Temperature and Salinity relationship...

[Dom]

Is there a relationship between temperature and salinity? Will salinity change depending on the temperature of the water?

 

[redfishbluefish]

The simple answer is YES. When I first make up my water from a cooled basement (not sure of the temperature), the refractometer reading is about 1.030. But when I warm it up to 78 degrees, it's now at 1.026.

If you're using an "old school" glass hyrdrometer, you need to follow the temperature requires of that hydrometer. Otherwise, you're getting false numbers.

 

[Dom]

The simple answer is YES. When I first make up my water from a cooled basement (not sure of the temperature), the refractometer reading is about 1.030. But when I warm it up to 78 degrees, it's now at 1.026.

If you're using an "old school" glass hyrdrometer, you need to follow the temperature requires of that hydrometer. Otherwise, you're getting false numbers.

I've had my refractometer about 7 years. Hoping there is a model or something on it that will help me to figure this out.

The reason I asked is because is seemed to me that as my water temp goes up, salinity goes down. This means that temperature fluctuations in the DT will = fluctuations in salinity?

 

[Dkeller_nc]

Technically, the actual salinity of sea water is insensitive to temperature, since salinity is defined as the percent by weight of salt in a given weight of water, which will not change with temperature.

However, the refractive index of seawater is a relatively strong function of temperature, which is why proper equilibration of the measuring instrument, the calibration solution and the sample is important.

 

[Blutspitze]

The refractive index aspect is going to be the main piece to this puzzle, but you can also point to evaporation, as well. While not a direct line, the increase in temperature causing more evaporation will increase the salinity, hence needing top off.

 

[Dom]

So I looked up my refractometer (Model RHS-10ATC). Where temperature is concerned, it says the device has a "built-in automatic temperature compensation system for field use".

It doesn't say anything about a temperature range of operation or anything about temperature requirements.

 

[Blutspitze]

So I looked up my refractometer (Model RHS-10ATC). Where temperature is concerned, it says the device has a "built-in automatic temperature compensation system for field use".

It doesn't say anything about a temperature range of operation or anything about temperature requirements.

ATC is a really common feature among refractometers. Aquarium ones will typically be calibrated to read water at ~ 77-80 F, and can compensate for a difference of a few degrees. With yours, it sounds more like it's designed for testing ocean water or the like - in the field. It'd have a MUCH wider range of temperatures where it's accurate. Generally the instructions will tell you to wait 15-30 seconds or so for the temperatures to equalize properly before reading to ensure the ATC does its job.

 

[Dom]

I'm open to recommendations. What do you use?

 

[Martin Kuhn]

The correct answer schould be
- if you are using a refrak water temperature does NOT affect the reading of this tool (the drop of water immediately adapts to the temperature of the tool which has ATC). You then read the psu scale of the refrak which eG shows 34,5 psu and should not use the spec. Gravity also shown which shows ~ 1,0255 then

- if you are using a spindle or conductivity Probe the water temperature DOES affect the reading. You are measuring „in“ the water which has a different volume per weight (density) depending if it is hotter/colder! Also other elements than water have this behavior. As already written here: if you measure in „colder water“ the reading is higher. If you measure in „hotter water“ of the same tank the reading is lower. Exactly for that reason you should always measure the temperature together with the reading of the spindle/Probe and then calculate the salinity (psu) of this pair of measurement results. If you do this with hotter / colder water the calculated salinity (psu) is again the same!

In other words
- measurements in salinity are TEMPERATURE INDEPENDENT and can be used directly
- measurements in density and conductivity are “Temperature depending” and thus make sense only together with the temperature where they have been measured. To compare this results over time you have to either measure each time at exactly the same temperature, or better, calculate the salinity from the measurement + its temperature.

 

[Dom]

The correct answer schould be
- if you are using a refrak water temperature does NOT affect the reading of this tool (the drop of water immediately adapts to the temperature of the tool which has ATC). You then read the psu scale of the refrak which eG shows 34,5 psu and should not use the spec. Gravity also shown which shows ~ 1,0255 then

- if you are using a spindle or conductivity Probe the water temperature DOES affect the reading. You are measuring „in“ the water which has a different volume per weight (density) depending if it is hotter/colder! Also other elements than water have this behavior. As already written here: if you measure in „colder water“ the reading is higher. If you measure in „hotter water“ of the same tank the reading is lower. Exactly for that reason you should always measure the temperature together with the reading of the spindle/Probe and then calculate the salinity (psu) of this pair of measurement results. If you do this with hotter / colder water the calculated salinity (psu) is again the same!

In other words
- measurements in salinity are TEMPERATURE INDEPENDENT and can be used directly
- measurements in density and conductivity are “Temperature depending” and thus make sense only together with the temperature where they have been measured. To compare this results over time you have to either measure each time at exactly the same temperature, or better, calculate the salinity from the measurement + its temperature.

When you speak of density you are speaking of salt density?

Isn't salinity the ratio of salt to water in the tank?

 

[Dkeller_nc]

The correct answer schould be
- if you are using a refrak water temperature does NOT affect the reading of this tool (the drop of water immediately adapts to the temperature of the tool which has ATC). You then read the psu scale of the refrak which eG shows 34,5 psu and should not use the spec. Gravity also shown which shows ~ 1,0255 then

- if you are using a spindle or conductivity Probe the water temperature DOES affect the reading. You are measuring „in“ the water which has a different volume per weight (density) depending if it is hotter/colder! Also other elements than water have this behavior. As already written here: if you measure in „colder water“ the reading is higher. If you measure in „hotter water“ of the same tank the reading is lower. Exactly for that reason you should always measure the temperature together with the reading of the spindle/Probe and then calculate the salinity (psu) of this pair of measurement results. If you do this with hotter / colder water the calculated salinity (psu) is again the same!

In other words
- measurements in salinity are TEMPERATURE INDEPENDENT and can be used directly
- measurements in density and conductivity are “Temperature depending” and thus make sense only together with the temperature where they have been measured. To compare this results over time you have to either measure each time at exactly the same temperature, or better, calculate the salinity from the measurement + its temperature.

Just for the record, there are several potential misinterpretations here. While in theory the ATC of a refractometer will indeed compensate for the temperature difference between when the refractometer is calibrated and when it is used, the range over which this will be accurate is fairly limited. And refractive index is quite a strong function of temperature, so to ensure accuracy it's wise to calibrate the refractometer with a calibration solution that matches 35 ppt seawater's refractive index close to the temperature at which you intend to use it. Randy Holmes Farley wrote an excellent article on using refractive index to estimate salinity here.

And while it is true that the density of water, or a salt solution in water, is a function of temperature, it's a comparatively weak function of temperature. So weak, in fact, that the average aquarist will have difficulty measuring this effect without standardized volumetric glassware and an accurate scale. For all intents and purposes, we can ignore that effect when it comes to keeping animals in an aquarium.

 

[Martin Kuhn]

And while it is true that the density of water, or a salt solution in water, is a function of temperature, it's a comparatively weak function of temperature. So weak, in fact, that the average aquarist will have difficulty measuring this effect without standardized volumetric glassware and an accurate scale. For all intents and purposes, we can ignore that effect when it comes to keeping animals in an aquarium.

Have a look on page 114 within this FAQ where i show a table of salinity as a function of measured density (spindle) & temperature

http://mathgame.de/Aqua/WaterParameter_FAQ_eng.pdf

I wouldn't say that the effect of temperature is as weak. Using eg a normal calibrated spindle (calibrated to density in g/cm^3) you get a quite different result if measuring water in a typical range for reef aquariums of 23°C and 26°C (which is 73,5 / 79 ° F)
a measurement result of 1,023 g/cm^3 @ 23°C -> 33,8 psu, whereas
a measurement result of 1,023 g/cm^3 @ 26°C -> 35,0 psu
for me this is already some difference.

I noticed some cases where users here in Europe thought they have changing salinity in their tanks , where at the end it was due to misunderstanding of this physical behavior of (salt)water.

 

[Martin Kuhn]

When you speak of density you are speaking of salt density?

Isn't salinity the ratio of salt to water in the tank?

Hi Dom

the thing is:
1) Salinity is the measurement unit which makes most sense for reefers (as beeing temperature independent and measured in [psu]) but

2) it is also commonly used like "i measure the salinity" .... not saying" with which measurement method your measure it"
a) With a Refrak in [psu]
measuring the braking index of light

b) with a spindle in either ([g/cm^3] if using a spindle calibrated to "desnity" or
([-] if using a spindle calibrated to "spec. gravity")
measuring the buoyancy of the (lighter) spindle in (heavier) salt water

c) with a conductivity sensor (in ms/cm)
measuring the eletrical resistance that goes down the more salt our saltwater contains

 

[infinite0180]

I use the hanna salinity checker. It works great. Ive heated water to >80 and tested salinity at 35ppt. Then ive let the water cool down to 78 and rechecked. It still reads 35ppt. The compensation works excellent. Basically you can see it working when its first placed in water. The salinity reading wont stabilize until the temp reading is stable.

 

[Dkeller_nc]

Using eg a normal calibrated spindle (calibrated to density in g/cm^3) you get a quite different result if measuring water in a typical range for reef aquariums of 23°C and 26°C (which is 73,5 / 79 ° F)
a measurement result of 1,023 g/cm^3 @ 23°C -> 33,8 psu, whereas
a measurement result of 1,023 g/cm^3 @ 26°C -> 35,0 psu
for me this is already some difference.

I believe you may be confusing something. The actual salinity of seawater is invariant with temperature because it is defined as (g of dissolved salt/ kg of water). Since mass doesn't change with temperature, this dimensionless number is temperature invariant.

What you're noting in your post above is psu, which is defined as:

"The use of electrical conductivity measurements to estimate the ionic content of seawater led to the development of the scale called the practical salinity scale 1978 (PSS-78).[9][10] Salinities measured using PSS-78 do not have units. The suffix psu or PSU (denoting practical salinity unit) is sometimes added to PSS-78 measurement values.[11] "

That's from wikipedia's page on salinity.

What you're showing in your table is the change in electrical conductivity with temperature at a constant density. That is most certainly not the same as stating that the actual density of constant-composition seawater is changing in a like manner.

 

[vetteguy53081]

I have found that warmer the water, the higher the salinity and vice-versa

 

[Randy Ruiz]

I'm trying to follow all this, but frankly (as I'm guessing many have) been getting lost in much of it. I passed chemistry years ago with a "C". So, for many of us if we are using a refractometer, temperature should not make much difference, right? Recently I was mixing up a batch of water in my garage on a hot day & the temp got up to 95°F. I had to do an ice bath (put 3 med bags of ice in a large plastic garbage bag) to bring the temp down of 25 gallons to 78°F over about 30-45 min. The SG had not changed significantly from 1.025-1.026 during the ice bath. It would seem that if SG was temp dependent that such a great variation in temp would happen then.

 

[Dom]

I'll be paying closer attention to this as I move forward. My experience has been that using a remeasured cup to mix 5 gallon water changes, I have been getting higher readings since I added a heater to my mixing water as opposed to mixing at room temperature.

 

[Randy Holmes-Farley Verified Community Expert]

I'll be paying closer attention to this as I move forward. My experience has been that using a remeasured cup to mix 5 gallon water changes, I have been getting higher readings since I added a heater to my mixing water as opposed to mixing at room temperature.

Which likely reflects your measuring device and its temperature effects and errors, not the actual salinity.

 

[Dom]

Which likely reflects your measuring device and its temperature effects and errors, not the actual salinity.

Can you recommend a refractometer?