Salinity is the amount of salt dissolved in a body of water. Since this is measured in grams per kilogram of water, it ends up without units - the grams and kilograms cancel each other out. But, because that is annoying, folks invented a unit called the "Practical Salinity Unit" or PSU for short. Typical seawater has a salinity of about 35 PSU. It can also be measured in parts-per-trillion which also ends up being about 35.
There is another way to measure the salinity of water and that is called specific gravity. Typical seawater has a specific gravity of about 1.025. The problem with this way of measuring salinity is that it depends on the temperature of the water. So, most scales are given for a reference temperature of 77°F (or 25°C).
That may seem daunting, but is worth explaining. Just know that if you see strange variances when measuring salinity, temperature could be a factor and that no matter how we measure it, we're going to be targeting 35 PSU or 1.025 sg.
Instruments to Measure Salinity
There are several ways to measure salinity in a water sample and correct and stable salinity is a crucial part of a reef tank. Because of that, I strongly encourage you to have at least two ways of measuring it so you don't rely on a single instrument that could be giving you incorrect readings.
No matter what instruments you use, you must always read the documentation and calibrate them frequently and correctly: following the manufacturer's instructions. I cannot stress this enough. We all get lazy and neglect to calibrate often and that's when bad things happen.
A hydrometer is a little plastic device you fill with water. It has a needle that floats to show you the specific gravity of the water.
These are cheap and available at most pet shops, but they are not recommended for our purposes. Do not use these.
It sounds intimidating, but it's not so bad. A refractometer uses a prism and light to show you the salinity of water - you just place a few drops of water onto its lens and then look into the eye piece, where a line will tell you the result.
I firmly believe this should be your number one way to measure salinity, always cross-checked with another instrument.
All refractometers are not created equal, however, so it is a good idea to spend enough to get a good one. After buying cheap a one from Amazon and regretting it, I ended up with a Red Sea Seawater Refractometer which has worked very well so far.
Make sure it is always calibrated and kept clean.
Hannah Marine Salinity Tester
This is a very attractive device that is easy to use: you just turn it on, dip it in the water and wait a few seconds to get both salinity and temperature readings.
I started my first tank with one of these and I ran into problems. After some time, it was giving me inacurate readings and I let my salinity get too high which negatively affected my corals. That being said, a lot of people rely on this instrument and, as long as you're using another instrument to cross-check, it can be a good option.
Milwaukee Digital Salinity Refractometer
This is another refractometer but it reads the salinity digitally and displays it on an LCD screen so you don't have to peer through an eye piece. This is, in my opinion, the best instrument for the job, so I highly recommend it although it is a bit pricey. It has automatic temperature correction and you can easily switch the display units between PSU, ppt and SG.
With the Milwaukee and the refractometer in hand, we're now ready to test the salinity of our freshly mixed saltwater.