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A hydrometer measures the specific gravity and density of a liquid based on Archimedes’ principle of buoyancy. It consists of a sealed glass bulb with a long thin tube, weighted with mercury or lead shot, and calibrated for use with liquids of a certain temperature. The specific gravity is the ratio of the mass of the liquid to that of an equal volume of pure water, and the scale varies depending on the intended use. Different types of hydrometers include heavy liquid, light liquid, battery-operated, alcoholometers, and saccharometers.
A hydrometer is a scientific instrument used to measure both the specific gravity and density of a liquid, based on Archimedes’ principle of buoyancy. It is composed of a sealed glass bulb with a long thin tube attached to one end, and is weighted with mercury or lead shot under the bulb so it can float upright. The portion of the tube is labeled with a scale so that when it is placed in a liquid, the measured specific gravity can be seen. The meter is typically calibrated for use with liquids of a certain temperature, and the scale on it may vary depending on its intended use.
Archimedes’ principle of buoyancy states that an object floating in a liquid is lifted up by a force equal to the weight of the liquid displaced by the object. This means that the lower the density, the more the hydrometer will sink in the liquid sample. On the other hand, with higher density, there is a higher force that will lift the bulb higher in the liquid.
To use a hydrometer, the liquid sample must be poured into a tall glass cylinder or similar container. The hydrometer is placed vertically in the liquid with the bulb end pointing down so that it sways and floats in the sample. The buoyancy depth of the bulb is measured using the scale on the neck of the tube and the reading should be taken at the plane of the water level, not where the liquid clings to the side of the neck. The scale is typically measured in terms of specific gravity, with a range from just below 1 to just above it, such as 0.95 to 1.05.
The specific gravity of a liquid is the ratio of the mass of that liquid to that of an equal volume of pure water. In simpler terms, it’s how many times the liquid is lighter or heavier than water. Since it is a ratio of mass, specific gravity has no units. For example, a specific gravity of 1 means the liquid has the same mass and density as the same volume of pure water, or if the measurement is less than 1, the liquid is less dense with a lower specific gravity.
Because the density of a liquid generally changes with temperature, a hydrometer is typically calibrated for use with a liquid at a certain temperature. Traditionally, a hydrometer has been calibrated to measure specific gravity using the density of water at 39°F (4°C) because this is where pure water reaches its maximum density. Recently, however, it’s more common for one to be calibrated at 60°F (15.5°C) or 68°F (20°C). The instrument calibration will usually be labeled on the neck of the tube and the liquid sample should be at the calibration temperature when the measurement is taken.
There are different types of hydrometers, which have different scales that vary according to their use. A heavy liquid hydrometer usually has a scale starting at 1 and beyond, and a light liquid one might have a scale starting far below 1. A battery-operated hydrometer, which often comes as a digital model, measures the specific gravity of battery fluid and can be used to determine the condition and charge of a battery. Other hydrometers include alcoholometers, which measure in terms of the proof of alcohol, and saccharometers, which measure the density of sugar in a sugar solution.
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