What is a Hydrometer?

Hydrometers measure a liquid’s density and specific gravity, and rely on Archimedes’ principle of buoyancy. They are made up of a sealed glass bulb with a long, narrow tube connected to one end and a lead or mercury shot under the bulb so it can float in the upright position. The tube has a scale that displays the specific gravity of the liquid being measured. Hydrometers are normally calibrated to measure a liquid’s density at a predesignated temperature and the scale varies based on the instrument’s intended use.

What is Archimedes’ Principle of Buoyancy?

Archimedes’ principle of buoyancy states that an object floating in liquid is lifted up by a force equal to the weight of the liquid being displaced by the object. This means that the lower the density, the more the hydrometer sinks into the liquid sample. On the other hand, with higher density, there is greater force that lifts the bulb higher in the liquid.

How to Use a Hydrometer

Step 1 – Pour the liquid sample into a tall container or glass cylinder.

Step 2 – Place the hydrometer vertically in the liquid with the bulb end down to allow it to float.

Step 3 – Measure the depth of bulb flotation with the scale on the tube’s neck.  Take the measurement at the water level’s plane. The scale normally measures in specific gravity terms ranging from .95 to 1.05.

Step 4 – Repeat the measurement as many times as required.

What is a Liquid’s Specific Gravity?

A liquid’s specific gravity is the ratio of its mass to an equal volume of water. It is the number of times heavier or lighter the liquid is compared to water. Since the measurement is a ratio of the two masses, there are no units associated with it. If a liquid’s specific gravity is one, then the liquid has the same density and mass as the same volume of pure water. Hydrometers are calibrated to measure a liquid’s density at a predefined temperature. Historically, this was set to 39° F (4° C) since that is where pure water has the maximum density. However, the devices have been set to 60° F (15.5° C) or 68° F (20° C) more recently. The temperature the hydrometer has been calibrated to is labeled on the tube’s neck.

Types of Hydrometers

There are a few hydrometer types in use today with scales that vary based on their intended use. Hydrometers meant to be used with heavy liquids have a scale that starts at one (1) and goes higher. The ones that measure lighter liquids start significantly below the number one (1). Battery hydrometers measure the battery liquid’s specific gravity and assess the battery’s overall charge and condition. Alcoholometers measure the overall proof of alcohol and saccharometers measure the density of sugar in various sugar solutions.