Salinity

Hi!

How can we measure the correct specific density of our aquarium water?
All hydrometers are calibrated to measure water at around 25ºC (or 75ºF), even the Swing Arm hydrometers that are more tolerant regarding temperature will not give correct readings at 15-16ºC. 

I have found that refractometer gives correct reading regardless the temperature. 
This spring I read 10 PSU from the Baltic sea. Normally it is max. 8 PSU but usually only 5 PSU. We had hard time believing the reading we got and took the sample to a friend who is a hydrobiologist and the intendent of a public aquarium here. He read the salinity and got the same reading with the university's HiTech gear!

The sample was only 13°C and my refractometer is calibrated to tropical temperatures, yet we got accurate reading. I have had 35 PSU readings from tropical and cold water with my refractometer. I measure 350g TM salt per 10 liter when I do a water change, and the reading is always the same regardless the temperature... 
Hydrometers don't give accurate readings in cold water, I wouldn't trust them...

Hi Rapide!

Thank you for your answer.
After reading a lot about it i've come to the same conclusion, and i'm buying a refractometer.

In the meantime i'm looking for some way to use my floating hydrometer, there are some conversion s that we can make, but i'm having some trouble finding the correct formulas.

Can you measure with a refractometer then fill the hydrometer and put a corresponding mark on it?

That's a good idea... But I would test it a couple times before I would trust it
Some hydrometers are real crap. I have some cheap one somewhere that gave reading of 33‰ when my calibrated refractometer gave 38‰