Salt mixing temperature
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True salinity does NOT change with temperature.
But almost all MEASUREMENTS do change if not properly compensated.
Salinity is based on grams of salt dissolved in a kilogram of water.
Temperature does not affect mass, so that ratio cannot change with temperature.
Unfortunately measurements (conductivity and refractivity) are actually based on density, since it is obviously very hard to measure the different masses.
Temperature does affect density since the water expands.
Hence, the real salinity does not change, but it looks like it does.
True salinity does NOT change with temperature.
But almost all MEASUREMENTS do change if not properly compensated.
Salinity is based on grams of salt dissolved in a kilogram of water.
Temperature does not affect mass, so that ratio cannot change with temperature.
Unfortunately measurements (conductivity and refractivity) are actually based on density, since it is obviously very hard to measure the different masses.
Temperature does affect density since the water expands.
Hence, the real salinity does not change, but it looks like it does.
sfsuphysics
Supporting Member
Well I really want to poke a bear right now... (better than saying I want to stir something up
)But volume is dependent on density, so the amount of salt that we add to a fixed volume of water will be different mass based on the temperature. So the salinty we are making would change based on temperature. a 5 gallon bucket of water at 50° weighs different than a 5 gallon bucket at 80° because they have different masses
Ok, that's my one cabin fever moment of the day
Well I really want to poke a bear right now... (better than saying I want to stir something up
But volume is dependent on density, so the amount of salt that we add to a fixed volume of water will be different mass based on the temperature. So the salinty we are making would change based on temperature. a 5 gallon bucket of water at 50° weighs different than a 5 gallon bucket at 80° because they have different masses
Ok, that's my one cabin fever moment of the day
Sure, but temperature can also affect humidity, which may affect the amount of moisture that may be in the salt mix, affecting the mass of what you mix in,
and blah blah blah.
I do want to emphasize for everyone: If you sea a salinity measurement changing with temperature, it is WRONG.
Yes, that Apex is WRONG. Simple as that. (Note that there is a setting in the apex for temp compensation)
True salinity is mass based, and thus cannot really change on temperature.
ofzakaria
Guest
Am confused about the last paragraph. Are we saying temp do not impact salinity? I saw couple of tech papers link temp to density, and density to salinity. The papers say when temp change it change density which change salinity. That's what happening in the oceans now with global warming..is this understanding incorrect?Sure, but temperature can also affect humidity, which may affect the amount of moisture that may be in the salt mix, affecting the mass of what you mix in,
and blah blah blah.
I do want to emphasize for everyone: If you sea a salinity measurement changing with temperature, it is WRONG.
Yes, that Apex is WRONG. Simple as that. (Note that there is a setting in the apex for temp compensation)
True salinity is mass based, and thus cannot really change on temperature.
Appreciate the input.
sfsuphysics
Supporting Member
Hey I said ONE cabin fever moment of the day, you have to wait until tomorrow to post something new for me to respondSure, but temperature can also affect humidity, which may affect the amount of moisture that may be in the salt mix, affecting the mass of what you mix in,
and blah blah blah.
So "climate change" is truly affecting the oceans salinity.
Various ratios and number of ions is changing due to various climate change processes.
But it is not directly due to density only. It is more about ocean chemistry changing.
Raw uncompensated PSU and mS/cm2 do change, and that is what people usually talk about and use in tables and graphs.
So my guess is that most of the papers you see are doing that.
I would not call it wrong. Just a bit sloppy at most.
If a paper directly states that salinity changes on density, it is wrong.
Think about the endless confusion between mass and weight.
ofzakaria
Guest
That's great input. Thank you.
So are we saying that when salinity of the same water sample change when its temperature change is due to the measurment method?
Yes.
ofzakaria
Guest
In your opinion what's the best way to get the target salinity measurment during mixing?
Sorry for the 21 question ha ha..
Refractometers have temperature compensation in them.
To maximize accuracy, make sure to calibrate and measure in the same place/temp as you store the device.
So as long as you test/adjust, you should be fine.
Just do not trust Apex.
ofzakaria
Guest
You are awesome thank you for all this info and explanation!
I trust refractometers the most but I feel I need to be extra careful with calibration procedure. As you said I care alot about the calibration solution temp, the water mix temp and the refracto temp when I do the measurment to have relatively similar conditions between call solution and the water mix..
Then I use the digital refracto for spot check and or frequent tank water checks..
Thanks again.
(Not disagreeing with anyone - just saying it out loud for clarification after some research)
So, true salinity is a mass measurement, which is not affected by temperature. Seawater has a salinity of 35 ppt.
Conductivity (Apex) and refraction measurements are affected by temperature because they are density based and only a proxy for true salinity.
The only way to measure true salinity is to boil the salt out of the water and divide it’s weight by the weight of the water. This will be the same no matter the temp.
So, true salinity is a mass measurement, which is not affected by temperature. Seawater has a salinity of 35 ppt.
Conductivity (Apex) and refraction measurements are affected by temperature because they are density based and only a proxy for true salinity.
The only way to measure true salinity is to boil the salt out of the water and divide it’s weight by the weight of the water. This will be the same no matter the temp.
Flagg37
Guest
I think about it this way. 35 ppt means you have 35 parts of salt for every 1000 parts of water (you could also say it’s 3.5%). So if I take 1000 cups of water and 35 cups of salt and mix them together it will be 35 ppt. Adding or taking away water or salt from the tank will change this since it will no longer be 35 parts salt to 1000 parts water but temperature has no bearing on the ratio.
ofzakaria
Guest
Great analogy
No, because cups are VOLUME, and the grams of water per cup will vary by temperature due to expansion and change in density.
The only way to avoid temperature issues is by carefully weighing salt and water.
Since that is impractical, your next bet is to measure it with a temperature calibrated device and adjust.
ofzakaria
Guest
Man the more I read here and outside the forum, look like the best way to avoid issues are:
1- as you said weigh in the salt to be used if you know how much would make the salinity you looking for
2- eliminate temp factor by trying our best to have all conditions within similar or close temp to your system temp.
3- if we are depending on refracto to reach the salinity we want by adding cups of salt to rodi, we should try to have the cal solution similar temp to the refracto and the rodi water so measurments during calibration and during testing the water mix are within close proximity so the atc take care of other compensation..
@rygh do you see a hole in the above?
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