Hydrometers are often used to measure salt water.
In addition to concentration (%), the specific gravity and Baume scale are also often used to manage the concentration of salt water.
A hydrometer is a glass areometer like the one pictured.
The hydrometer is floated in a glass cylinder or tank filled with salt water, and the marking corresponding to the surface of the liquid is read.
There are some hydrometers that include markings for the Baume scale as well as the specific gravity scale, and these are called Baume meters.
Hydrometers simply and conveniently measure the concentration (specific gravity or Baume scale) of salt water. An advantage of hydrometers is their low cost.
On the other hand, they are made of delicate glass, and broken glass can get into the salt water. It's also difficult to read which marking corresponds to the surface of the liquid, and some people are not able to prepare a large volume of salt water. Because of this, many people choose salt meters that use the refractive method, which can take measurements with only a few drops of salt water and are easy to read (PAL models have a digital display).
Specific gravity is the weight of 1 cm3 of the sample in grams divided by the weight of 1 cm3 of the water in grams. For example, if the weight of 1 cm3 of the sample is 1.65 g, and the weight of 1 cm3 of water is 1.00 g, the specific gravity of the sample would be 1.65 (1.65 ÷ 1.00 = 1.65). Specific gravity changes based on temperature, causing differences in specific gravity such as the following:
The specific gravity (20℃/20℃) = the weight of 1 cm3 of sample at 20℃ divided by the weight of 1 cm3 of water at 20℃
The specific gravity (20℃/4℃) = the weight of 1 cm3 of sample at 20℃ divided by the weight of 1 cm3 of water at 4℃
Specific gravity is measured by a hydrometer (glass areometer), but for each liquid there is a correlation between specific gravity and refractive index, leading many people to calculate the specific gravity of a sample using its refractive index. The reason why they do this is that hydrometers (glass areometers) require a large amount of liquid, whereas refractometers require a comparatively small amount for measurement.
The Baume scale is a user-friendly scale that was created because of how detailed and difficult to understand specific gravity measurements with a hydrometer are. The Baume scale and specific gravity have the following relationship:
Baume scale = 144.3 – (144.3 ÷ specific gravity)
This formula can be used for liquids heavier than water. The Baume scale of water is 0.0°.
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