity in the aquarium business nowadays is specific gravity. Specific gravity is a one-dimensional number with no dimensions. You compare the density with waters density at 4 degree C. At that temperature - density and specific gravity is nearly the same in numbers but density have the dimension kg/L.

at 25 degree C - there is difference between the number. A specific gravity of 1.026 - 25 degree C as reference will give a density of 1.023 kg/L (1022.97 g/l is around 1.023 kg/L)

You use specific gravity and name it density and its here its going wrong.

Sincerely Lasse

No. I wonder why the density converted to either conductivity or PSU converted does show same conductivity/PSU as the probe - or reversed.

 

[arking_mark]

Interesting that you mention the TM hydrometer. It is from Germany and comes in two different models. One for german speaking countries where density in kg/l is given and one for the rest of the world that give the result as specific gravity. Which did you use?

Sincerely Lasse

Specific Gravity.

 

[kilnakorr]

Forgive me if you already understand this, but density and specific gravity values WILL be different. The density of fresh water is not exactly 1.000000. Most people do not report density on this forum.

Instead of a calculator, a table might be easier to understand. I show some relative values in this article:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

​

Table 3. Density and specific gravity as a function of salinity of seawater.3 The darker blue rows represent the range usually encountered in the open ocean.​
Salinity (PSU)​	Density (25° C)​	Specific Gravity (25° C)​
0​	997.05​	1.0000​
29​	1018.8​	1.0218​
30​	1019.6​	1.0226​
31​	1020.3​	1.0233​
32​	1021.1​	1.0241​
33​	1021.8​	1.0249​
34​	1022.6​	1.0256​
35​	1023.3​	1.0264​
36​	1024.1​	1.0271​
37​	1024.9​	1.0279​
38​	1025.6​	1.0286​
39​	1026.4​	1.0294​

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?

 

[arking_mark]

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?

What is the temp reading of your tank?

What is the ms/cm reading of the probe?

 

[kilnakorr]

What is the temp reading of your tank?

25,0 C - although calibration shouldn't matter it was done using 25C, 50ms fluid as directions states.

 

[arking_mark]

25,0 C - although calibration shouldn't matter it was done using 25C, 50ms fluid as directions states.

What is the ms/cm reading of the probe?

 

[kilnakorr]

What is the ms/cm reading of the probe?

49,9

 

[Lasse]

I'm going to try and simplify the frustration of using GHL salinity measurements.

Most people would agree that the target salinity number that they would like to maintain is that of NSW.

The accepted Specific Gravity of NSW @77f is roughly 1.0263.

When measured with an EC probe, 53ms/cm.

When measured by density, 1.0233 kg/L.

When measured by PSU, 35 ppt.

This is what most salinity calculators show.

The issue with GHL salinity is that for it's EC probe measurements, it does a significantly different calculation for PSU and Density. In GHL land, 53ms/cm does NOT convert to accepted norms.

These different different GHL salinity conversions cause confusion.

If you trust the EC measurement, then great...don't worry about anything else.

I wanted to get to ground truth. I used a high precision TM hydrometer with a NIST certified thermometer to pinpoint what I believed was my actual salinity. With a freshly calibrated GHL EC probe, I found that the only measurement that was accurate for my tank was the GHL density calculation.

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

 

[Randy Holmes-Farley Verified Community Expert]

25,0 C - although calibration shouldn't matter it was done using 25C, 50ms fluid as directions states.

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

 

[arking_mark]

49,9

Here is what I would expect...so your 32.7 looks accurately converted, but the density is calculated differently by GHL, 1.0299 vs 1.0216.