Whenever I prepare a new batch of salt water for my volcano shrimp (opae ula) tank, I need a way to measure the salinity.
Too much salt and it will kill the volcano shrimp. Too little salt and the volcano shrimp (opae ula) will not breed.
So the exact amount is required.
I use an inexpensive analogue specific gravity (S.G.) meter, also called a salinity meter or hydrometer.
There is also a digital salt meter called Koi Medic salt meter which measures the salinity in percentage or parts per thousand (ppt).
It is a bit more expensive but you can read the result instantly and not get your hand wet.
The drawback is the limited salinity range, between 0% to 2% for this model.
This is a simple analogue salinity meter.
I like it because I do not need to deal with flat batteries and leaking batteries.
It has a wide salinity range from 0 to 100 ppt (parts per thousand) and is inexpensive.