Salinity and temperatures - measuring.

[kilnakorr]

From what I can read on various threads, including some bro-science nonsense, salinity does not change with temperature.
Seems logic as 35 ppt won't change if temperature is change.
But salinity does change.
Since I got a digital reader, I tested some cooler freshly made saltmix and it reads 35,0 ppt / 1.028 sg. I tested the temperature on the device which reads roughly 1.5 C to high.

If specific gravity changes with temperature, and device temperature is misleading, I should still be able to trust the salinity readings, regarding of temperature right?
The salinity matches my refractometer perfectly, but the salinity/gravity seems of (or I cannot find any understable charts for salinity, sg, and temperature corelation).

 

[Reef.]

What are you using to measure the salinity? If a refractometer you are measuring the refractive index not SG, check to see if your refractometer has ATC written on it, if so it will correct the reading to the temp.

You need to calibrate it with 35ppt solution to trust the reading, I would advise to made your own, very easy to do, also look to buy a Tropic Marin hydrometer, no calibration needed and it will always give you the same result, very useful to have along side a good refractometer.

 

[kilnakorr]

What are you using to measure the salinity? If a refractometer you are measuring the refractive index not SG, check to see if your refractometer has ATC written on it, if so it will correct the reading to the temp.

You need to calibrate it with 35ppt solution to trust the reading, I would advise to made your own, very easy to do, also look to buy a Tropic Marin hydrometer, no calibration needed and it will always give you the same result, very useful to have along side a good refractometer.

The refractometer has ppt and sg (I think most do) and ATC.
The other is a 'probe' type you just stick in the water. As mentioned, they both agree on the salinity, but not on the specific gravity part.
I just wonder what 35 ppt relates to in specific gravity at specific temperature. This way I could check if the digital probe is showing wierd values, or simply auto corrects with temperature.

 

[Reef.]

The refractometer has ppt and sg (I think most do) and ATC.
The other is a 'probe' type you just stick in the water. As mentioned, they both agree on the salinity, but not on the specific gravity part.
I just wonder what 35 ppt relates to in specific gravity at specific temperature. This way I could check if the digital probe is showing wierd values, or simply auto corrects with temperature.

I could be wrong but this is how I understand it, salinity, SG and refractive index are all used interchangeably incorrectly, they all are measuring something different, the amount of the salt in the water doesn’t change with temp but the equipment we use to measure that amount of salt in the water does change with temp, so a refractometer will refract light differently at different temps, a hydrometer will also give different results at different temps, it will show a higher or lower gravity pull at different temps, that is not the same as saying it is showing more or less salt in the water.

 

[kilnakorr]

I could be wrong but this is how I understand it, salinity, SG and refractive index are all used interchangeably incorrectly, they all are measuring something different, the amount of the salt in the water doesn’t change with temp but the equipment we use to measure that amount of salt in the water does change with temp, so a refractometer will refract light differently at different temps, a hydrometer will also give different results at different temps, it will show a higher or lower gravity pull at different temps, that is not the same as saying it is showing more or less salt in the water.

I think thats correct. 35 ppt will be 35 ppt regardsless of temperature, but the gravity will change.
Ofcourse the good @Randy Holmes-Farley already did an article on this. I just needed the right search terms:

http://reefkeeping.com/issues/2004-07/rhf/index.php

Including a nice table.

 

[kilnakorr]

I thought I'd just do some tests with same sample of water, and change the temperature:
Temp(C) Sg PPT
24.0 1.027 35.1
35.6 1.023 35.8
24.9 1.026 35.0
22.2 1.026 35.0
15.1 1.028 34.1
12.5 1.029 33.7

It seems to accurate around it's calibrated temp (was 25 C), but not at high/low temperatures.
I need to study the above table to see if these numbers make any sense....

 

[Reef.]

I thought I'd just do some tests with same sample of water, and change the temperature:
Temp(C) Sg PPT
24.0 1.027 35.1
35.6 1.023 35.8
24.9 1.026 35.0
22.2 1.026 35.0
15.1 1.028 34.1
12.5 1.029 33.7

It seems to accurate around it's calibrated temp (was 25 C), but not at high/low temperatures.
I need to study the above table to see if these numbers make any sense....

I use a hydrometer, I don’t like using charts, I make sure my water is at 25c, I then know the SG reading is correct, I’ve found with charts the further away from the calibrated temp the charts become less reliable.

 

[kilnakorr]

I use a hydrometer, I don’t like using charts, I make sure my water is at 25c, I then know the SG reading is correct, I’ve found with charts the further away from the calibrated temp the charts become less reliable.

I wasn't trying to rely on charts at all. Just curious on how these things where connected.
Using a hydrometer seems tedious, and will also require a thermometer that can be trusted. I find most digital thermometers often are a few degrees celcius off, so another thing that can give false readings.

 

[Reef.]

I wasn't trying to rely on charts at all. Just curious on how these things where connected.
Using a hydrometer seems tedious, and will also require a thermometer that can be trusted. I find most digital thermometers often are a few degrees celcius off, so another thing that can give false readings.

aren’t you already relying on a thermometer or the refractometer ATC and maybe charts, buying 2-3 cheap digital thermometers will give you a very good idea what the correct temp is if they are within .1c of each other.

Measuring the correct salinity with all the options can be tedious if you want the reading to be accurate. Calibration, making the calibration fluid or buying one and hoping it’s correct...

Adding 500ml of tank water to a measuring cylinder, letting the temp drop to 25c over a couple of minutes, then reading the salinity and knowing it’s probably as good as you are going to get doesn’t seem that much effort, then using the same water to calibrate your refractometer doesn’t seem that tedious?

Each to their own I guess, I’ll leave you to read your very interesting salinity charts, I’ll stick with my tedious hydrometer

 

[kilnakorr]

aren’t you already relying on a thermometer or the refractometer ATC and maybe charts, buying 2-3 cheap digital thermometers will give you a very good idea what the correct temp is if they are within .1c of each other.

Measuring the correct salinity with all the options can be tedious if you want the reading to be accurate. Calibration, making the calibration fluid or buying one and hoping it’s correct...

Adding 500ml of tank water to a measuring cylinder, letting the temp drop to 25c over a couple of minutes, then reading the salinity and knowing it’s probably as good as you are going to get doesn’t seem that much effort, then using the same water to calibrate your refractometer doesn’t seem that tedious?

Each to their own I guess, I’ll leave you to read your very interesting salinity charts, I’ll stick with my tedious hydrometer

I see your point. Charts was just because I was curious of the relationsship between sg and temperature.
Taking 500mL of tank water, will result in 500 mL ATO water as it kicks in (you could turn it off, and on again but room for error).

My point/question was merely based on the checker with an incorrect thermometer and if salinity measurements could be trusted.
Putting a 'stick' in the tank is a lot easier than pulling out water from the tank

 

[Reef.]

I see your point. Charts was just because I was curious of the relationsship between sg and temperature.
Taking 500mL of tank water, will result in 500 mL ATO water as it kicks in (you could turn it off, and on again but room for error).

My point/question was merely based on the checker with an incorrect thermometer and if salinity measurements could be trusted.
Putting a 'stick' in the tank is a lot easier than pulling out water from the tank

Putting a stick it works if your tank is big enough, mine isn’t, you also need the flow off.

When checking, just add 500ml of water then take out the 500ml for testing, heck you can even put it back in after testing.

I change water daily so it’s not an issue for me, either way, salinity doesn’t need checking daily, at water change day works too.
As said I also use a refractometer, but the hydrometer is the gold standard for me, even after calibrating the refractometer it’s normally off the next time I come to use it.

Not meaning to hammer the point home, you just seemed interested in getting your salinity as good as you possibly can, I disregarded hydrometers at the start, I soon realised how important it was, not just for the animals but it affects all your other parameters too, if salinity is off, so is your cal, mag and alk.

You can make all the methods work, some just need more checks than others.

 

[Randy Holmes-Farley]

From what I can read on various threads, including some bro-science nonsense, salinity does not change with temperature.
Seems logic as 35 ppt won't change if temperature is change.
But salinity does change.
Since I got a digital reader, I tested some cooler freshly made saltmix and it reads 35,0 ppt / 1.028 sg. I tested the temperature on the device which reads roughly 1.5 C to high.

If specific gravity changes with temperature, and device temperature is misleading, I should still be able to trust the salinity readings, regarding of temperature right?
The salinity matches my refractometer perfectly, but the salinity/gravity seems of (or I cannot find any understable charts for salinity, sg, and temperature corelation).

Bro science? What are you talking about? You may be referring to my posts, as I have had to post that exact phrase many times in the past couple of weeks. Too many folks are posting non-science nonsense that salinity changes with temperature.

Salinity is measured in PPT or PSU and is UNCHANGING with temperature.

"digital reader" is an ambiguous term. Please tell us what it is. It might be a refractometer or a conductivity meter.

 

[Randy Holmes-Farley]

The refractometer has ppt and sg (I think most do) and ATC.
The other is a 'probe' type you just stick in the water. As mentioned, they both agree on the salinity, but not on the specific gravity part.
I just wonder what 35 ppt relates to in specific gravity at specific temperature. This way I could check if the digital probe is showing wierd values, or simply auto corrects with temperature.

It sounds as if you are using a conductivity meter. Clarifying that will help.

Conductivity meters generally try to correct for temperature changes. Exactly how that is done will vary, and the best meters even give you choices.

If you used the same meter in the same water at different temps, and got different answers in ppt, then the meter is not doing as especially good job at the correction, and I'd only use it closer to the temp that you calibrate/check it.

Note that IF you are using the Hanna conductivity meter HI98319:

Marine Salinity Tester - HI98319

Hanna Instruments HI98319 Marine Salinity Tester measures conductivity (EC) to determine the salinity of natural and artificial seawater within seconds.

www.hannainst.com

It only claims to be accurate +/- 1 ppt, while the whole range of numbers you post is 34.75 +/- 1.05 ppt.

Thus, what you are seeing may be mostly random error, but it may also be imperfect temp correction.

 

[Randy Holmes-Farley]

I think thats correct. 35 ppt will be 35 ppt regardsless of temperature, but the gravity will change.
Ofcourse the good @Randy Holmes-Farley already did an article on this. I just needed the right search terms:

http://reefkeeping.com/issues/2004-07/rhf/index.php

Including a nice table.

That sort of table IS NOT an indication that specific gravity changes with temperature. It is an indication that a hydrometer measuring specific gravity will give incorrect readings that need to be corrected.

I discuss how specific gravity actually changes here:

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!

This month, Randy explains and reviews specific gravity.

reefs.com

 

[kilnakorr]

Bro science? What are you talking about? You may be referring to my posts, as I have had to post that exact phrase many times in the past couple of weeks. Too many folks are posting non-science nonsense that salinity changes with temperature.

Yes. I came across several posts with far fetched explanations.

To try and sum up my issue:
I tested the temp of my tank with 3 different thermometers:
25.6
25.6-25.7(changing between these two)
25.6
So seems accurate.
Tested with the salinity 'stick': 26.4

So if the stick reads an incorrect temperature, and uses that temperature to adjust the reading, it could be inaccurate.

Some pictures of the 'stick' with SG/PPT measurements.

the hydrometer is the gold standard for me

True. The most accurate way, and will pick one up. I can see many people have noticed commercial '35 ppt' calibration fluids often are a bit of. Making your own calibration fluid would probably be a good thing.

 

[kilnakorr]

Salinity is measured in PPT or PSU and is UNCHANGING with temperature.

Yes. I don't question this at all.

I discuss how specific gravity actually changes here:

Reread it. And SG DOES change with temperature.

My stick, not sure how it works, should have meassured 35 ppt at any temperature, and only shown changes in SG readings.

 

[Randy Holmes-Farley]

Yes. I don't question this at all.


Reread it. And SG DOES change with temperature.

My stick, not sure how it works, should have meassured 35 ppt at any temperature, and only shown changes in SG readings.

SG changes hardly at all with temperature. That is because specific gravity is the ratio of seawater density to the freshwater density at that temperature. Both change, and change very similarly, cancelling each other out (almost).

The value reported by a hydrometer incorrectly shows a big change in sg with temperature. That is not a correct representation of the change in sg with temperature. It is an attribute of the measuring device giving incorrect readings.

Your stick looks like the scionix device, and it claims a tolerance of +/- 2 ppt, which means all the values you reported are within tolerance for the same value.

The Scionix Salinity Digital Meter is a must have tool for marine aquariums

Scionix Salinity Digital Meter eliminates the uncertainty associated with hydrometers or refractometers with this portable, rugged, water resistant device

aquarays.co.nz

 

[kilnakorr]

SG changes hardly at all with temperature. That is because specific gravity is the ratio of seawater density to the freshwater density at that temperature. Both change, and change very similarly, cancelling each other out (almost).

The value reported by a hydrometer incorrectly shows a big change in sg with temperature. That is not a correct representation of the change in sg with temperature. It is an attribute of the measuring device giving incorrect readings.

Your stick looks like the scionix device, and it claims a tolerance of +/- 2 ppt, which means all the values you reported are within tolerance for the same value.

The Scionix Salinity Digital Meter is a must have tool for marine aquariums

Scionix Salinity Digital Meter eliminates the uncertainty associated with hydrometers or refractometers with this portable, rugged, water resistant device

aquarays.co.nz

I think the reason I'm was a bit confused is that alot of refractometers have auto temp correction, hydrometer needs specific temp etc.
Why all this temperature correction if neither sg or ppt changes?

From your article:

For example, the specific gravity of natural seawater (S =35) is 1.0278 using the 3.98 °C standard, 1.0269 using the 60 °F standard, 1.0266 using the 20 °C standard, and 1.0264 using the 77 °F standard.

Unless I'm misunderstanding this, there's a different in sg with different temperatures.


I guess I'll just accept, that temperature somehow impacts most devices and needs to adjust for it.

 

[Randy Holmes-Farley]

I think the reason I'm was a bit confused is that alot of refractometers have auto temp correction, hydrometer needs specific temp etc.
Why all this temperature correction if neither sg or ppt changes?

Because the measuring method itself is imperfect and changes a lot with temperature.

Conductivity of 35 ppt seawater changes a lot with temperature. If you are trying to use the conductivity to determine salinity, you need to correct for that.

The refractive index of 35 ppt seawater changes a lot with temperature. If you are trying to use the refractive index to determine salinity, you need to correct for that.

The density of 35 ppt seawater changes a lot with temperature. If you are trying to use the density to determine salinity, you need to correct for that. FWIW, a hydrometer actually measures/responds to density, even if it reports in other units.

 

[Randy Holmes-Farley]

Unless I'm misunderstanding this, there's a different in sg with different temperatures.

Yes, but as you can see, that effect is pretty small. Much smaller than the corrections all of those devices require. Far smaller than the hydrometer correction table would suggest.

specific gravity of natural seawater (S =35) is
1.0278 at 3.98 deg C
1.0269 at 60 °F
1.0266 at 20 °C
1.0264 at 77 °F