Hydrometer

A hydrometer is a device that measures certain characteristics such as density or specific gravity of liquids. It works on the buoyancy and fluid displacement principles. Hydrometers are hollow bodies with a constant volume calibrated over a particular range. 

They are calibrated according to their usage. For example, a hydrometer is calibrated as a lactometer for measuring milk density, a saccharometer to measure a liquid’s sugar density, or an alcohol metre to measure alcohol levels in spirits.

The hydrometer works on Archimedes’ principle – “When a solid is suspended in a fluid, the buoyant force acting on the solid equals the displaced fluid’s weight.”

Hydrometer construction and working

A hydrometer comprises a plastic or thin glass tube with both ends sealed, a printed or graduated scale, and calibrated to specific gravity. One end of this tube is bulb-shaped and weighted with either a lead shot or steel shot ballast. The instrument floats upright because of the ballast, just like a fishing bobber.

Another plastic or glass cylinder, known as a hydrometer jar, is filled with the liquid to be measured. The tube is then placed inside the jar containing the liquid to be measured.

When the sample liquid level in the jar aligns to a point on the hydrometer scale, the sample liquid’s specific gravity is indicated. How heavy or light the sample liquid is compared to water is recorded based on the scale used.

Scales of hydrometer

Hydrometers can be scaled to different scales for measuring specific gravities of other liquids. Some of these scales are as follows:

• Brix scale

Brix scale reads in Brix or Balling degrees. It measures the purity percent of a liquid compared to pure sucrose by weight.

• Baume scale

Baume scale hydrometers are calibrated to measure degrees of Baume. It comprises two scales – one for liquids lighter than water and the other for liquids heavier than water.

• Alcohol scale

Alcohol scale hydrometers are standard specific gravity metres that measure the specific gravity of the liquid before and after fermentation. The difference between the two is referenced to determine alcohol concentration by weight on an alcohol scale.

• API scale

API, also known as American Petroleum Institute Index, measures the heaviness or lightness of petroleum-based liquids compared to water. It is designed to compare the densities of petroleum liquids.

How to read hydrometer efficiently

Many hydrometers have more than one scale printed on them. For using a hydrometer:

1. Fill the jar with sample liquid.
2. Place the hydrometer in the jar and twirl it quickly to dislodge air bubbles.
3. When the hydrometer settles, record readings from the correct scale.

For accurate measurements, ensure that the temperature of the sample liquid is 15.55°C (60°F). If the liquid temperature varies, adjust the measurement accordingly.

The eye should be slightly below the surface plane of the liquid and raised slowly to the surface for reading transparent liquids. The surface looks elliptical but appears to be a straight line. The point at which the line coincides on the hydrometer scale, that hydrometer reading should be considered.

It is not easy to read if the liquid is not clear; hence, one shall take the reading from above the surface. Factor in this change in observation point to estimate the reading accurately.

Accuracy of a hydrometer

Three factors determine the accuracy of a hydrometer.

• Cleanliness

Proper cleaning of the hydrometer and its components–hydrometer surface and stem–is necessary for accurate readings. The liquid can rise uniformly and merge into the invisible stem film if they are correctly cleaned.

• Temperature

The temperature of the hydrometer and liquid should be at room temperature. It prevents changes in density while testing.

• Proper immersion

The hydrometer jar’s inside diameter should be approximately 1 inch greater than the hydrometer’s outside diameter.

Range of hydrometer

A hydrometer sinks deep in liquids with low density and less deep in liquids with high density. Hydrometers with dense liquids mark 1.000 at the top (for water) and 1.000 at the bottom for light liquids.

Various industries use a set of hydrometers with specific gravities ranging from (1.0-0.95 and 0.95-) to measure specific gravity fluids.

Uses of hydrometer

• A hydrometer is used to determine a liquid’s specific gravity or density. Some of the unique uses of hydrometers are as follows.

• A saccharometer is a hydrometer that is used along with a thermometer to check sugar concentration in wine or beer sugar. 

• Alcohol metre or proof and tralle hydrometer measures the alcohol content in fermented liquids.

• Certain hydrometers are used to grade soil that cannot be sieved.

• A lactometer is a hydrometer specifically designed to test milk. Milk contains a variety of lighter and heavier substances than water, so the specific gravity does not provide an accurate reading. Therefore, one can combine the specific gravity with other tests to get detailed information about the milk’s composition. The fat content of milk is usually measured using this method.

Conclusion

A hydrometer is a device that measures a liquid’s specific gravity and density. It works on Archimedes’ buoyancy principle. It is widely used in various industries for measuring specific gravities of multiple fluids. It is even used to measure the density of soil and salt. Furthermore, a hydrometer helps determine alcohol content, fat in milk, etc. It is an instrument that requires a physical recording of values. Thus, there are myriad uses of hydrometers, and they are essential scientific instruments.