A hydrometer measures how dense the water is. The more salt that is in the water, the more dense it is. The hydrometer we use might not be like the ones you have seen. The one we use is a kind that is designed to test the salinity in an aquarium. This is what it looks like:
To use it you fill it to the top, dipping it below the surface of the water. After you gently dip the hydrometer in to the water, tap the pointer with a pencil. You do that to avoid air bubbles that may lift the scale, Read the scale. Be sure you fill the water to the top. Then rinse it after you use it..
Periodic cleaning is recommended to keep the dust from making the pointer
to read incorrectly. Do not take it apart. Just soak it in vinegar (acid)
for 30 minutes and this will also remove dust. Rinse with water and
dry.