Hydrometers are used to determine the specific gravity of the water. Two types available; floating glass and plastic swing arm. They are advantageous as they are relatively cheap, easy and quick to use. However, care must be taken with the readings they provide, as some are notoriously inaccurate in the results they provide. Ensure that no bubbles or debris attached to them, which will significantly alter the reading provided.
Most, if not all, hydrometers used in the hobby give specific gravity as the ratio of the density of the sample at 20ºC to the density of pure water at 4ºC (d20/4). Seawater with a salinity of 35‰ (35 parts per thousand) will have a specific gravity (@ d20/4) of 1.0234. The target salinity should be 35‰, so a specific gravity from one of these hydrometers of 1.023 to 1.024 is fine, depending on errors in reading or even a bit of evaporation.
Floating glass hydrometers consist of a calibrate glass tube with a specific gravity scale towards the top. They work by placing it within a calm sample of water (it will not work by placing it in the tank with pumps still moving water around) and reading the value off the scale where the water surface intersects with the scale.
They require a largish water sample, of sufficient depth to allow it to float without touching the bottom of the vessel. They are difficult to use in a tank, and it is best practice to take water sample out of tank into a container and use that. For example a jug or cut off soft drink bottle. Care must be taken, since they are made from glass and can break easily. Ensure that there are no bubbles, debris or precipitate on the surface of the glass float hydrometer, as these will change the reading.
Plastic Swing Arm
These are made from plastic and contain a plastic swing arm who's height changes with density of the water and are quick and easy to use. Beware of any bubbles on the swing arm as these will increase the specific gravity reading provided, and any films or precipitate on the arm which will decrease the value. Care must be also taken with the factory calibration of the unit, since some many provide a highly erroneous reading. It is best to calibrate it against the reading from a refractometer, however filling it with pure water (which should read 1.000) is a good first step.
Scale for determining specific gravity with floating hydrometer. Value taken at point water surface, not the meniscus, aligns with the scale.