Is the salinity of water higher or lower as temperature drops? Meaning if you mix water at 77 degrees and get a reading would that reading be higher or lower at 74 degrees? At what temp do you keep your tanks?
if you are using a hydrometer rather than a refractometer the water temperature will have an effect on your reading. Even refractometers say to wait 30 seconds for the temp of the water in the sample to normalize to room temp. Unfortunately, I am not sure which way it will go, but my intuition tells me that since cold water is denser that it will cause the "needle" of a hydrometer to raise more.
salinity and temperature are independent, except for the secondary effect of raised water temperature increasing evaporation which was mentioned earlier
salinity is a measure of the ratio between total dissolved salt weight and pure water weight. while it is true that temperature and density have an inverse relationship generally speaking (a notable exception is around phase shifts with polar molecules). I doesn't technically matter. Instantaneously in an open system or generally in a closed system, the number of water molecules and number of the various salt molecules do not change with variation in temperature.
Of course we don't want to find ourselves using scales or stills every time we need to check our salinity -- that would be silly. We use quicker and more efficient ways to measure salinity, and those measurements depend on temperature.
For example a conductivity probe will always give a reading that is temperature dependent. This is because waters conductivity increases with temperature. A lot of controllers, like my apex, using this type of probe will do the adjustment for you somewhat accurately, but in my opinion not accurate enough. There are a variety of factors that effect this type of measurement.
Both analog refractometers and digital refractometer like mine work by analyzing the light emitted through a fluid. Particularly how the light bends as the photons go through and interact with the molecules in your tank water. Since colder temperatures increases density the photons will interact with more salt water molecules per distance travelled. The opposite is true as well. Since bending of the light is the basis of the measurement density can plainly be seen as a variable.
A hydrometers readings also greatly depend on temperature, because they look at the relative density of pure water and your sample. Temperature directly effects density for both pure water and your water with dissolved salts. Although, it is not true that a given change in temperature in both pure water and aquarium water will yield an equal change in density between the two samples.
Last edited by willienelson; 01-31-2019 at 12:54 AM.