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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.
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.
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.
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.
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.
When you speak of density you are speaking of salt density?
Isn't salinity the ratio of salt to water in the tank?
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'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.